linux-kernel - Re: [PATCH v2] drm/bridge: ti-sn65dsi86: fix REFCLK setting

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Message-ID: <CAD=FV=Xt-DPHRddZM=t-KTWiGouNfm_QpPps3MA6csd+j=YO+A@mail.gmail.com>
Date: Thu, 21 Aug 2025 07:36:14 -0700
From: Doug Anderson <dianders@...omium.org>
To: Michael Walle <mwalle@...nel.org>
Cc: 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>, Simona Vetter <simona@...ll.ch>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	Jayesh Choudhary <j-choudhary@...com>, Devarsh Thakkar <devarsht@...com>
Subject: Re: [PATCH v2] drm/bridge: ti-sn65dsi86: fix REFCLK setting

Hi,

On Thu, Aug 21, 2025 at 5:23 AM Michael Walle <mwalle@...nel.org> wrote:
>
> The bridge has three bootstrap pins which are sampled to determine the
> frequency of the external reference clock. The driver will also
> (over)write that setting. But it seems this is racy after the bridge is
> enabled. It was observed that although the driver write the correct
> value (by sniffing on the I2C bus), the register has the wrong value.
> The datasheet states that the GPIO lines have to be stable for at least
> 5us after asserting the EN signal. Thus, there seems to be some logic
> which samples the GPIO lines and this logic appears to overwrite the
> register value which was set by the driver. Waiting 20us after
> asserting the EN line resolves this issue.
>
> Signed-off-by: Michael Walle <mwalle@...nel.org>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>

nit: officially you're supposed to move your Signed-off-by all the way
at the bottom of all the other tags any time you post a patch. I don't
think it's important enough to re-send, though.

In any case, thanks for re-posting. I guess it kinda stagnated. I'll
give this another week on the list and then plan to apply to
drm-misc-fixes unless there are any other comments.

-Doug