于 2024年6月30日 GMT+08:00 03:57:47，Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx> 写道：
>
>
>在2024年6月29日六月 上午6:22，Icenowy Zheng写道：
>[...]
>> @@ -302,6 +302,10 @@ pgprot_t ttm_io_prot(struct ttm_buffer_object *bo,
>> struct ttm_resource *res,
>> caching = res->bus.caching;
>> }
>>
>> + /* Downgrade cached mapping for non-snooping devices */
>> + if (!bo->bdev->dma_coherent && caching == ttm_cached)
>> + caching = ttm_write_combined;
>Hi Icenowy,
>
>Thanks for your patch! You saved many non-coh PCIe host implementations a day!.
>
>Unfortunately I don't think we can safely ttm_cached to ttm_write_comnined, we've
>had enough drama with write combine behaviour on all different platforms.
>
>See drm_arch_can_wc_memory in drm_cache.h.
>
Yes this really sounds like an issue.
Maybe the behavior of ttm_write_combined should furtherly be decided
by drm_arch_can_wc_memory() in case of quirks?
>Thanks
>
>> +
>> return ttm_prot_from_caching(caching, tmp);
>> }
>> EXPORT_SYMBOL(ttm_io_prot);
>> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
>> index 7b00ddf0ce49f..3335df45fba5e 100644
>> --- a/drivers/gpu/drm/ttm/ttm_tt.c
>> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
>> @@ -152,6 +152,10 @@ static void ttm_tt_init_fields(struct ttm_tt *ttm,
>> enum ttm_caching caching,
>> unsigned long extra_pages)
>> {
>> + /* Downgrade cached mapping for non-snooping devices */
>> + if (!bo->bdev->dma_coherent && caching == ttm_cached)
>> + caching = ttm_write_combined;
>> +
>> ttm->num_pages = (PAGE_ALIGN(bo->base.size) >> PAGE_SHIFT) + extra_pages;
>> ttm->page_flags = page_flags;
>> ttm->dma_address = NULL;
>> diff --git a/include/drm/ttm/ttm_caching.h b/include/drm/ttm/ttm_caching.h
>> index a18f43e93abab..f92d7911f50e4 100644
>> --- a/include/drm/ttm/ttm_caching.h
>> +++ b/include/drm/ttm/ttm_caching.h
>> @@ -47,7 +47,8 @@ enum ttm_caching {
>>
>> /**
>> * @ttm_cached: Fully cached like normal system memory, requires that
>> - * devices snoop the CPU cache on accesses.
>> + * devices snoop the CPU cache on accesses. Downgraded to
>> + * ttm_write_combined when the snooping capaiblity is missing.
>> */
>> ttm_cached
>> };
>> --
>> 2.45.2
>
