============================================Dynamic DMA mapping using the generic device============================================ :Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com> This document describes the DMA API. For a more gentle introductionof the API (and actual examples), see Documentation/core-api/dma-api-howto.rst. This API is split into two pieces. Part I describes the basic API.Part II describes extensions for supporting non-coherent memorymachines. Unless you know that your driver absolutely has to supportnon-coherent platforms (this is usually only legacy platforms) youshould only use the API described in part I. Part I - DMA API---------------- To get the DMA API, you must #include <linux/dma-mapping.h>. Thisprovides dma_addr_t and the interfaces described below. A dma_addr_t can hold any valid DMA address for the platform. It can begiven to a device to use as a DMA source or target. A CPU cannot referencea dma_addr_t directly because there may be translation between its physicaladdress space and the DMA address space. Part Ia - Using large DMA-coherent buffers------------------------------------------ :: void * dma_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_handle, gfp_t flag) Coherent memory is memory for which a write by either the device orthe processor can immediately be read by the processor or devicewithout having to worry about caching effects. (You may however needto make sure to flush the processor's write buffers before tellingdevices to read that memory.) This routine allocates a region of <size> bytes of coherent memory. It returns a pointer to the allocated region (in the processor's virtualaddress space) or NULL if the allocation failed. It also returns a <dma_handle> which may be cast to an unsigned integer thesame width as the bus and given to the device as the DMA address base ofthe region. Note: coherent memory can be expensive on some platforms, and theminimum allocation length may be as big as a page, so you shouldconsolidate your requests for coherent memory as much as possible.The simplest way to do that is to use the dma_pool calls (see below). The flag parameter allows the caller to specify the ``GFP_`` flags (seekmalloc()) for the allocation (the implementation may ignore flags that affectthe location of the returned memory, like GFP_DMA). :: void dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, dma_addr_t dma_handle) Free a previously allocated region of coherent memory. dev, size and dma_handlemust all be the same as those passed into dma_alloc_coherent(). cpu_addr mustbe the virtual address returned by dma_alloc_coherent(). Note that unlike the sibling allocation call, this routine may only be calledwith IRQs enabled. Part Ib - Using small DMA-coherent buffers------------------------------------------ To get this part of the DMA API, you must #include <linux/dmapool.h> Many drivers need lots of small DMA-coherent memory regions for DMAdescriptors or I/O buffers. Rather than allocating in units of a pageor more using dma_alloc_coherent(), you can use DMA pools. These workmuch like a struct kmem_cache, except that they use the DMA-coherent allocator,not __get_free_pages(). Also, they understand common hardware constraintsfor alignment, like queue heads needing to be aligned on N-byte boundaries. .. kernel-doc:: mm/dmapool.c :export: .. kernel-doc:: include/linux/dmapool.h Part Ic - DMA addressing limitations------------------------------------ DMA mask is a bit mask of the addressable region for the device. In other words,if applying the DMA mask (a bitwise AND operation) to the DMA address of amemory region does not clear any bits in the address, then the device canperform DMA to that memory region. All the below functions which set a DMA mask may fail if the requested maskcannot be used with the device, or if the device is not capable of doing DMA. :: int dma_set_mask_and_coherent(struct device *dev, u64 mask) Updates both streaming and coherent DMA masks. Returns: 0 if successful and a negative error if not. :: int dma_set_mask(struct device *dev, u64 mask) Updates only the streaming DMA mask. Returns: 0 if successful and a negative error if not. :: int dma_set_coherent_mask(struct device *dev, u64 mask) Updates only the coherent DMA mask. Returns: 0 if successful and a negative error if not. :: u64 dma_get_required_mask(struct device *dev) This API returns the mask that the platform requires tooperate efficiently. Usually this means the returned maskis the minimum required to cover all of memory. Examining therequired mask gives drivers with variable descriptor sizes theopportunity to use smaller descriptors as necessary. Requesting the required mask does not alter the current mask. If youwish to take advantage of it, you should issue a dma_set_mask()call to set the mask to the value returned. :: size_t dma_max_mapping_size(struct device *dev); Returns the maximum size of a mapping for the device. The size parameterof the mapping functions like dma_map_single(), dma_map_page() andothers should not be larger than the returned value. :: size_t dma_opt_mapping_size(struct device *dev); Returns the maximum optimal size of a mapping for the device. Mapping larger buffers may take much longer in certain scenarios. Inaddition, for high-rate short-lived streaming mappings, the upfront timespent on the mapping may account for an appreciable part of the totalrequest lifetime. As such, if splitting larger requests incurs nosignificant performance penalty, then device drivers are advised