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

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Date:   Tue, 25 Aug 2020 17:06:06 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Tim Gover <tim.gover@...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,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Phil Elwell <phil@...pberrypi.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 29/78] drm/vc4: crtc: Add a delay after disabling the
 PixelValve output

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?

Maxime

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