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

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Date:   Tue, 12 Dec 2023 04:29:00 -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.
>

For anyone who's following this,  I added a note which I apparently
forgot to save:

This adds support for 720p60 in the i.MX8M Plus.

NXP uses a look-up table in their downstream code to accomplish this.
Using this calculation, the driver can adjust without the need for a
complicated table and should be flexible for different timings and
resolutions depending on the monitor.

I don't have a DSI analyzer, and this appears to only work on
i.MX8M Plus but not Mini and Nano for some reason, despite their
having a similar DSI bridge.

When Frieder teste this, he reported no changes on the Kontrol BL
i.MX8MM:   "So at least there is no negative impact in this case"


If someone else has an i.MX8MP, I would appreciate any feedback.

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>
>
> 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
>