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

[Lukasz Majewski <l.majewski@majess.pl>]

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Hi Andrey,

> On Mon, Dec 4, 2017 at 1:12 PM, Lukasz Majewski
> <l.majewski@majess.pl> wrote:
> > Hi Andrey,
> >  
> >> Hi Lukasz,
> >>
> >> On Tue, Nov 14, 2017 at 1:45 AM, Lukasz Majewski
> >> <l.majewski@majess.pl> wrote:  
> >> > Hi Andrey,
> >> >  
> >> >> Hi Lukasz,
> >> >>
> >> >> On Thu, Nov 9, 2017 at 2:28 PM, Lukasz Majewski
> >> >> <l.majewski@majess.pl> wrote:  
> >> >> > On Thu, 9 Nov 2017 07:34:44 -0800
> >> >> > Andrey Yurovsky <yurovsky@gmail.com> wrote:
> >> >> >  
> >> >> >> On Thu, Nov 9, 2017 at 1:55 AM, Lukasz Majewski
> >> >> >> <l.majewski@majess.pl> wrote:  
> >> >> >> > Hi Andrey,
> >> >> >> >  
> >> >> >> >> Hi Otavio,
> >> >> >> >>
> >> >> >> >> On Wed, Nov 8, 2017 at 2:47 AM, Otavio Salvador
> >> >> >> >> <otavio.salvador@ossystems.com.br> wrote:  
> >> >> >> >> > On Tue, Nov 7, 2017 at 10:43 PM, your name
> >> >> >> >> > <yurovsky@gmail.com> wrote:  
> >> >> >> >> >> From: Andrey Yurovsky <yurovsky@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@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).

I see. I thought that you need start LBA.

>From your use case it seems like extending the 'gpt' command is the
right thing to do - since already some uuid handling is done there.

> 2. determine which file system to load a file from (ex: fatload) given
> a label. 

There is a group of generic commands - like load, ls, etc.

> 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.

If you are using GPT, then you also may want to extend the 'gpt'
command -> iterate through PTEs and when label matched, return the part
number. With it you can use e.g. load mmc 0 <part> file.

> 
> 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.



-- 
Best regards,

Łukasz Majewski
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