tolimit total DMA streaming mappings length to the returned value. :: bool dma_need_sync(struct device *dev, dma_addr_t dma_addr); Returns %true if dma_sync_single_for_{device,cpu} calls are required totransfer memory ownership. Returns %false if those calls can be skipped. :: unsigned long dma_get_merge_boundary(struct device *dev); Returns the DMA merge boundary. If the device cannot merge any DMA addresssegments, the function returns 0. Part Id - Streaming DMA mappings-------------------------------- Streaming DMA allows to map an existing buffer for DMA transfers and thenunmap it when finished. Map functions are not guaranteed to succeed, so thereturn value must be checked. .. note:: In particular, mapping may fail for memory not addressable by the device, e.g. if it is not within the DMA mask of the device and/or a connecting bus bridge. Streaming DMA functions try to overcome such addressing constraints, either by using an IOMMU (a device which maps I/O DMA addresses to physical memory addresses), or by copying the data to/from a bounce buffer if the kernel is configured with a :doc:`SWIOTLB <swiotlb>`. However, these methods are not always available, and even if they are, they may still fail for a number of reasons. In short, a device driver may need to be wary of where buffers are located in physical memory, especially if the DMA mask is less than 32 bits. :: dma_addr_t dma_map_single(struct device *dev, void *cpu_addr, size_t size, enum dma_data_direction direction) Maps a piece of processor virtual memory so it can be accessed by thedevice and returns the DMA address of the memory. The DMA API uses a strongly typed enumerator for its direction: ======================= =============================================DMA_NONE no direction (used for debugging)DMA_TO_DEVICE data is going from the memory to the deviceDMA_FROM_DEVICE data is coming from the device to the memoryDMA_BIDIRECTIONAL direction isn't known======================= ============================================= .. note:: Contiguous kernel virtual space may not be contiguous as physical memory. Since this API does not provide any scatter/gather capability, it will fail if the user tries to map a non-physically contiguous piece of memory. For this reason, memory to be mapped by this API should be obtained from sources which guarantee it to be physically contiguous (like kmalloc). .. warning:: Memory coherency operates at a granularity called the cache line width. In order for memory mapped by this API to operate correctly, the mapped region must begin exactly on a cache line boundary and end exactly on one (to prevent two separately mapped regions from sharing a single cache line). Since the cache line size may not be known at compile time, the API will not enforce this requirement. Therefore, it is recommended that driver writers who don't take special care to determine the cache line size at run time only map virtual regions that begin and end on page boundaries (which are guaranteed also to be cache line boundaries). DMA_TO_DEVICE synchronisation must be done after the last modification of the memory region by the software and before it is handed off to the device. Once this primitive is used, memory covered by this primitive should be treated as read-only by the device. If the device may write to it at any point, it should be DMA_BIDIRECTIONAL (see below). DMA_FROM_DEVICE synchronisation must be done before the driver accesses data that may be changed by the device. This memory should be treated as read-only by the driver. If the driver needs to write to it at any point, it should be DMA_BIDIRECTIONAL (see below). DMA_BIDIRECTIONAL requires special handling: it means that the driver isn't sure if the memory was modified before being handed off to the device and also isn't sure if the device will also modify it. Thus, you must always sync bidirectional memory twice: once before the memory is handed off to the device (to make sure all memory changes are flushed from the processor) and once before the data may be accessed after being used by the device (to make sure any processor cache lines are updated with data that the device may have changed). :: void dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size, enum dma_data_direction direction) Unmaps the region previously mapped. All the parameters passed inmust be identical to those passed to (and returned by) dma_map_single(). :: dma_addr_t dma_map_page(struct device *dev, struct page *page, unsigned long offset, size_t size, enum dma_data_direction direction) void dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size, enum dma_data_direction direction) API for mapping and unmapping for pages. All the notes and warningsfor the other mapping APIs apply here. Also, although the <offset>and <size> parameters are provided to do partial page mapping, it isrecommended that you never use these unless you really know what thecache width is. :: dma_addr_t dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) void dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size, enum dma_data_direction dir, unsigned long attrs) API for mapping and unmapping for MMIO resources. All the notes andwarnings for the other mapping APIs apply here. The API should only beused to map device MMIO resources, mapping of RAM is not permitted. :: int dma_mapping_error(struct device *dev, dma_addr_t dma_addr) In some circumstances dma_map_single(), dma_map_page() and dma_map_resource()will fail to create a mapping. A driver can check for these