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

[Andrey Yurovsky]

On Mon, Dec 4, 2017 at 1:12 PM, Lukasz Majewski <l.majewski at majess.pl> wrote:
> Hi Andrey,
>
>> Hi Lukasz,
>>
>> On Tue, Nov 14, 2017 at 1:45 AM, Lukasz Majewski
>> <l.majewski at majess.pl> wrote:
>> > Hi Andrey,
>> >
>> >> 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.
>> >
>> > Maybe you can set some env variable with proper data?
>> >
>> > For example, please refer to ./cmd/part.c do_part_start() function.
>> >
>> > Example call from envs (include/configs/display5.h):
>> >         "part start mmc ${mmcdev} ${kernel_part} lba_start; " \
>> >
>>
>> Again that assumes the partition is referred to by number, I need it
>> to be by label, and the part/disk interface does not seem to have any
>> way to utilize labels. Unfortunately it looks like my original
>> approach with the gpt command is the only way to implement this with
>> the current design (at least from what I see here). Please let me know
>> if I've missed something. Thanks!
>
> Please correct me if I'm wrong - you need the starting LBA of the
> partition named e.g. "FOO" in gpt ?
>
> Conceptually it would be correct to have:
>
> part start <interface> <dev> <NAME - e.g.'FOO'> <env to set>
> gpt start <interface> <dev> <NAME- e.g. 'FOO'> <env to set>
>
> If your code is really _small_ and can be used only with GPT, then lets
> go for the second option.

The use cases I have in mind:

1. determine which root file system to use by label, let's say
"rootfs1" and pass its UUID to the Linux kernel via the command line,
ex: "PART-UUID=${uuid}". To do this we need a way to ask for a UUID
corresponding to a label in the partition table (given duplicate
labels, assume it gives us the first or last match).
2. determine which file system to load a file from (ex: fatload) given
a label. I'm not sure that the starting LBA is helpful here, we really
are looking to map a label to a partition number in the table.

The implementation of "part start" interprets the argument as a
partition number, so I can get the stating LBA if I know the partition
number but I don't see a way (via cmd/disk.c) to get anything useful
if all I know is a label, and that is what I'm trying to solve.