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From:   Dave Stevenson <dave.stevenson@raspberrypi.com>
Date:   Wed, 29 Jul 2020 15:09:21 +0100
Message-ID: <CAPY8ntBLWrfagZ5-kQz+5Mkw4_KaaTP63_L3D4owJAfA5kFpzw@mail.gmail.com>
Subject: Re: [PATCH v4 29/78] drm/vc4: crtc: Add a delay after disabling the
 PixelValve output
To:     Maxime Ripard <maxime@cerno.tech>
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>
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Sender: linux-kernel-owner@vger.kernel.org
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Hi Maxime

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 output
> 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_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?

  Dave

> +
>         if (vc4_encoder->post_crtc_disable)
>                 vc4_encoder->post_crtc_disable(encoder);
>
> --
> git-series 0.9.1