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Date:   Wed, 29 Jul 2020 16:42:51 +0200
From:   Maxime Ripard <maxime@cerno.tech>
To:     Dave Stevenson <dave.stevenson@raspberrypi.com>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
        Eric Anholt <eric@anholt.net>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        linux-rpi-kernel@lists.infradead.org,
        bcm-kernel-feedback-list@broadcom.com,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>,
        Tim Gover <tim.gover@raspberrypi.com>,
        Phil Elwell <phil@raspberrypi.com>
Subject: Re: [PATCH v4 29/78] drm/vc4: crtc: Add a delay after disabling the
 PixelValve output
Message-ID: <20200729144251.us6a2pgkjjmm53ov@gilmour.lan>
References: <cover.7a1aa1784976093af26cb31fd283cf5b3ed568bb.1594230107.git-series.maxime@cerno.tech>
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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@cerno.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 ou=
tput
> > and before disabling the HDMI controller. 20ms seems to be good enough =
so
> > let's use that.
> >
> > Signed-off-by: Maxime Ripard <maxime@cerno.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_c=
rtc.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 =3D wait_for(!(CRTC_READ(PV_V_CONTROL) & PV_VCONTROL_VIDEN)=
, 1);
> >         WARN_ONCE(ret, "Timeout waiting for !PV_VCONTROL_VIDEN\n");
> >
> > +       mdelay(20);
>=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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