Re: [Linaro-mm-sig] [PATCH] drm/msm: Add fence->wait() op

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 21.07.21 um 21:03 schrieb Daniel Vetter:
> On Wed, Jul 21, 2021 at 09:34:43AM -0700, Rob Clark wrote:
>> On Wed, Jul 21, 2021 at 12:59 AM Daniel Vetter <daniel@ffwll.ch> wrote:
>>> On Wed, Jul 21, 2021 at 12:32 AM Rob Clark <robdclark@gmail.com> wrote:
>>>> On Tue, Jul 20, 2021 at 1:55 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>>>>> On Tue, Jul 20, 2021 at 8:26 PM Rob Clark <robdclark@gmail.com> wrote:
>>>>>> On Tue, Jul 20, 2021 at 11:03 AM Christian König
>>>>>> <ckoenig.leichtzumerken@gmail.com> wrote:
>>>>>>> Hi Rob,
>>>>>>>
>>>>>>> Am 20.07.21 um 17:07 schrieb Rob Clark:
>>>>>>>> From: Rob Clark <robdclark@chromium.org>
>>>>>>>>
>>>>>>>> Somehow we had neither ->wait() nor dma_fence_signal() calls, and no
>>>>>>>> one noticed.  Oops.
>>>>>>>
>>>>>>> I'm not sure if that is a good idea.
>>>>>>>
>>>>>>> The dma_fence->wait() callback is pretty much deprecated and should not
>>>>>>> be used any more.
>>>>>>>
>>>>>>> What exactly do you need that for?
>>>>>> Well, the alternative is to track the set of fences which have
>>>>>> signalling enabled, and then figure out which ones to signal, which
>>>>>> seems like a lot more work, vs just re-purposing the wait
>>>>>> implementation we already have for non-dma_fence cases ;-)
>>>>>>
>>>>>> Why is the ->wait() callback (pretty much) deprecated?
>>>>> Because if you need it that means for your driver dma_fence_add_cb is
>>>>> broken, which means a _lot_ of things don't work. Like dma_buf poll
>>>>> (compositors have patches to start using that), and I think
>>>>> drm/scheduler also becomes rather unhappy.
>>>> I'm starting to page back in how this works.. fence cb's aren't broken
>>>> (which is also why dma_fence_wait() was not completely broken),
>>>> because in retire_submits() we call
>>>> dma_fence_is_signaled(submit->hw_fence).
>>>>
>>>> But the reason that the custom wait function cleans up a tiny bit of
>>>> jank is that the wait_queue_head_t gets signaled earlier, before we
>>>> start iterating the submits and doing all that retire_submit() stuff
>>>> (unpin/unref bo's, etc).  I suppose I could just split things up to
>>>> call dma_fence_signal() earlier, and *then* do the retire_submits()
>>>> stuff.
>>> Yeah reducing the latency there sounds like a good idea.
>>> -Daniel
>>>
>> Hmm, no, turns out that isn't the problem.. or, well, it is probably a
>> good idea to call drm_fence_signal() earlier.  But it seems like
>> waking up from wait_event_* is faster than wake_up_state(wait->task,
>> TASK_NORMAL).  I suppose the wake_up_state() approach still needs for
>> the scheduler to get around to schedule the runnable task.

As far as I know wake_up_state() tries to run the thread on the CPU it 
was scheduled last, while wait_event_* makes the thread run on the CPU 
who issues the wake by default.

And yes I've also noticed this already and it was one of the reason why 
I suggested to use a wait_queue instead of the hand wired dma_fence_wait 
implementation.

>>
>> So for now, I'm going back to my own wait function (plus earlier
>> drm_fence_signal())
>>
>> Before removing dma_fence_opps::wait(), I guess we want to re-think
>> dma_fence_default_wait().. but I think that would require a
>> dma_fence_context base class (rather than just a raw integer).
> Uh that's not great ... can't we fix this instead of papering over it in
> drivers? Aside from maybe different wakeup flags it all is supposed to
> work exactly the same underneath, and whether using a wait queue or not
> really shouldn't matter.

Well it would have been nicer if we used the existing infrastructure 
instead of re-inventing stuff for dma_fence, but that chance is long gone.

And you don't need a dma_fence_context base class, but rather just a 
flag in the dma_fence_ops if you want to change the behavior.

Regards,
Christian.

