linux-kernel - Re: [PATCH] drm/vc4: Fix HDMI mode validation

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Date:   Thu, 26 Mar 2020 18:19:41 +0100
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Eric Anholt <eric@...olt.net>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     maxime@...no.tech, linux-rpi-kernel@...ts.infradead.org,
        f.fainelli@...il.com,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/vc4: Fix HDMI mode validation

Am 26.03.20 um 13:20 schrieb Nicolas Saenz Julienne:
> Current mode validation impedes setting up some video modes which should
> be supported otherwise. Namely 1920x1200@...z.
>
> Fix this by lowering the minimum HDMI state machine clock to pixel clock
> ratio allowed.
>
> Fixes: 32e823c63e90 ("drm/vc4: Reject HDMI modes with too high of clocks")
> Reported-by: Stefan Wahren <stefan.wahren@...e.com>
> Suggested-by: Dave Stevenson <dave.stevenson@...pberrypi.com>
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index cea18dc15f77..340719238753 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -681,11 +681,23 @@ static enum drm_mode_status
>  vc4_hdmi_encoder_mode_valid(struct drm_encoder *crtc,
>  			    const struct drm_display_mode *mode)
>  {
> -	/* HSM clock must be 108% of the pixel clock.  Additionally,
> -	 * the AXI clock needs to be at least 25% of pixel clock, but
> -	 * HSM ends up being the limiting factor.
> +	/*
> +	 * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must
> +	 * be faster than pixel clock, infinitesimally faster, tested in
> +	 * simulation. Otherwise, exact value is unimportant for HDMI
> +	 * operation." This conflicts with bcm2835's vc4 documentation, which
> +	 * states HSM's clock has to be at least 108% of the pixel clock.
> +	 *
> +	 * Real life tests reveal that vc4's firmware statement holds up, and
> +	 * users are able to use pixel clocks closer to HSM's, namely for
> +	 * 1920x1200@...z. So it was decided to have leave a 1% margin between
> +	 * both clocks. Which, for RPi0-3 implies a maximum pixel clock of

s/RPi0-3/bcm2835/ ?

Beside this nit:

Tested-by: Stefan Wahren <stefan.wahre@...e.com>

Thanks