errors by testingthe returned DMA address with dma_mapping_error(). A non-zero return valuemeans the mapping could not be created and the driver should take appropriateaction (e.g. reduce current DMA mapping usage or delay and try again later). :: int dma_map_sg(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) Maps a scatter/gather list for DMA. Returns the number of DMA address segmentsmapped, which may be smaller than <nents> passed in if several consecutivesglist entries are merged (e.g. with an IOMMU, or if some adjacent segmentsjust happen to be physically contiguous). Please note that the sg cannot be mapped again if it has been mapped once.The mapping process is allowed to destroy information in the sg. As with the other mapping interfaces, dma_map_sg() can fail. When itdoes, 0 is returned and a driver must take appropriate action. It iscritical that the driver do something, in the case of a block driveraborting the request or even oopsing is better than doing nothing andcorrupting the filesystem. With scatterlists, you use the resulting mapping like this:: int i, count = dma_map_sg(dev, sglist, nents, direction); struct scatterlist *sg; for_each_sg(sglist, sg, count, i) { hw_address[i] = sg_dma_address(sg); hw_len[i] = sg_dma_len(sg); } where nents is the number of entries in the sglist. The implementation is free to merge several consecutive sglist entriesinto one. The returned number is the actual number of sg entries itmapped them to. On failure, 0 is returned. Then you should loop count times (note: this can be less than nents times)and use sg_dma_address() and sg_dma_len() macros where you previouslyaccessed sg->address and sg->length as shown above. :: void dma_unmap_sg(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) Unmap the previously mapped scatter/gather list. All the parametersmust be the same as those and passed in to the scatter/gather mappingAPI. Note: <nents> must be the number you passed in, *not* the number ofDMA address entries returned. :: void dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size, enum dma_data_direction direction) void dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size, enum dma_data_direction direction) void dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) void dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) Synchronise a single contiguous or scatter/gather mapping for the CPUand device. With the sync_sg API, all the parameters must be the sameas those passed into the sg mapping API. With the sync_single API,you can use dma_handle and size parameters that aren't identical tothose passed into the single mapping API to do a partial sync. .. note:: You must do this: - Before reading values that have been written by DMA from the device (use the DMA_FROM_DEVICE direction) - After writing values that will be written to the device using DMA (use the DMA_TO_DEVICE) direction - before *and* after handing memory to the device if the memory is DMA_BIDIRECTIONAL See also dma_map_single(). :: dma_addr_t dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) void dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) int dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl, int nents, enum dma_data_direction dir, unsigned long attrs) void dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl, int nents, enum dma_data_direction dir, unsigned long attrs) The four functions above are just like the counterpart functionswithout the _attrs suffixes, except that they pass an optionaldma_attrs. The interpretation of DMA attributes is architecture-specific, andeach attribute should be documented inDocumentation/core-api/dma-attributes.rst. If dma_attrs are 0, the semantics of each of these functionsis identical to those of the corresponding functionwithout the _attrs suffix. As a result dma_map_single_attrs()can generally replace dma_map_single(), etc. As an example of the use of the ``*_attrs`` functions, here's howyou could pass an attribute DMA_ATTR_FOO when mapping memoryfor DMA:: #include <linux/dma-mapping.h> /* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and * documented in Documentation/core-api/dma-attributes.rst */ ... unsigned long attr; attr |= DMA_ATTR_FOO; .... n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr); .... Architectures that care about DMA_ATTR_FOO would check for itspresence in their implementations of the mapping and unmappingroutines, e.g.::: void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) { .... if (attrs & DMA_ATTR_FOO) /* twizzle the frobnozzle */ .... } Part Ie - IOVA-based DMA mappings--------------------------------- These APIs allow a very efficient mapping when using an IOMMU. They are anoptional path that requires extra code and are only recommended for driverswhere DMA mapping performance, or the space usage for storing the DMA addressesmatter. All the considerations from the previous section apply here as well. :: bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state, phys_addr_t phys, size_t size); Is used to try to allocate IOVA space for mapping operation. If it returnsfalse this API can't be used for the given device and the normal streamingDMA mapping API should be used. The ``struct dma_iova_state`` is allocatedby the driver and must be kept around until unmap time. :: static inline bool dma_use_iova(struct dma_iova_state *state) Can be used by the driver to check if the IOVA-based API is used after aLines 1–500 of 868. Load more
