linux-kernel - Re: [RFC] drm: bridge: samsung-dsim: Recalculate timings when rounding HFP up

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Date:   Thu, 26 Oct 2023 20:12:33 +0200
From:   Frieder Schrempf <frieder.schrempf@...tron.de>
To:     Adam Ford <aford173@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     marex@...x.de, Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jonas Karlman <jonas@...boo.se>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        aford@...conembedded.com,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maxime Ripard <mripard@...nel.org>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        linux-kernel@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [RFC] drm: bridge: samsung-dsim: Recalculate timings when
 rounding HFP up

Hi Adam,

On 13.10.23 05:10, Adam Ford 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
> and it gets rounded down which can cause certain resolutions and
> refresh rates to not sync.
> 
> ie. 720p60 on some monitors show hss of 1390 and hdisplay of 1280.  This
> yields an HFP of 110. When taking the HFP of 110 divides along 4 lanes,
> the result is 27.5 which rounds down to 27 and causes some monitors not
> to sync.
> 
> The solultion is to round HFP up by finding the remainder of HFP /
> the number of lanes and increasing the hsync_start, hsync_end, and
> htotal to compensate when there is a remainder.
> 
> Signed-off-by: Adam Ford <aford173@...il.com>
> ---
> 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.

I just want to report that I have tested this patch (on top of current
linux-next) on our Kontron BL i.MX8MM board with the ADV7535 bridge and
I don't see any change when trying the 30 different modes the monitor
reports as supported. 18 of those work and 12 don't work.

So at least there is no negative impact in this case.

Tested-by: Frieder Schrempf <frieder.schrempf@...tron.de> # Kontron BL
i.MX8MM with HDMI monitor

Thanks
Frieder