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Date:   Mon, 24 Feb 2020 17:33:42 +0100
From:   Karel Zak <kzak@redhat.com>
To:     Stephen Warren <swarren@wwwdotorg.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Dmitry Osipenko <digetx@gmail.com>,
        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
Subject: Re: [PATCH v1] partitions/efi: Add 'gpt_sector' kernel cmdline
 parameter
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 <20200219162738.GA10644@infradead.org>
 <f9e41108-7811-0deb-6977-be0f60e23b52@wwwdotorg.org>
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On Wed, Feb 19, 2020 at 09:59:54AM -0700, Stephen Warren wrote:
> 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.

 +1

> I thought that the backup GPT was always present in the standard location;

If they have proprietary stuff on begin of the device and valid backup
GPT at the end of the device then designer of this junk is crazy, because
many GPT fdisk-like tools will try to recover from the backup header and 
overwrite the unknown (invalid) stuff at the begin of the device...

    Karel


-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com