============================================ Dynamic DMA mapping using the generic device ============================================ :Author: James E.J. Bottomley <James.Bottomley@HansenPartnership.com> This document describes the DMA API. For a more gentle introduction of the API (and actual examples), see Documentation/core-api/dma-api-howto.rst. This API is split into two pieces. Part I describes the basic API. Part II describes extensions for supporting non-coherent memory machines. Unless you know that your driver absolutely has to support non-coherent platforms (this is usually only legacy platforms) you should only use the API described in part I. Part I - DMA API ---------------- To get the DMA API, you must #include <linux/dma-mapping.h>. This provides dma_addr_t and the interfaces described below. A dma_addr_t can hold any valid DMA address for the platform. It can be given to a device to use as a DMA source or target. A CPU cannot reference a dma_addr_t directly because there may be translation between its physical address space and the DMA address space. Part Ia - Using large DMA-coherent buffers ------------------------------------------ :: void * dma_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_handle, gfp_t flag) Coherent memory is memory for which a write by either the device or the processor can immediately be read by the processor or device without having to worry about caching effects. (You may however need to make sure to flush the processor's write buffers before telling devices to read that memory.) This routine allocates a region of <size> bytes of coherent memory. It returns a pointer to the allocated region (in the processor's virtual address space) or NULL if the allocation failed. It also returns a <dma_handle> which may be cast to an unsigned integer the same width as the bus and given to the device as the DMA address base of the region. Note: coherent memory can be expensive on some platforms, and the minimum allocation length may be as big as a page, so you should consolidate your requests for coherent memory as much as possible. The simplest way to do that is to use the dma_pool calls (see below). The flag parameter allows the caller to specify the ``GFP_`` flags (see kmalloc()) for the allocation (the implementation may ignore flags that affect the location of the returned memory, like GFP_DMA). :: void dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, dma_addr_t dma_handle) Free a previously allocated region of coherent memory. dev, size and dma_handle must all be the same as those passed into dma_alloc_coherent(). cpu_addr must be the virtual address returned by dma_alloc_coherent(). Note that unlike the sibling allocation call, this routine may only be called with IRQs enabled. Part Ib - Using small DMA-coherent buffers ------------------------------------------ To get this part of the DMA API, you must #include <linux/dmapool.h> Many drivers need lots of small DMA-coherent memory regions for DMA descriptors or I/O buffers. Rather than allocating in units of a page or more using dma_alloc_coherent(), you can use DMA pools. These work much like a struct kmem_cache, except that they use the DMA-coherent allocator, not __get_free_pages(). Also, they understand common hardware constraints for alignment, like queue heads needing to be aligned on N-byte boundaries. .. kernel-doc:: mm/dmapool.c :export: .. kernel-doc:: include/linux/dmapool.h Part Ic - DMA addressing limitations ------------------------------------ DMA mask is a bit mask of the addressable region for the device. In other words, if applying the DMA mask (a bitwise AND operation) to the DMA address of a memory region does not clear any bits in the address, then the device can perform DMA to that memory region. All the below functions which set a DMA mask may fail if the requested mask cannot be used with the device, or if the device is not capable of doing DMA. :: int dma_set_mask_and_coherent(struct device *dev, u64 mask) Updates both streaming and coherent DMA masks. Returns: 0 if successful and a negative error if not. :: int dma_set_mask(struct device *dev, u64 mask) Updates only the streaming DMA mask. Returns: 0 if successful and a negative error if not. :: int dma_set_coherent_mask(struct device *dev, u64 mask) Updates only the coherent DMA mask. Returns: 0 if successful and a negative error if not. :: u64 dma_get_required_mask(struct device *dev) This API returns the mask that the platform requires to operate efficiently. Usually this means the returned mask is the minimum required to cover all of memory. Examining the required mask gives drivers with variable descriptor sizes the opportunity to use smaller descriptors as necessary. Requesting the required mask does not alter the current mask. If you wish to take advantage of it, you should issue a dma_set_mask() call to set the mask to the value returned. :: size_t dma_max_mapping_size(struct device *dev); Returns the maximum size of a mapping for the device. The size parameter of the mapping functions like dma_map_single(), dma_map_page() and others should not be larger than the returned value. :: size_t dma_opt_mapping_size(struct device *dev); Returns the maximum optimal size of a mapping for the device. Mapping larger buffers may take much longer in certain scenarios. In addition, for high-rate short-lived streaming mappings, the upfront time spent on the mapping may account for an appreciable part of the total request lifetime. As such, if splitting larger requests incurs no significant performance penalty, then device drivers are advised to limit