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From: Doug Anderson <dianders@chromium.org>
To: Wolfram Sang <wsa@kernel.org>
Cc: Lyude Paul <lyude@redhat.com>,
	Steev Klimaszewski <steev@kali.org>,
	Stephen Boyd <swboyd@chromium.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>,
	Linus W <linus.walleij@linaro.org>,
	Rob Clark <robdclark@chromium.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Thierry Reding <treding@nvidia.com>,
	linux-i2c <linux-i2c@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andrzej Hajda <a.hajda@samsung.com>,
	Neil Armstrong <narmstrong@baylibre.com>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Jonas Karlman <jonas@kwiboo.se>,
	Jernej Skrabec <jernej.skrabec@siol.net>,
	Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH v6 1/5] i2c: i2c-core-of: Fix corner case of finding adapter by node
Date: Mon, 17 May 2021 13:16:29 -0700	[thread overview]
Message-ID: <CAD=FV=UuBo_T_FiPWT3Wz1SGtcAkwnsKKAtxtLkGDnzeYaybtg@mail.gmail.com> (raw)
In-Reply-To: <20210503145750.v6.1.Ib7e3a4af2f3e2cb3bd8e4adbac3bcfc966f27791@changeid>

Hi,

On Mon, May 3, 2021 at 2:59 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> The of_find_i2c_adapter_by_node() could end up failing to find an
> adapter in certain conditions. Specifically it's possible that
> of_dev_or_parent_node_match() could end up finding an I2C client in
> the list and cause bus_find_device() to stop early even though an I2C
> adapter was present later in the list.
>
> Let's move the i2c_verify_adapter() into the predicate function to
> prevent this. Now we'll properly skip over the I2C client and be able
> to find the I2C adapter.
>
> This issue has always been a potential problem if a single device tree
> node could represent both an I2C client and an adapter. I believe this
> is a sane thing to do if, for instance, an I2C-connected DP bridge
> chip is present. The bridge chip is an I2C client but it can also
> provide an I2C adapter (DDC tunneled over AUX channel). We don't want
> to have to create a sub-node just so a panel can link to it with the
> "ddc-i2c-bus" property.
>
> I believe that this problem got worse, however, with commit
> e814e688413a ("i2c: of: Try to find an I2C adapter matching the
> parent"). Starting at that commit it would be even easier to
> accidentally miss finding the adapter.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---
> Later patches in this series won't work right without this one, but
> they won't crash. If we can't find the i2c bus we'll just fall back to
> the hardcoded panel modes which, at least today, all panels have.
>
> I'll also note that part of me wonders if we should actually fix this
> further to run two passes through everything: first look to see if we
> find an exact match and only look at the parent pointer if there is no
> match. I don't currently have a need for that and it's a slightly
> bigger change, but it seems conceivable that it could affect someone?
>
> (no changes since v1)
>
>  drivers/i2c/i2c-core-of.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)

FYI that I've just posted v7 of this series and I've dropped
${SUBJECT} patch from my series.

I think that ${SUBJECT} patch is still correct and could be useful to
land, but it's no longer needed by my series since I'm getting access
to the DDC bus in a different way. If this patch needs to be spun,
please let me know. ...or, feel free to land it! :-)

-Doug

  reply	other threads:[~2021-05-17 20:17 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-03 21:58 [PATCH v6 0/5] drm: Fix EDID reading on ti-sn65dsi86; solve some chicken-and-egg problems Douglas Anderson
2021-05-03 21:58 ` [PATCH v6 1/5] i2c: i2c-core-of: Fix corner case of finding adapter by node Douglas Anderson
2021-05-17 20:16   ` Doug Anderson [this message]
2021-05-03 21:58 ` [PATCH v6 2/5] drm/dp: Allow an early call to register DDC i2c bus Douglas Anderson
2021-05-07 21:18   ` Lyude Paul
2021-05-07 22:00     ` Bjorn Andersson
2021-05-07 22:09       ` Lyude Paul
2021-05-14 11:16         ` Jani Nikula
2021-05-17 20:17           ` Doug Anderson
2021-05-03 21:58 ` [PATCH v6 3/5] drm/bridge: ti-sn65dsi86: Promote the AUX channel to its own sub-dev Douglas Anderson
2021-05-03 21:58 ` [PATCH v6 4/5] drm/bridge: ti-sn65dsi86: Don't read EDID blob over DDC Douglas Anderson
2021-05-03 21:58 ` [PATCH v6 5/5] arm64: dts: qcom: Link the panel to the bridge's DDC bus Douglas Anderson

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