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

[Jiaxun Yang]

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.

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

-- 
- Jiaxun