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

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Message-Id: <DCH8VNHTN1LM.3068OZU8IMK3Q@kernel.org>
Date: Mon, 01 Sep 2025 08:20:19 +0200
From: "Michael Walle" <mwalle@...nel.org>
To: "Doug Anderson" <dianders@...omium.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>, "Devarsh
 Thakkar" <devarsht@...com>
Subject: Re: [PATCH v2] drm/bridge: ti-sn65dsi86: fix REFCLK setting

Hi,

On Fri Aug 29, 2025 at 12:52 AM CEST, Doug Anderson wrote:
> > 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.
>
> I realized that this is lacking a Fixes: tag. I went back and
> confirmed that even in the first version of the driver, AKA commit
> a095f15c00e2 ("drm/bridge: add support for sn65dsi86 bridge driver"),
> we still had no delay between these two lines:
>
> pm_runtime_get_sync(pdata->dev);
>
> /* configure bridge ref_clk */
> ti_sn_bridge_set_refclk_freq(pdata);
>
> ...and the last line of the runtime resume function was turning on the
> enable. So I believe this means that the bug has always been there.
> Does that sound right to others? If so, I'll add that Fixes tag when
> applying.,..

Yes, that's right. Thanks for amending the patch.

-michael

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