Re: [Intel-gfx] [PATCH] drm/i915: fix i915 running as dom0 under Xen

[Tvrtko Ursulin]

On 02/02/2017 09:47, Juergen Gross wrote:
> Commit 920cf4194954ec ("drm/i915: Introduce an internal allocator for
> disposable private objects") introduced a regression for the kernel
> running as Xen dom0: when switching to graphics mode a GPU HANG
> occurred.
>
> Reason seems to be a missing adaption similar to that done in
> commit 7453c549f5f648 ("swiotlb: Export swiotlb_max_segment to users")
> to i915_gem_object_get_pages_internal().
>
> So limit the maximum page order to be used according to the maximum
> swiotlb segment size instead to the complete swiotlb size.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
> Please consider for 4.10 as otherwise 4.10 will be unusable as Xen dom0
> with i915 graphics.
> ---
>  drivers/gpu/drm/i915/i915_gem_internal.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_internal.c b/drivers/gpu/drm/i915/i915_gem_internal.c
> index 4b3ff3e..d09c749 100644
> --- a/drivers/gpu/drm/i915/i915_gem_internal.c
> +++ b/drivers/gpu/drm/i915/i915_gem_internal.c
> @@ -66,8 +66,16 @@ i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
>
>  	max_order = MAX_ORDER;
>  #ifdef CONFIG_SWIOTLB
> -	if (swiotlb_nr_tbl()) /* minimum max swiotlb size is IO_TLB_SEGSIZE */
> -		max_order = min(max_order, ilog2(IO_TLB_SEGPAGES));
> +	if (swiotlb_nr_tbl()) {
> +		unsigned int max_segment;
> +
> +		max_segment = swiotlb_max_segment();
> +		if (max_segment) {
> +			max_segment = max_t(unsigned int, max_segment,
> +					    PAGE_SIZE) >> PAGE_SHIFT;
> +			max_order = min(max_order, ilog2(max_segment));
> +		}
> +	}
>  #endif
>
>  	gfp = GFP_KERNEL | __GFP_HIGHMEM | __GFP_RECLAIMABLE;

Looks OK to me. We could bikeshed it to only use swiotlb_max_segment() 
which I think was the intention when that API was added but can leave 
that for the future.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

Regards,

Tvrtko