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Subject: Re: [PATCH] pstore/ram: Improve backward compatibility with older
 Chromebooks
To:     Doug Anderson <dianders@chromium.org>
Cc:     Kees Cook <keescook@chromium.org>, Rob Herring <robh@kernel.org>,
        Anton Vorontsov <anton@enomsg.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@lists.infradead.org>,
        Julius Werner <jwerner@chromium.org>,
        Guenter Roeck <groeck@chromium.org>,
        Matthias Kaehlcke <mka@chromium.org>,
        Brian Norris <briannorris@chromium.org>,
        Colin Cross <ccross@android.com>,
        Tony Luck <tony.luck@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
References: <20190503174730.245762-1-dianders@chromium.org>
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From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <90974ece-ab3a-7f5a-7d71-bd8a0d1d5aec@gmail.com>
Date:   Tue, 7 May 2019 15:23:38 -0700
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Hi Doug,

On 5/7/19 3:19 PM, Doug Anderson wrote:
> Hi,
> 
> On Tue, May 7, 2019 at 3:17 PM Frank Rowand <frowand.list@gmail.com> wrote:
>>
>> On 5/6/19 4:58 PM, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Mon, May 6, 2019 at 2:10 PM Kees Cook <keescook@chromium.org> wrote:
>>>>
>>>> From: Douglas Anderson <dianders@chromium.org>
>>>> Date: Fri, May 3, 2019 at 10:48 AM
>>>> To: Kees Cook, Anton Vorontsov
>>>> Cc: <linux-rockchip@lists.infradead.org>, <jwerner@chromium.org>,
>>>> <groeck@chromium.org>, <mka@chromium.org>, <briannorris@chromium.org>,
>>>> Douglas Anderson, Colin Cross, Tony Luck,
>>>> <linux-kernel@vger.kernel.org>
>>>>
>>>>> When you try to run an upstream kernel on an old ARM-based Chromebook
>>>>> you'll find that console-ramoops doesn't work.
>>>>>
>>>>> Old ARM-based Chromebooks, before <https://crrev.com/c/439792>
>>>>> ("ramoops: support upstream {console,pmsg,ftrace}-size properties")
>>>>> used to create a "ramoops" node at the top level that looked like:
>>>>>
>>>>> / {
>>>>>   ramoops {
>>>>>     compatible = "ramoops";
>>>>>     reg = <...>;
>>>>>     record-size = <...>;
>>>>>     dump-oops;
>>>>>   };
>>>>> };
>>>>>
>>>>> ...and these Chromebooks assumed that the downstream kernel would make
>>>>> console_size / pmsg_size match the record size.  The above ramoops
>>>>> node was added by the firmware so it's not easy to make any changes.
>>>>>
>>>>> Let's match the expected behavior, but only for those using the old
>>>>> backward-compatible way of working where ramoops is right under the
>>>>> root node.
>>>>>
>>>>> NOTE: if there are some out-of-tree devices that had ramoops at the
>>>>> top level, left everything but the record size as 0, and somehow
>>>>> doesn't want this behavior, we can try to add more conditions here.
>>>>>
>>>>> Signed-off-by: Douglas Anderson <dianders@chromium.org>
>>>>
>>>> I like this; thanks! Rob is this okay by you? I just want to
>>>> double-check since it's part of the DT parsing logic.
>>>>
>>>> I'll pick it up and add a Cc: stable.
>>>
>>> Hold off a second--I may need to send out a v2 but out of time for the
>>> day.  I think I need a #include file to fix errors on x86:
>>>
>>>> implicit declaration of function 'of_node_is_root' [-Werror,-Wimplicit-function-declaration
>>
>> Instead of checking "of_node_is_root(parent_node)" the patch could check
>> for parent_node not "/reserved-memory".  Then the x86 error would not
>> occur.
>>
>> The check I am suggesting is not as precise, but it should be good enough
>> for this case, correct?
> 
> Sure, there are a million different ways to slice it.  If you prefer
> that instead of adding a dummy of_node_is_root() I'm happy to do that.

Yes, I would prefer to avoid adding a dummy of_node_is_root() if the
alternative is reasonable (and if I understand, you are saying the
alternative is reasonable).

Thanks,

Frank