Date:   Mon, 13 Sep 2021 06:57:20 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "# 4.0+" <stable@...r.kernel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.14 147/334] drm/bridge: ti-sn65dsi86: Dont read EDID
 blob over DDC

Hi,

On Mon, Sep 13, 2021 at 6:51 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> From: Douglas Anderson <dianders@...omium.org>
>
> [ Upstream commit a70e558c151043ce46a5e5999f4310e0b3551f57 ]
>
> This is really just a revert of commit 58074b08c04a ("drm/bridge:
> ti-sn65dsi86: Read EDID blob over DDC"), resolving conflicts.
>
> The old code failed to read the EDID properly in a very important
> case: before the bridge's pre_enable() was called. The way things need
> to work:
> 1. Read the EDID.
> 2. Based on the EDID, decide on video settings and pixel clock.
> 3. Enable the bridge w/ the desired settings.
>
> The way things were working:
> 1. Try to read the EDID but fail; fall back to hardcoded values.
> 2. Based on hardcoded values, decide on video settings and pixel clock.
> 3. Enable the bridge w/ the desired settings.
> 4. Try again to read the EDID, it works now!
> 5. Realize that the hardcoded settings weren't quite right.
> 6. Disable / reenable the bridge w/ the right settings.
>
> The reasons for the failures were twofold:
> a) Since we never ran the bridge chip's pre-enable then we never set
>    the bit to ignore HPD. This meant the bridge chip didn't even _try_
>    to go out on the bus and communicate with the panel.
> b) Even if we fixed things to ignore HPD, the EDID still wouldn't read
>    if the panel wasn't on.
>
> Instead of reverting the code, we could fix it to set the HPD bit and
> also power on the panel. However, it also works nicely to just let the
> panel code read the EDID. Now that we've split the driver up we can
> expose the DDC AUX channel bus to the panel node. The panel can take
> charge of reading the EDID.
>
> NOTE: in order for things to work, anyone that needs to read the EDID
> will need to instantiate their panel using the new DP AUX bus (AKA by
> listing their panel under the "aux-bus" node of the bridge chip in the
> device tree).
>
> In the future if we want to use the bridge chip to provide a full
> external DP port (which won't have a panel) then we will have to
> conditinally add EDID reading back in.
>
> Suggested-by: Andrzej Hajda <a.hajda@...sung.com>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> Link: https://patchwork.freedesktop.org/patch/msgid/20210611101711.v10.9.I9330684c25f65bb318eff57f0616500f83eac3cc@changeid
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 22 ----------------------
>  1 file changed, 22 deletions(-)

I guess it's not a huge deal, but I did respond to Sasha and request
that this patch be dropped from the stable queue unless the whole big
pile of patches was being backported. See:

https://lore.kernel.org/lkml/CAD=FV=U2dGjeEzp+K1vnLTj8oPJ-GKBTTKz2XQ1OZ7QF_sTHuw@mail.gmail.com/

I said:

> I would suggest against backporting this one unless you're going to
> backport the whole pile of DP AUX bus patches, which probably doesn't
> make sense for stable. Even though the old EDID reading was broken for
> the first read, it still worked for later reads. ...and the first read
. didn't crash or anything--it just timed out.


-Doug

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