[Intel-gfx] [PATCH v6 1/3] drm/i915/migrate: Account for the reserved_space

[Andrzej Hajda]

Hi,

I messed-up with versions, my prev comment landed in v2, so I put it 
here to clean things up.

On 02.12.2022 13:28, Matthew Auld wrote:
> From: Chris Wilson <chris.p.wilson at intel.com>
>
> If the ring is nearly full when calling into emit_pte(), we might
> incorrectly trample the reserved_space when constructing the packet to
> emit the PTEs. This then triggers the GEM_BUG_ON(rq->reserved_space >
> ring->space) when later submitting the request, since the request itself
> doesn't have enough space left in the ring to emit things like
> workarounds, breadcrumbs etc.
>
> v2: Fix the whitespace errors
>
> Testcase: igt at i915_selftests@live_emit_pte_full_ring
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7535
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6889
> Fixes: cf586021642d ("drm/i915/gt: Pipelined page migration")
> Signed-off-by: Chris Wilson <chris.p.wilson at intel.com>
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Andrzej Hajda <andrzej.hajda at intel.com>
> Cc: Andi Shyti <andi.shyti at linux.intel.com>
> Cc: Nirmoy Das <nirmoy.das at intel.com>
> Cc: <stable at vger.kernel.org> # v5.15+
> Tested-by: Nirmoy Das <nirmoy.das at intel.com>
> Reviewed-by: Nirmoy Das <nirmoy.das at intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_migrate.c | 16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_migrate.c b/drivers/gpu/drm/i915/gt/intel_migrate.c
> index b405a04135ca..b783f6f740c8 100644
> --- a/drivers/gpu/drm/i915/gt/intel_migrate.c
> +++ b/drivers/gpu/drm/i915/gt/intel_migrate.c
> @@ -342,6 +342,16 @@ static int emit_no_arbitration(struct i915_request *rq)
>   	return 0;
>   }
>   
> +static int max_pte_pkt_size(struct i915_request *rq, int pkt)
> +{
> +	struct intel_ring *ring = rq->ring;
> +
> +	pkt = min_t(int, pkt, (ring->space - rq->reserved_space) / sizeof(u32) + 5);
> +	pkt = min_t(int, pkt, (ring->size - ring->emit) / sizeof(u32) + 5);
> +
> +	return pkt;
> +}
> +

I guess, the assumption that subtractions of u32 values do not 
overflows, is valid.
Then I guess more natural would be use u32 for all vars involved, this 
way we can use min instead of min_t, minor nit.

Anyway:
Reviewed-by: Andrzej Hajda <andrzej.hajda at intel.com>

Regards
Andrzej

>   static int emit_pte(struct i915_request *rq,
>   		    struct sgt_dma *it,
>   		    enum i915_cache_level cache_level,
> @@ -388,8 +398,7 @@ static int emit_pte(struct i915_request *rq,
>   		return PTR_ERR(cs);
>   
>   	/* Pack as many PTE updates as possible into a single MI command */
> -	pkt = min_t(int, dword_length, ring->space / sizeof(u32) + 5);
> -	pkt = min_t(int, pkt, (ring->size - ring->emit) / sizeof(u32) + 5);
> +	pkt = max_pte_pkt_size(rq, dword_length);
>   
>   	hdr = cs;
>   	*cs++ = MI_STORE_DATA_IMM | REG_BIT(21); /* as qword elements */
> @@ -422,8 +431,7 @@ static int emit_pte(struct i915_request *rq,
>   				}
>   			}
>   
> -			pkt = min_t(int, dword_rem, ring->space / sizeof(u32) + 5);
> -			pkt = min_t(int, pkt, (ring->size - ring->emit) / sizeof(u32) + 5);
> +			pkt = max_pte_pkt_size(rq, dword_rem);
>   
>   			hdr = cs;
>   			*cs++ = MI_STORE_DATA_IMM | REG_BIT(21);