linux-kernel - Re: [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

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Date: Tue, 16 Apr 2024 07:15:10 -0500
From: Adam Ford <aford173@...il.com>
To: dri-devel@...ts.freedesktop.org
Cc: marex@...x.de, aford@...conembedded.com, 
	Frieder Schrempf <frieder.schrempf@...tron.de>, Inki Dae <inki.dae@...sung.com>, 
	Jagan Teki <jagan@...rulasolutions.com>, Marek Szyprowski <m.szyprowski@...sung.com>, 
	Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	Robert Foss <rfoss@...nel.org>, Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Marco Felsch <m.felsch@...gutronix.de>, Michael Tretter <m.tretter@...gutronix.de>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

On Sun, Feb 11, 2024 at 5:09 PM Adam Ford <aford173@...il.com> wrote:
>
> When using video sync pulses, the HFP, HBP, and HSA are divided between
> the available lanes if there is more than one lane.  For certain
> timings and lane configurations, the HFP may not be evenly divisible.
> If the HFP is rounded down, it ends up being too small which can cause
> some monitors to not sync properly. In these instances, adjust htotal
> and hsync to round the HFP up, and recalculate the htotal.
>

Marek V and Marek S,

Would either of you be willing to test that this doesn't break your
applications?

thanks

adam

> Tested-by: Frieder Schrempf <frieder.schrempf@...tron.de> # Kontron BL i.MX8MM with HDMI monitor
> Signed-off-by: Adam Ford <aford173@...il.com>
> ---
> V2:  No changes
>
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index 8476650c477c..52939211fe93 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -1606,6 +1606,27 @@ static int samsung_dsim_atomic_check(struct drm_bridge *bridge,
>                 adjusted_mode->flags |= (DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_PVSYNC);
>         }
>
> +       /*
> +        * When using video sync pulses, the HFP, HBP, and HSA are divided between
> +        * the available lanes if there is more than one lane.  For certain
> +        * timings and lane configurations, the HFP may not be evenly divisible.
> +        * If the HFP is rounded down, it ends up being too small which can cause
> +        * some monitors to not sync properly. In these instances, adjust htotal
> +        * and hsync to round the HFP up, and recalculate the htotal. Through trial
> +        * and error, it appears that the HBP and HSA do not appearto need the same
> +        * correction that HFP does.
> +        */
> +       if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE && dsi->lanes > 1) {
> +               int hfp = adjusted_mode->hsync_start - adjusted_mode->hdisplay;
> +               int remainder = hfp % dsi->lanes;
> +
> +               if (remainder) {
> +                       adjusted_mode->hsync_start += remainder;
> +                       adjusted_mode->hsync_end   += remainder;
> +                       adjusted_mode->htotal      += remainder;
> +               }
> +       }
> +
>         return 0;
>  }
>
> --
> 2.43.0
>