[U-Boot] [PATCH] gpt: add part-uuid and part-num subcommands

[Andrey Yurovsky]

Hi Lukasz,

On Thu, Nov 9, 2017 at 2:28 PM, Lukasz Majewski <l.majewski at majess.pl> wrote:
> On Thu, 9 Nov 2017 07:34:44 -0800
> Andrey Yurovsky <yurovsky at gmail.com> wrote:
>
>> On Thu, Nov 9, 2017 at 1:55 AM, Lukasz Majewski
>> <l.majewski at majess.pl> wrote:
>> > Hi Andrey,
>> >
>> >> Hi Otavio,
>> >>
>> >> On Wed, Nov 8, 2017 at 2:47 AM, Otavio Salvador
>> >> <otavio.salvador at ossystems.com.br> wrote:
>> >> > On Tue, Nov 7, 2017 at 10:43 PM, your name <yurovsky at gmail.com>
>> >> > wrote:
>> >> >> From: Andrey Yurovsky <yurovsky at gmail.com>
>> >> >>
>> >> >> It is useful to be able to retrieve a partition UUID or number
>> >> >> given the partition label, for instance some systems use the
>> >> >> partition label to indicate the purpose of the partition (such
>> >> >> as "rootfs0" being the 0th root file system in an A/B image
>> >> >> scheme).
>> >> >>
>> >> >> Add "gpt part-uuid" to retrieve the partition UUID for a given
>> >> >> label and "gpt part-num" to retrieve the partition number for a
>> >> >> given label along with some documentation.
>> >> >>
>> >> >> Signed-off-by: Andrey Yurovsky <yurovsky at gmail.com>
>> >> >
>> >> > Why not use the 'part' cmd? it provides it.
>> >>
>> >> Sorry, I missed the part cmd, it doesn't seem to be documented in
>> >> doc/ and it's unclear what <dev> means there.
>> >
>> > If I may ask - Andrey, if you are now on this "topic" - would you
>> > dare to add some ./doc entry for 'part' command?
>>
>> Yes, I will do that.
>
> Thanks :-)

On further investigation I am not sure that it's possible to extend
the part command to retrieve UUIDs by label because of the design of
the partition type drivers. Here is how I understand it to work:
1. the "part" command uses part_get_info() and in turn gets a
partition driver and can call print() there (which is how EFI/GPT
disks are printed with "part list"). The right information (including
label) is printed but it's not tied to the caller in any way.
2. "part uuid"  blk_get_device_part_str() which in turn can get
information but referenced by partition number (and only partition
number, due to how part_get_info() works). There's nothing at the
'part' layer tying a label to a number though so one would already
need to know the number, which is the problem I'm trying to solve.

This layering keeps partitioning generalized and decoupled from the
underlying partition "driver" but there's no concept of a label in the
"driver" API. I'm not sure if it's reasonable to extend it since
part_get_info() and similar really don't have a way to map labels at
this layer so perhaps extending the GPT-specific command is the only
reasonable approach?