total DMA streaming mappings length to the returned value. :: bool dma_need_sync(struct device *dev, dma_addr_t dma_addr); Returns %true if dma_sync_single_for_{device,cpu} calls are required to transfer memory ownership. Returns %false if those calls can be skipped. :: unsigned long dma_get_merge_boundary(struct device *dev); Returns the DMA merge boundary. If the device cannot merge any DMA address segments, the function returns 0. Part Id - Streaming DMA mappings -------------------------------- Streaming DMA allows to map an existing buffer for DMA transfers and then unmap it when finished. Map functions are not guaranteed to succeed, so the return value must be checked. .. note:: In particular, mapping may fail for memory not addressable by the device, e.g. if it is not within the DMA mask of the device and/or a connecting bus bridge. Streaming DMA functions try to overcome such addressing constraints, either by using an IOMMU (a device which maps I/O DMA addresses to physical memory addresses), or by copying the data to/from a bounce buffer if the kernel is configured with a :doc:`SWIOTLB <swiotlb>`. However, these methods are not always available, and even if they are, they may still fail for a number of reasons. In short, a device driver may need to be wary of where buffers are located in physical memory, especially if the DMA mask is less than 32 bits. :: dma_addr_t dma_map_single(struct device *dev, void *cpu_addr, size_t size, enum dma_data_direction direction) Maps a piece of processor virtual memory so it can be accessed by the device and returns the DMA address of the memory. The DMA API uses a strongly typed enumerator for its direction: ======================= ============================================= DMA_NONE no direction (used for debugging) DMA_TO_DEVICE data is going from the memory to the device DMA_FROM_DEVICE data is coming from the device to the memory DMA_BIDIRECTIONAL direction isn't known ======================= ============================================= .. note:: Contiguous kernel virtual space may not be contiguous as physical memory. Since this API does not provide any scatter/gather capability, it will fail if the user tries to map a non-physically contiguous piece of memory. For this reason, memory to be mapped by this API should be obtained from sources which guarantee it to be physically contiguous (like kmalloc). .. warning:: Memory coherency operates at a granularity called the cache line width. In order for memory mapped by this API to operate correctly, the mapped region must begin exactly on a cache line boundary and end exactly on one (to prevent two separately mapped regions from sharing a single cache line). Since the cache line size may not be known at compile time, the API will not enforce this requirement. Therefore, it is recommended that driver writers who don't take special care to determine the cache line size at run time only map virtual regions that begin and end on page boundaries (which are guaranteed also to be cache line boundaries). DMA_TO_DEVICE synchronisation must be done after the last modification of the memory region by the software and before it is handed off to the device. Once this primitive is used, memory covered by this primitive should be treated as read-only by the device. If the device may write to it at any point, it should be DMA_BIDIRECTIONAL (see below). DMA_FROM_DEVICE synchronisation must be done before the driver accesses data that may be changed by the device. This memory should be treated as read-only by the driver. If the driver needs to write to it at any point, it should be DMA_BIDIRECTIONAL (see below). DMA_BIDIRECTIONAL requires special handling: it means that the driver isn't sure if the memory was modified before being handed off to the device and also isn't sure if the device will also modify it. Thus, you must always sync bidirectional memory twice: once before the memory is handed off to the device (to make sure all memory changes are flushed from the processor) and once before the data may be accessed after being used by the device (to make sure any processor cache lines are updated with data that the device may have changed). :: void dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size, enum dma_data_direction direction) Unmaps the region previously mapped. All the parameters passed in must be identical to those passed to (and returned by) dma_map_single(). :: dma_addr_t dma_map_page(struct device *dev, struct page *page, unsigned long offset, size_t size, enum dma_data_direction direction) void dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size, enum dma_data_direction direction) API for mapping and unmapping for pages. All the notes and warnings for the other mapping APIs apply here. Also, although the <offset> and <size> parameters are provided to do partial page mapping, it is recommended that you never use these unless you really know what the cache width is. :: dma_addr_t dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) void dma_unmap_resource(struct device *dev, dma_addr_t addr, size_t size, enum dma_data_direction dir, unsigned long attrs) API for mapping and unmapping for MMIO resources. All the notes and warnings for the other mapping APIs apply here. The API should only be used to map device MMIO resources, mapping of RAM is not permitted. :: int dma_mapping_error(struct device *dev, dma_addr_t dma_addr) In some circumstances dma_map_single(), dma_map_page() and dma_map_resource() will fail to create a mapping. A driver can