linux-kernel - Re: [PATCH v1] partitions/efi: Add 'gpt

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Date:   Mon, 24 Feb 2020 17:33:42 +0100
From:   Karel Zak <kzak@...hat.com>
To:     Stephen Warren <swarren@...dotorg.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Dmitry Osipenko <digetx@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Davidlohr Bueso <dave@...olabs.net>,
        Colin Cross <ccross@...roid.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        David Heidelberg <david@...t.cz>,
        Peter Geis <pgwipeout@...il.com>, linux-efi@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-block@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] partitions/efi: Add 'gpt_sector' kernel cmdline
 parameter

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@...hat.com>
 http://karelzak.blogspot.com