linux-kernel - [PATCH 3/3] drm/i915: Fix 0 return value from DMA fence wait on i915 requests

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Date:   Wed,  9 Nov 2022 20:09:37 +0100
From:   Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>
To:     Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Wilson <chris.p.wilson@...el.com>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] drm/i915: Fix 0 return value from DMA fence wait on i915 requests

According to the docs, dma_fence_wait_timeout() returns "0 if the wait
timed out," ... "Other error values may be returned on custom
implementations."  While it is not quite clear if a custom implementation
is allowed to return "other error" instead of 0, it is rather clear that 0
return value should always mean that the wait timed out before the fence
got signaled.

i915 implementation of dma_fence_ops.wait() used with request fences,
which is a transparent wrapper around i915_request_wait_timeout(), returns
-ETIME if the wait has timed out -- that may be considered as acceptable.
However, it can return 0 in a rare case when the fence has been found
signaled right after no more wait time was left, and that's not compatible
with expectations of dma-fence and its users.

Since other users of i915_request_wait_timeout() may interpret 0 return
value as success, don't touch it, update the i915_fence_wait() wrapper
instead.  Return 1 instead of 0, but keep -ETIME in case of timeout since
some i915 users of dma_fence_wait_timeout() may expect it.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>
---
 drivers/gpu/drm/i915/i915_request.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index f949a9495758a..451456ab1ddef 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -102,7 +102,7 @@ static signed long i915_fence_wait(struct dma_fence *fence,
 {
 	return i915_request_wait_timeout(to_request(fence),
 					 interruptible | I915_WAIT_PRIORITY,
-					 timeout);
+					 timeout) ?: 1;
 }
 
 struct kmem_cache *i915_request_slab_cache(void)
-- 
2.25.1