linux-kernel - Re: [PATCH v4 29/78] drm/vc4: crtc: Add a delay after disabling the PixelValve output

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Message-ID: <27a549fe-e38d-f9a2-2677-ae5b5dc1aa45@i2se.com>
Date:   Tue, 1 Sep 2020 18:31:07 +0200
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Tim Gover <tim.gover@...pberrypi.com>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Eric Anholt <eric@...olt.net>,
        bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org,
        Phil Elwell <phil@...pberrypi.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 29/78] drm/vc4: crtc: Add a delay after disabling the
 PixelValve output

Hi Maxime,

Am 01.09.20 um 11:58 schrieb Maxime Ripard:
> Hi Stefan
>
> On Tue, Aug 25, 2020 at 11:30:58PM +0200, Stefan Wahren wrote:
>> Am 25.08.20 um 17:06 schrieb Maxime Ripard:
>>> Hi Stefan,
>>>
>>> On Wed, Jul 29, 2020 at 05:50:31PM +0200, Stefan Wahren wrote:
>>>> Am 29.07.20 um 16:42 schrieb Maxime Ripard:
>>>>> Hi,
>>>>>
>>>>> On Wed, Jul 29, 2020 at 03:09:21PM +0100, Dave Stevenson wrote:
>>>>>> On Wed, 8 Jul 2020 at 18:43, Maxime Ripard <maxime@...no.tech> wrote:
>>>>>>> In order to avoid pixels getting stuck in the (unflushable) FIFO between
>>>>>>> the HVS and the PV, we need to add some delay after disabling the PV output
>>>>>>> and before disabling the HDMI controller. 20ms seems to be good enough so
>>>>>>> let's use that.
>>>>>>>
>>>>>>> Signed-off-by: Maxime Ripard <maxime@...no.tech>
>>>>>>> ---
>>>>>>>  drivers/gpu/drm/vc4/vc4_crtc.c | 2 ++
>>>>>>>  1 file changed, 2 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
>>>>>>> index d0b326e1df0a..7b178d67187f 100644
>>>>>>> --- a/drivers/gpu/drm/vc4/vc4_crtc.c
>>>>>>> +++ b/drivers/gpu/drm/vc4/vc4_crtc.c
>>>>>>> @@ -403,6 +403,8 @@ static void vc4_crtc_atomic_disable(struct drm_crtc *crtc,
>>>>>>>         ret = wait_for(!(CRTC_READ(PV_V_CONTROL) & PV_VCONTROL_VIDEN), 1);
>>>>>>>         WARN_ONCE(ret, "Timeout waiting for !PV_VCONTROL_VIDEN\n");
>>>>>>>
>>>>>>> +       mdelay(20);
>>>>>> mdelay for 20ms seems a touch unfriendly as it's a busy wait. Can we
>>>>>> not msleep instead?
>>>>> Since the timing was fairly critical, sleeping didn't seem like a good
>>>>> solution since there's definitely some chance you overshoot and end up
>>>>> with a higher time than the one you targeted.
>>>> usleep_range(min, max) isn't a solution?
>>> My understanding of usleep_range was that you can still overshoot, even
>>> though it's backed by an HR timer so the resolution is not a jiffy. Are
>>> we certain that we're going to be in that range?
>> you are right there is no guarantee about the upper wake up time.
>>
>> And it's not worth the effort to poll the FIFO state until its empty
>> (using 20 ms as timeout)?
> I know this isn't really a great argument there, but getting this to
> work has been quite painful, and the timing is very sensitive. If we
> fail to wait for enough time, there's going to be a pixel shift that we
> can't get rid of unless we reboot, which is pretty bad (and would fail
> any CI test that checks for the output integrity).
>
> I know busy-looping for 20ms isn't ideal, but it's not really in a
> hot-path (it's only done when changing a mode), with the sync time of
> the display likely to be much more than that, and if it can avoid having
> to look into it ever again or avoid random failures, I'd say it's worth
> it.

i don't want to delay this series.

Could you please add a small comment to the delay to clarify the timing
is very sensitive?

Thanks

>
> Maxime