check for these errors by testing the returned DMA address with dma_mapping_error(). A non-zero return value means the mapping could not be created and the driver should take appropriate action (e.g. reduce current DMA mapping usage or delay and try again later). :: int dma_map_sg(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) Maps a scatter/gather list for DMA. Returns the number of DMA address segments mapped, which may be smaller than <nents> passed in if several consecutive sglist entries are merged (e.g. with an IOMMU, or if some adjacent segments just happen to be physically contiguous). Please note that the sg cannot be mapped again if it has been mapped once. The mapping process is allowed to destroy information in the sg. As with the other mapping interfaces, dma_map_sg() can fail. When it does, 0 is returned and a driver must take appropriate action. It is critical that the driver do something, in the case of a block driver aborting the request or even oopsing is better than doing nothing and corrupting the filesystem. With scatterlists, you use the resulting mapping like this:: int i, count = dma_map_sg(dev, sglist, nents, direction); struct scatterlist *sg; for_each_sg(sglist, sg, count, i) { hw_address[i] = sg_dma_address(sg); hw_len[i] = sg_dma_len(sg); } where nents is the number of entries in the sglist. The implementation is free to merge several consecutive sglist entries into one. The returned number is the actual number of sg entries it mapped them to. On failure, 0 is returned. Then you should loop count times (note: this can be less than nents times) and use sg_dma_address() and sg_dma_len() macros where you previously accessed sg->address and sg->length as shown above. :: void dma_unmap_sg(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) Unmap the previously mapped scatter/gather list. All the parameters must be the same as those and passed in to the scatter/gather mapping API. Note: <nents> must be the number you passed in, *not* the number of DMA address entries returned. :: void dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size, enum dma_data_direction direction) void dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size, enum dma_data_direction direction) void dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) void dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nents, enum dma_data_direction direction) Synchronise a single contiguous or scatter/gather mapping for the CPU and device. With the sync_sg API, all the parameters must be the same as those passed into the sg mapping API. With the sync_single API, you can use dma_handle and size parameters that aren't identical to those passed into the single mapping API to do a partial sync. .. note:: You must do this: - Before reading values that have been written by DMA from the device (use the DMA_FROM_DEVICE direction) - After writing values that will be written to the device using DMA (use the DMA_TO_DEVICE) direction - before *and* after handing memory to the device if the memory is DMA_BIDIRECTIONAL See also dma_map_single(). :: dma_addr_t dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) void dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) int dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl, int nents, enum dma_data_direction dir, unsigned long attrs) void dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl, int nents, enum dma_data_direction dir, unsigned long attrs) The four functions above are just like the counterpart functions without the _attrs suffixes, except that they pass an optional dma_attrs. The interpretation of DMA attributes is architecture-specific, and each attribute should be documented in Documentation/core-api/dma-attributes.rst. If dma_attrs are 0, the semantics of each of these functions is identical to those of the corresponding function without the _attrs suffix. As a result dma_map_single_attrs() can generally replace dma_map_single(), etc. As an example of the use of the ``*_attrs`` functions, here's how you could pass an attribute DMA_ATTR_FOO when mapping memory for DMA:: #include <linux/dma-mapping.h> /* DMA_ATTR_FOO should be defined in linux/dma-mapping.h and * documented in Documentation/core-api/dma-attributes.rst */ ... unsigned long attr; attr |= DMA_ATTR_FOO; .... n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, attr); .... Architectures that care about DMA_ATTR_FOO would check for its presence in their implementations of the mapping and unmapping routines, e.g.::: void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) { .... if (attrs & DMA_ATTR_FOO) /* twizzle the frobnozzle */ .... } Part Ie - IOVA-based DMA mappings --------------------------------- These APIs allow a very efficient mapping when using an IOMMU. They are an optional path that requires extra code and are only recommended for drivers where DMA mapping performance, or the space usage for storing the DMA addresses matter. All the considerations from the previous section apply here as well. :: bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state, phys_addr_t phys, size_t size); Is used to try to allocate IOVA space for mapping operation. If it returns false this API can't be used for the given device and the normal streaming DMA mapping API should be used. The ``struct dma_iova_state`` is allocated by the driver and must be kept around until unmap time. :: static inline bool dma_use_iova(struct dma_iova_state *state) Can be used by the driver to check if the IOVA-based API is used after a