19.02.2020 19:59, Stephen Warren пишет:

On 2/19/20 9:27 AM, Christoph Hellwig wrote:

quoted

On Wed, Feb 19, 2020 at 07:23:39PM +0300, Dmitry Osipenko wrote:

quoted

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?

The backup GPT not always presents in the standard location. For example
Tegra30 ASUS Google Nexus 7 has a backup GPT in the proper location and
this is what KMSG prints:

[    1.722888] Primary GPT is invalid, using alternate GPT.
[    1.723076]  mmcblk1: p1 p2 p3 p4 p5 p6 p7 p8 p9 p10

But this doesn't work for Tegra20 Acer A500 and (IIRC) Tegra30 Ouya
because both primary and backup GPTs are invalid at the standard locations.