> -Daniel
>
>> BR,
>> -R
>>
>>>> BR,
>>>> -R
>>>>
>>>>> It essentially exists only for old drivers where ->enable_signalling
>>>>> is unreliable and we paper over that with a retry loop in ->wait and
>>>>> pray no one notices that it's too butchered. The proper fix is to have
>>>>> a driver thread to guarantee that ->enable_signalling works reliable,
>>>>> so you don't need a ->wait.
>>>>>
>>>>> Can you type up a kerneldoc patch for dma_fence_ops->wait to hammer
>>>>> this in please?
>>>>> -Daniel
>>>>>
>>>>>> BR,
>>>>>> -R
>>>>>>
>>>>>>> Regards,
>>>>>>> Christian.
>>>>>>>
>>>>>>>> Note that this removes the !timeout case, which has not been used in
>>>>>>>> a long time.
>>>>>>>
>>>>>>>> Signed-off-by: Rob Clark <robdclark@chromium.org>
>>>>>>>> ---
>>>>>>>>    drivers/gpu/drm/msm/msm_fence.c | 59 +++++++++++++++++++--------------
>>>>>>>>    1 file changed, 34 insertions(+), 25 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/gpu/drm/msm/msm_fence.c b/drivers/gpu/drm/msm/msm_fence.c
>>>>>>>> index cd59a5918038..8ee96b90ded6 100644
>>>>>>>> --- a/drivers/gpu/drm/msm/msm_fence.c
>>>>>>>> +++ b/drivers/gpu/drm/msm/msm_fence.c
>>>>>>>> @@ -38,11 +38,10 @@ static inline bool fence_completed(struct msm_fence_context *fctx, uint32_t fenc
>>>>>>>>        return (int32_t)(fctx->completed_fence - fence) >= 0;
>>>>>>>>    }
>>>>>>>>
>>>>>>>> -/* legacy path for WAIT_FENCE ioctl: */
>>>>>>>> -int msm_wait_fence(struct msm_fence_context *fctx, uint32_t fence,
>>>>>>>> -             ktime_t *timeout, bool interruptible)
>>>>>>>> +static signed long wait_fence(struct msm_fence_context *fctx, uint32_t fence,
>>>>>>>> +             signed long remaining_jiffies, bool interruptible)
>>>>>>>>    {
>>>>>>>> -     int ret;
>>>>>>>> +     signed long ret;
>>>>>>>>
>>>>>>>>        if (fence > fctx->last_fence) {
>>>>>>>>                DRM_ERROR_RATELIMITED("%s: waiting on invalid fence: %u (of %u)\n",
>>>>>>>> @@ -50,33 +49,34 @@ int msm_wait_fence(struct msm_fence_context *fctx, uint32_t fence,
>>>>>>>>                return -EINVAL;
>>>>>>>>        }
>>>>>>>>
>>>>>>>> -     if (!timeout) {
>>>>>>>> -             /* no-wait: */
>>>>>>>> -             ret = fence_completed(fctx, fence) ? 0 : -EBUSY;
>>>>>>>> +     if (interruptible) {
>>>>>>>> +             ret = wait_event_interruptible_timeout(fctx->event,
>>>>>>>> +                     fence_completed(fctx, fence),
>>>>>>>> +                     remaining_jiffies);
>>>>>>>>        } else {
>>>>>>>> -             unsigned long remaining_jiffies = timeout_to_jiffies(timeout);
>>>>>>>> -
>>>>>>>> -             if (interruptible)
>>>>>>>> -                     ret = wait_event_interruptible_timeout(fctx->event,
>>>>>>>> -                             fence_completed(fctx, fence),
>>>>>>>> -                             remaining_jiffies);
>>>>>>>> -             else
>>>>>>>> -                     ret = wait_event_timeout(fctx->event,
>>>>>>>> -                             fence_completed(fctx, fence),
>>>>>>>> -                             remaining_jiffies);
>>>>>>>> -
>>>>>>>> -             if (ret == 0) {
>>>>>>>> -                     DBG("timeout waiting for fence: %u (completed: %u)",
>>>>>>>> -                                     fence, fctx->completed_fence);
>>>>>>>> -                     ret = -ETIMEDOUT;
>>>>>>>> -             } else if (ret != -ERESTARTSYS) {
>>>>>>>> -                     ret = 0;
>>>>>>>> -             }
>>>>>>>> +             ret = wait_event_timeout(fctx->event,
>>>>>>>> +                     fence_completed(fctx, fence),
>>>>>>>> +                     remaining_jiffies);
>>>>>>>> +     }
>>>>>>>> +
>>>>>>>> +     if (ret == 0) {
>>>>>>>> +             DBG("timeout waiting for fence: %u (completed: %u)",
>>>>>>>> +                             fence, fctx->completed_fence);
>>>>>>>> +             ret = -ETIMEDOUT;
>>>>>>>> +     } else if (ret != -ERESTARTSYS) {
>>>>>>>> +             ret = 0;
>>>>>>>>        }
>>>>>>>>
>>>>>>>>        return ret;
>>>>>>>>    }
>>>>>>>>
>>>>>>>> +/* legacy path for WAIT_FENCE ioctl: */
>>>>>>>> +int msm_wait_fence(struct msm_fence_context *fctx, uint32_t fence,
>>>>>>>> +             ktime_t *timeout, bool interruptible)
>>>>>>>> +{
>>>>>>>> +     return wait_fence(fctx, fence, timeout_to_jiffies(timeout), interruptible);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>    /* called from workqueue */
>>>>>>>>    void msm_update_fence(struct msm_fence_context *fctx, uint32_t fence)
>>>>>>>>    {
>>>>>>>> @@ -114,10 +114,19 @@ static bool msm_fence_signaled(struct dma_fence *fence)
>>>>>>>>        return fence_completed(f->fctx, f->base.seqno);
>>>>>>>>    }
>>>>>>>>
>>>>>>>> +static signed long msm_fence_wait(struct dma_fence *fence, bool intr,
>>>>>>>> +             signed long timeout)
>>>>>>>> +{
>>>>>>>> +     struct msm_fence *f = to_msm_fence(fence);
>>>>>>>> +
>>>>>>>> +     return wait_fence(f->fctx, fence->seqno, timeout, intr);
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>    static const struct dma_fence_ops msm_fence_ops = {
>>>>>>>>        .get_driver_name = msm_fence_get_driver_name,
>>>>>>>>        .get_timeline_name = msm_fence_get_timeline_name,
>>>>>>>>        .signaled = msm_fence_signaled,
>>>>>>>> +     .wait = msm_fence_wait,
>>>>>>>>    };
>>>>>>>>
>>>>>>>>    struct dma_fence *
>>>>>
>>>>>
>>>>> --
>>>>> Daniel Vetter
>>>>> Software Engineer, Intel Corporation
>>>>> http://blog.ffwll.ch
>>>
>>>
>>> --
>>> Daniel Vetter
>>> Software Engineer, Intel Corporation
>>> http://blog.ffwll.ch