Re: [RFC PATCH 2/2] drm/ttm: downgrade cached to write_combined when snooping not available

[Icenowy Zheng]

于 2024年6月30日 GMT+08:00 03:57:47，Jiaxun Yang <jiaxun.yang at flygoat.com> 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
>