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

[Alexander Graf]

On 11/16/2017 12:41 PM, Andre Przywara wrote:
> 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?

I'm not sure what the envisioned use of this is either. In general, it 
makes sense to keep the env on a partition and to mark the firmware 
residing on eMMC as off limits to an OS installer. So some sort of 
partitioning scheme is very useful and good to have.

>
>>> 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.

Right, in our case ~16MB would be enough, because we only store grub on 
the ESP. But there are other boot loaders out there like systemd-boot 
which put the kernel images and initrds onto the ESP.

>
> 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.

The reason I'm not fully comfortable with prepopulated system partitions 
is mostly because I'm not sure all installers will deal with them 
properly. Some might decide you're better off resizing a system 
partition rather than removing it - and if there's nothing useful inside 
that may be the wrong choice.

But that's nothing earth shattering. If you do need a system partition 
to have other installers work well, that's ok too I guess.

>
>>> 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.

Yup, exactly that :)


Alex