linux-kernel - Re: [PATCH v6 1/5] i2c: i2c-core-of: Fix corner case of finding adapter by node

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Date:   Mon, 17 May 2021 13:16:29 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Wolfram Sang <wsa@...nel.org>
Cc:     Lyude Paul <lyude@...hat.com>, Steev Klimaszewski <steev@...i.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Stanislav Lisovskiy <stanislav.lisovskiy@...el.com>,
        Linus W <linus.walleij@...aro.org>,
        Rob Clark <robdclark@...omium.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thierry Reding <treding@...dia.com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH v6 1/5] i2c: i2c-core-of: Fix corner case of finding
 adapter by node

Hi,

On Mon, May 3, 2021 at 2:59 PM Douglas Anderson <dianders@...omium.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@...omium.org>
> Reviewed-by: Bjorn Andersson <bjorn.andersson@...aro.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