[PATCH 13/17] drm/radeon: Remove custom dma_fence_ops->wait implementation

[Daniel Vetter]

On Mon, Apr 30, 2018 at 8:26 PM, Christian König
<ckoenig.leichtzumerken at gmail.com> wrote:
> Am 30.04.2018 um 17:38 schrieb Daniel Vetter:
>>
>> On Sun, Apr 29, 2018 at 09:08:31AM +0200, Christian König wrote:
>>>
>>> NAK, there is a subtitle but major difference:
>>>
>>>> -               if (rdev->needs_reset) {
>>>> -                       t = -EDEADLK;
>>>> -                       break;
>>>> -               }
>>>
>>> Without that the whole radeon GPU reset code breaks.
>>
>> Oops I've missed that. How does this work when you register a callback
>> using ->enable_signaling and then block on it? Everything just dies?
>
>
> The short answer is we simply avoid using enable_signaling() from inside
> driver IOCTLs.
>
>> We have lots of users of that for buffer/fence sharing. A really ugly, but
>> probably working fix for this would be a kthread worker that just looks
>> for ->needs_reset and force-completes all fences with
>> dma_fence_set_error(-EIO), which is kinda what's supposed to happen here
>> anyway.
>
>
> That actually won't help. Radeon does this dance to return an error from
> dma_fence_wait() when the GPU needs a reset.
>
> This way all IOCTLs should return to userspace with -EAGAIN and when they
> are restarted we block for the running GPU reset to finish.
>
> I was against this approach, but it works as long as radeon only has to deal
> with it's own fences.

Yeah, as soon as you mix in a 2nd driver it goes boom, since you can
easily construct loops (even if they go through ->enable_signaling and
maybe just an irq handler to fire off something somewhere else).
Currently we're really bad a detecting these loops (aka there's
nothing), but I hope that the cross-release lockdep stuff gets enabled
soon. Then we could annotate dma_fences and lockdep should complain
about a lot of these issues.

Alas, problem exists already, but I'm not going to attempt to fix it.

Anyway, I'll drop this patch here.
-Daniel

>
> Christian.
>
>
>> -Daniel
>>
>>> Regards,
>>> Christian.
>>>
>>>
>>> Am 27.04.2018 um 08:17 schrieb Daniel Vetter:
>>>>
>>>> It's a copy of dma_fence_default_wait, written slightly differently.
>>>>
>>>> Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>>>> Cc: Alex Deucher <alexander.deucher at amd.com>
>>>> Cc: "Christian König" <christian.koenig at amd.com>
>>>> Cc: "David (ChunMing) Zhou" <David1.Zhou at amd.com>
>>>> Cc: amd-gfx at lists.freedesktop.org
>>>> ---
>>>>    drivers/gpu/drm/radeon/radeon_fence.c | 63
>>>> ---------------------------
>>>>    1 file changed, 63 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/radeon/radeon_fence.c
>>>> b/drivers/gpu/drm/radeon/radeon_fence.c
>>>> index e86f2bd38410..32690a525bfc 100644
>>>> --- a/drivers/gpu/drm/radeon/radeon_fence.c
>>>> +++ b/drivers/gpu/drm/radeon/radeon_fence.c
>>>> @@ -1051,72 +1051,9 @@ static const char
>>>> *radeon_fence_get_timeline_name(struct dma_fence *f)
>>>>         }
>>>>    }
>>>> -static inline bool radeon_test_signaled(struct radeon_fence *fence)
>>>> -{
>>>> -       return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
>>>> &fence->base.flags);
>>>> -}
>>>> -
>>>> -struct radeon_wait_cb {
>>>> -       struct dma_fence_cb base;
>>>> -       struct task_struct *task;
>>>> -};
>>>> -
>>>> -static void
>>>> -radeon_fence_wait_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
>>>> -{
>>>> -       struct radeon_wait_cb *wait =
>>>> -               container_of(cb, struct radeon_wait_cb, base);
>>>> -
>>>> -       wake_up_process(wait->task);
>>>> -}
>>>> -
>>>> -static signed long radeon_fence_default_wait(struct dma_fence *f, bool
>>>> intr,
>>>> -                                            signed long t)
>>>> -{
>>>> -       struct radeon_fence *fence = to_radeon_fence(f);
>>>> -       struct radeon_device *rdev = fence->rdev;
>>>> -       struct radeon_wait_cb cb;
>>>> -
>>>> -       cb.task = current;
>>>> -
>>>> -       if (dma_fence_add_callback(f, &cb.base, radeon_fence_wait_cb))
>>>> -               return t;
>>>> -
>>>> -       while (t > 0) {
>>>> -               if (intr)
>>>> -                       set_current_state(TASK_INTERRUPTIBLE);
>>>> -               else
>>>> -                       set_current_state(TASK_UNINTERRUPTIBLE);
>>>> -
>>>> -               /*
>>>> -                * radeon_test_signaled must be called after
>>>> -                * set_current_state to prevent a race with
>>>> wake_up_process
>>>> -                */
>>>> -               if (radeon_test_signaled(fence))
>>>> -                       break;
>>>> -
>>>> -               if (rdev->needs_reset) {
>>>> -                       t = -EDEADLK;
>>>> -                       break;
>>>> -               }
>>>> -
>>>> -               t = schedule_timeout(t);
>>>> -
>>>> -               if (t > 0 && intr && signal_pending(current))
>>>> -                       t = -ERESTARTSYS;
>>>> -       }
>>>> -
>>>> -       __set_current_state(TASK_RUNNING);
>>>> -       dma_fence_remove_callback(f, &cb.base);
>>>> -
>>>> -       return t;
>>>> -}
>>>> -
>>>>    const struct dma_fence_ops radeon_fence_ops = {
>>>>         .get_driver_name = radeon_fence_get_driver_name,
>>>>         .get_timeline_name = radeon_fence_get_timeline_name,
>>>>         .enable_signaling = radeon_fence_enable_signaling,
>>>>         .signaled = radeon_fence_is_signaled,
>>>> -       .wait = radeon_fence_default_wait,
>>>> -       .release = NULL,
>>>>    };
>
>



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch