Re: [PATCH 1/4] i915/gem: drop wbinvd_on_all_cpus usage

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 19/03/2022 19:42, Michael Cheng wrote:
> Previous concern with using drm_clflush_sg was that we don't know what the
> sg_table is pointing to, thus the usage of wbinvd_on_all_cpus to flush
> everything at once to avoid paranoia.

And now we know, or we know it is not a concern?

> To make i915 more architecture-neutral and be less paranoid, lets attempt to

"Lets attempt" as we don't know if this will work and/or what can/will 
break?

> use drm_clflush_sg to flush the pages for when the GPU wants to read
> from main memory.
> 
> Signed-off-by: Michael Cheng <michael.cheng@intel.com>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 9 ++-------
>   1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> index f5062d0c6333..b0a5baaebc43 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
> @@ -8,6 +8,7 @@
>   #include <linux/highmem.h>
>   #include <linux/dma-resv.h>
>   #include <linux/module.h>
> +#include <drm/drm_cache.h>
>   
>   #include <asm/smp.h>
>   
> @@ -250,16 +251,10 @@ static int i915_gem_object_get_pages_dmabuf(struct drm_i915_gem_object *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();
> +		drm_clflush_sg(pages);

And as noticed before, drm_clfush_sg still can call wbinvd_on_all_cpus 
so are you just punting the issue somewhere else? How will it be solved 
there?

Regards,

Tvrtko

>   
>   	sg_page_sizes = i915_sg_dma_sizes(pages->sgl);
>   	__i915_gem_object_set_pages(obj, pages, sg_page_sizes);