A recent change to the mm code in: 87744ab3832b mm: fix cache mode tracking in vm_insert_mixed() started enforcing checking the memory type against the registered list for amixed pfn insertion mappings. It happens that the drm drivers for a number of gpus relied on this being broken. Currently the driver only inserted VRAM mappings into the tracking table when they came from the kernel, and userspace mappings never landed in the table. This led to a regression where all the mapping end up as UC instead of WC now. I've considered a number of solutions but since this needs to be fixed in fixes and not next, and some of the solutions were going to introduce overhead that hadn't been there before I didn't consider them viable at this stage. These mainly concerned hooking into the TTM io reserve APIs, but these API have a bunch of fast paths I didn't want to unwind to add this to. The solution I've decided on is to add a new API like the arch_phys_wc APIs (these would have worked but wc_del didn't take a range), and use them from the drivers to add a WC compatible mapping to the table for all VRAM on those GPUs. This means we can then create userspace mapping that won't get degraded to UC. v1.1: use CONFIG_X86_PAT + add some comments in io.h
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| @@ -141,4 +141,26 @@ enum { · 140 unmodified lines | ||
| 141 | 141 | void *memremap(resource_size_t offset, size_t size, unsigned long flags); |
| 142 | 142 | void memunmap(void *addr); |
| 143 | 143 | |
| 144 | +/* | |
| 145 | + * On x86 PAT systems we have memory tracking that keeps track of | |
| 146 | + * the allowed mappings on memory ranges. This tracking works for | |
| 147 | + * all the in-kernel mapping APIs (ioremap*), but where the user | |
| 148 | + * wishes to map a range from a physical device into user memory | |
| 149 | + * the tracking won't be updated. This API is to be used by | |
| 150 | + * drivers which remap physical device pages into userspace, | |
| 151 | + * and wants to make sure they are mapped WC and not UC. | |
| 152 | + */ | |
| 153 | +#ifndef arch_io_reserve_memtype_wc | |
| 154 | +static inline int arch_io_reserve_memtype_wc(resource_size_t base, | |
| 155 | + resource_size_t size) | |
| 156 | +{ | |
| 157 | + return 0; | |
| 158 | +} | |
| 159 | + | |
| 160 | +static inline void arch_io_free_memtype_wc(resource_size_t base, | |
| 161 | + resource_size_t size) | |
| 162 | +{ | |
| 163 | +} | |
| 164 | +#endif | |
| 165 | + | |
| 144 | 166 | #endif /* _LINUX_IO_H */ |