Re: [Intel-gfx] [PATCH 3/6] drm/i915: Update ring position from request on retiring

[Chris Wilson]

Quoting Mika Kuoppala (2018-03-09 13:38:37)
> Chris Wilson  writes:
> 
> > When wedged, we do not update the ring->tail as we submit the requests
> > causing us to leak the ring->space upon cleaning up the wedged driver.
> > We can just use the value stored in rq->tail, and keep the submission
> > backend details away from set-wedge.
> >
> > Signed-off-by: Chris Wilson 
> > Cc: Mika Kuoppala 
> > ---
> >  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 efa9ee557f31..69b378a323fc 100644
> > --- a/drivers/gpu/drm/i915/i915_request.c
> > +++ b/drivers/gpu/drm/i915/i915_request.c
> > @@ -358,7 +358,7 @@ static void advance_ring(struct i915_request *request)
> >* is just about to be. Either works, if we miss the last two
> >* noops - they are safe to be replayed on a reset.
> >*/
> > - tail = READ_ONCE(request->ring->tail);
> > + tail = READ_ONCE(request->tail);
> 
> I tried to think if we need the READ_ONCE here anymore.

I tried as well to see if the comment was still correct. It still is due
to that we can retire before we see the context-switch interrupt.
 
> But as this is the safest version,
> Reviewed-by: Mika Kuoppala 
> 
> Noticed that request->tail is not cleared on i915_request_alloc.
> 
> If we would set rq->head = rq->tail = rq->ring->emit
> we could use rq->head == rq->tail to assert that
> we screw up something major during the request lifetime.

Heh, if we get a stray write here, we're going to get stray writes
everywhere :)
-Chris
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Re: [Intel-gfx] [PATCH 3/6] drm/i915: Update ring position from request on retiring

[Mika Kuoppala]

Chris Wilson  writes:

> When wedged, we do not update the ring->tail as we submit the requests
> causing us to leak the ring->space upon cleaning up the wedged driver.
> We can just use the value stored in rq->tail, and keep the submission
> backend details away from set-wedge.
>
> Signed-off-by: Chris Wilson 
> Cc: Mika Kuoppala 
> ---
>  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 efa9ee557f31..69b378a323fc 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -358,7 +358,7 @@ static void advance_ring(struct i915_request *request)
>* is just about to be. Either works, if we miss the last two
>* noops - they are safe to be replayed on a reset.
>*/
> - tail = READ_ONCE(request->ring->tail);
> + tail = READ_ONCE(request->tail);

I tried to think if we need the READ_ONCE here anymore.

But as this is the safest version,
Reviewed-by: Mika Kuoppala 

Noticed that request->tail is not cleared on i915_request_alloc.

If we would set rq->head = rq->tail = rq->ring->emit
we could use rq->head == rq->tail to assert that
we screw up something major during the request lifetime.

-Mika


>   } else {
>   tail = request->postfix;
>   }
> -- 
> 2.16.2
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[Intel-gfx] [PATCH 3/6] drm/i915: Update ring position from request on retiring

[Chris Wilson]

When wedged, we do not update the ring->tail as we submit the requests
causing us to leak the ring->space upon cleaning up the wedged driver.
We can just use the value stored in rq->tail, and keep the submission
backend details away from set-wedge.

Signed-off-by: Chris Wilson 
Cc: Mika Kuoppala 
---
 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 efa9ee557f31..69b378a323fc 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -358,7 +358,7 @@ static void advance_ring(struct i915_request *request)
 * is just about to be. Either works, if we miss the last two
 * noops - they are safe to be replayed on a reset.
 */
-   tail = READ_ONCE(request->ring->tail);
+   tail = READ_ONCE(request->tail);
} else {
tail = request->postfix;
}
-- 
2.16.2

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