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[209.85.166.53]) by smtp.gmail.com with ESMTPSA id b10sm4713638ils.13.2021.09.13.06.57.34 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Sep 2021 06:57:35 -0700 (PDT) Received: by mail-io1-f53.google.com with SMTP id q3so12151469iot.3 for ; Mon, 13 Sep 2021 06:57:34 -0700 (PDT) X-Received: by 2002:a02:3b15:: with SMTP id c21mr10044237jaa.54.1631541454478; Mon, 13 Sep 2021 06:57:34 -0700 (PDT) MIME-Version: 1.0 References: <20210913131113.390368911@linuxfoundation.org> <20210913131118.330293390@linuxfoundation.org> In-Reply-To: <20210913131118.330293390@linuxfoundation.org> From: Doug Anderson Date: Mon, 13 Sep 2021 06:57:20 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 5.14 147/334] drm/bridge: ti-sn65dsi86: Dont read EDID blob over DDC To: Greg Kroah-Hartman Cc: LKML , "# 4.0+" , Andrzej Hajda , Bjorn Andersson , Sasha Levin Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Mon, Sep 13, 2021 at 6:51 AM Greg Kroah-Hartman wrote: > > From: Douglas Anderson > > [ 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 > Signed-off-by: Douglas Anderson > Reviewed-by: Bjorn Andersson > Link: https://patchwork.freedesktop.org/patch/msgid/20210611101711.v10.9.I9330684c25f65bb318eff57f0616500f83eac3cc@changeid > Signed-off-by: Sasha Levin > --- > 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