Re: [PATCH v2] drm/i915/dmabuf: drop the flush on discrete

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On 10/29/21 14:21, Matthew Auld wrote:
> We were overzealous here; even though discrete is non-LLC, it should
> still be always coherent.
>
> v2(Thomas & Daniel)
>    - Be extra cautious and limit to DG1
>    - Add some more commentary
>
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Daniel Vetter <daniel@ffwll.ch>

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>


> ---
>   drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> index a45d0ec2c5b6..a2b485a1be8c 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> @@ -250,8 +250,19 @@ static int i915_gem_object_get_pages_dmabuf(struct drm_i915_gem_object *obj)
>   	if (IS_ERR(pages))
>   		return PTR_ERR(pages);
>   
> -	/* XXX: consider doing a vmap flush or something */
> -	if (!HAS_LLC(i915) || i915_gem_object_can_bypass_llc(obj))
> +	/*
> +	 * DG1 is special here since it still snoops transactions even with
> +	 * CACHE_NONE. This is not the case with other HAS_SNOOP platforms. We
> +	 * might need to revisit this as we add new discrete platforms.
> +	 *
> +	 * XXX: Consider doing a vmap flush or something, where possible.
> +	 * Currently we just do a heavy handed wbinvd_on_all_cpus() here since
> +	 * the underlying sg_table might not even point to struct pages, so we
> +	 * can't just call drm_clflush_sg or similar, like we do elsewhere in
> +	 * the driver.
> +	 */
> +	if (i915_gem_object_can_bypass_llc(obj) ||
> +	    (!HAS_LLC(i915) && !IS_DG1(i915)))
>   		wbinvd_on_all_cpus();
>   
>   	sg_page_sizes = i915_sg_dma_sizes(pages->sgl);