[U-Boot] [PATCH 3/4] sunxi: Add default partition scheme

[Andre Przywara]

Hi,

On 16/11/17 11:21, Maxime Ripard wrote:
> On Thu, Nov 16, 2017 at 10:30:38AM +0000, Andre Przywara wrote:
>> Hi,
>>
>> On 15/11/17 21:03, Alexander Graf wrote:
>>>
>>>
>>> On 15.11.17 11:11, Maxime Ripard wrote:
>>>> The partitions variable is especially useful to create a partition table
>>>> from U-Boot, either directly from the U-Boot shell, or through flashing
>>>> tools like fastboot and its oem format command.
>>>>
>>>> This is especially useful on devices with an eMMC you can't take out to
>>>> flash from another system, and booting a Linux system first to flash our
>>>> system then is not really practical.
>>>>
>>>> Signed-off-by: Maxime Ripard <maxime.ripard at free-electrons.com>
>>>> ---
>>>>  include/configs/sunxi-common.h | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/include/configs/sunxi-common.h b/include/configs/sunxi-common.h
>>>> index 4391a8cbc824..11da6ccfbf54 100644
>>>> --- a/include/configs/sunxi-common.h
>>>> +++ b/include/configs/sunxi-common.h
>>>> @@ -493,6 +493,12 @@ extern int soft_i2c_gpio_scl;
>>>>  #define SUNXI_MTDPARTS_DEFAULT
>>>>  #endif
>>>>  
>>>> +#define PARTS_DEFAULT \
>>>> +	"name=loader1,start=8k,size=32k;" \
>>>> +	"name=loader2,size=984k;" \
>>>> +	"name=boot,size=128M,bootable;" \
>>>> +	"name=system,size=-;"
>>>
>>> Is there a particular reason you're creating a boot and system
>>> partition? In a normal distro world, the distro installer will take care
>>> of creating ESP + root + swap + whatever for you - and they (or the user
>>> driving the installation) usually know best what they need :)
>>
>> But do we actually care about this?
> 
> I do.

I know, this was a misunderstanding, sorry. By "we" I meant Alex and
Karsten's generic distribution point of view. I was arguing that this
patch is of no big importance for them.

I think we agree that there are quite different use cases, and I don't
fight the usefulness of both.

>> If I understand this correctly, these are default settings for
>> U-Boot's "mtdparts default" command, which honestly I didn't even
>> know existed so far.
> 
> No, this has nothing to do with MTD. It's a default GPT partitioning
> scheme. And only when you want to create the table from U-Boot, it
> will not mangle with any pre-existing partition table if there is any
> (unless you tell U-Boot to overwrite it, of course).

This is what I tried to say: It only affects you if you use U-Boot's
partitioning command, which you probably won't do if you are running an
off-the-shelf distribution installer. Is that understanding correct?

>> So in a distribution scenario I wouldn't expect somebody to actually use
>> this. Instead you boot from a (possibly unpartitioned) SD card with just
>> U-Boot on it or from SPI flash, then launch an installer from somewhere
>> (PXE, USB drive) and let it do its job. No U-Boot partition involved.
>> And even if you use mtdpart, you can always override these default
>> settings on the command line.
> 
> Like I was telling Alexander, that makes a number of assumptions, the
> two most obvious one being that you have an installer and that you
> want to use it, both with reasonable reasons on why they wouldn't be
> true.
> 
> More tailored fit distros like ELBE, yocto or Buildroot will not have
> an installer in the first place but an image.
> 
> And even if you have an installer for the distro you want to use, if
> you ever go to production, you will not use it since the time spent to
> flash a pre-filled image compared to running the installer is
> significantly lower. And time is money :)
> 
> Just like plugging / unplugging microSD card isn't really realistic in
> that scenario.

I don't argue this (see above) and surely understand that generic
installers don't fly when it comes to bootstrapping devices.

But my understanding is that both Alex and Karsten don't really care
about this usage scenario, but instead are more looking into generic
distribution installers, which use U-Boot merely to launch grub.

Actually I wanted to help you out here by pointing out that their
concerns don't really apply to this patch ;-)

>> Does mtdparts even use partition tables (MBR/GPT)? mtd sounds quite
>> Android-y/embedded to me (passing partition information via command line).
>>
>> So apart from that I think it's good to have a default FAT/ESP
>> partition, also for storing the environment.
> 
> What is the typical size of the files you usually put in there? My
> actual question being is 128MB enough, way too big or too small? The
> environment is just 128kB big at the moment, so it looks wayyyyy to
> big for me, but I have no idea what is usually stored in an ESP
> partition.

128MB is actually quite fine. I tend to use 150MB or 100MB. The Ubuntu
arm64 kernel is around 20MB, and you may want to store more than one of
those on the ESP, along with an initrd. I understand that distributions
may not use the ESP for that, but their own /boot partition. But this is
their choice. Also other OSes (BSDs?) want to use the ESP, so being too
miserly here may backfire.

Do you feel that's too big? We are talking about at least 8GB eMMCs
mostly here, right?

>> It's debatable whether we need a system partition defined at this stage.
>> Can't this just left be unpartitioned, to be actually populated later?
> 
> This would break the cases I talked about earlier.

Fair enough.

>> In a MBR/GPT scenario I would expect a big partition covering the whole
>> device causes headache later on.
> 
> What kind of headaches?

Just thinking if an installer wants to add partitions (swap, /home, ...)
it might be easier if some space is actually left unpartitioned.
But that's just my non-embedded experience, where adding partitions is
easier and safer, compared to deleting or resizing an existing partition.

Cheers,
Andre.