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Subject: Re: [PATCH v1] partitions/efi: Add 'gpt_sector' kernel cmdline
 parameter
To:     Christoph Hellwig <hch@infradead.org>,
        Dmitry Osipenko <digetx@gmail.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Davidlohr Bueso <dave@stgolabs.net>,
        Colin Cross <ccross@android.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <mirq-linux@rere.qmqm.pl>,
        David Heidelberg <david@ixit.cz>,
        Peter Geis <pgwipeout@gmail.com>, linux-efi@vger.kernel.org,
        linux-tegra@vger.kernel.org, linux-block@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20200219162339.16192-1-digetx@gmail.com>
 <20200219162738.GA10644@infradead.org>
From:   Stephen Warren <swarren@wwwdotorg.org>
Message-ID: <f9e41108-7811-0deb-6977-be0f60e23b52@wwwdotorg.org>
Date:   Wed, 19 Feb 2020 09:59:54 -0700
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On 2/19/20 9:27 AM, Christoph Hellwig wrote:
> On Wed, Feb 19, 2020 at 07:23:39PM +0300, Dmitry Osipenko wrote:
>> The gpt_sector=<sector> causes the GPT partition search to look at the
>> specified sector for a valid GPT header if the GPT is not found at the
>> beginning or the end of block device.
>>
>> In particular this is needed for NVIDIA Tegra consumer-grade Android
>> devices in order to make them usable with the upstream kernel because
>> these devices use a proprietary / closed-source partition table format
>> for the EMMC and it's impossible to change the partition's format. Luckily
>> there is a GPT table in addition to the proprietary table, which is placed
>> in uncommon location of the EMMC storage and bootloader passes the
>> location to kernel using "gpt gpt_sector=<sector>" cmdline parameters.
>>
>> This patch is based on the original work done by Colin Cross for the
>> downstream Android kernel.
> 
> I don't think a magic command line is the way to go.  The best would be
> to reverse-engineer the proprietary partition table format.  If that is
> too hard we can at least key off the odd GPT location based of it's
> magic number.

I thought that the backup GPT was always present in the standard 
location; it's just the primary GPT that's in an odd location. So, this 
kernel parameter just forces the kernel to look first for the primary 
GPT in the unusual location, thus avoiding an error message when that's 
not there, and the system falls back to the backup GPT.

Or, do I misremember the layout, or the kernel's behaviour if primary 
GPT is missing?