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

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Date: Thu, 25 Jan 2024 12:44:23 -0600
From: Adam Ford <aford173@...il.com>
To: dri-devel@...ts.freedesktop.org
Cc: 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>, 
	Michael Tretter <m.tretter@...gutronix.de>, Marco Felsch <m.felsch@...gutronix.de>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

On Mon, Dec 11, 2023 at 9:33 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.
>
> Tested-by: Frieder Schrempf <frieder.schrempf@...tron.de> # Kontron BL i.MX8MM with HDMI monitor
> Signed-off-by: Adam Ford <aford173@...il.com>

Gentle nudge on this one.  Basically this fixes an issue with the 8MP,
but it's still unknown why it doesn't work on 8MM or 8MN, but Frieder
confirmed there are no regressions on 8MM or 8MN.

adam


>
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index 239d253a7d71..f5795da1d8bb 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -1628,6 +1628,27 @@ 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;
>  }
>
> --
> 2.40.1
>