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

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Message-ID: <20200729144251.us6a2pgkjjmm53ov@gilmour.lan>
Date:   Wed, 29 Jul 2020 16:42:51 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Dave Stevenson <dave.stevenson@...pberrypi.com>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Eric Anholt <eric@...olt.net>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        linux-rpi-kernel@...ts.infradead.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Phil Elwell <phil@...pberrypi.com>
Subject: Re: [PATCH v4 29/78] drm/vc4: crtc: Add a delay after disabling the
 PixelValve output

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.

Maxime

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