Re: [RFC] Raising the UBI version

[Richard Weinberger <richard@nod.at>]

Am 22.06.2016 um 16:39 schrieb Boris Brezillon:
> On Wed, 22 Jun 2016 16:21:36 +0200
> Richard Weinberger <richard@nod.at> wrote:
> 
>> Am 22.06.2016 um 16:13 schrieb Boris Brezillon:
>>>> /sys/class/ubi/version is the version of the UBI implementation,
>>>> not the version of the attached UBI image.
>>>> It will the here as soon you load the UBI module.  
>>>
>>> Do we have /sys/class/ubi/ubiX/version for the UBI image version?  
>>
>> No. That's why I plan to add /sys/class/ubi/ubiX/features_used to
>> show which features the attached UBI image requested.
>> And having a /sys/class/ubi/ubi/features which denotes what features
>> the _implementation_ supports.
> 
> Still the version and features are encoding different things IMO.
> Incrementing the on-flash version means that the on-flash format has
> changed in an incompatible way, while features denotes the fact that
> the existing format has been extended with new features but is backward
> compatible.

Yes. But now we have a mix of both. ;-\

>>
>>> This is still unclear to me why we need to version the
>>> user-space/kernel-space ABI, since it's supposed to be backward
>>> compatible, so adding new features requires adding new ioctls and
>>> keeping the old ones in a working state.
>>>
>>> What is /sys/class/ubi/version actually encoding? Isn't it encoding the
>>> fact that a specific UBI implementation is supporting all UBI on-flash
>>> formats up to format version X (that was my understanding)?
>>>   
>>
>> Well, /sys/class/ubi/version exports UBI_VERSION from ubi-media.h.
>> It is (ab)used to encode the ABI version *and* the on-flash version.
> 
> The on-flash versions supported by the implementation is a useful
> information.
> 
>> The problem is that mtd-utils libubi will refuse to work with
>> /sys/class/ubi/version unequal 1.
>>
>> Here the gem from libubi:
>>         if (read_positive_int(lib->ubi_version, &version))
>>                 goto out_error;
>>         if (version != LIBUBI_UBI_VERSION) {
>>                 errmsg("this library was made for UBI version %d, but UBI "
>>                        "version %d is detected\n", LIBUBI_UBI_VERSION, version);
>>                 goto out_error;
>>         }
> 
> And this is where the problem is: libubi does not make proper use of
> this information. We should either have
> 
> 	if (version >= LIBUBI_UBI_VERSION)
> 
> or, if we decide that version is a bitfield directly encoding which
> versions are supported by the implementation
> 
> #define VERSION_SUPPORTED(version, x)	((version) & BIT((x)-1)))
> 
> 	if (VERSION_SUPPORTED(version, 1))

Yep. But we cannot change already compiled and shipped code.

>>
>> This is why I want to hardcode it to 1.
>> Everything else will break existing user space in some way.
>> 10 years ago /sys/class/ubi/version seemed like a good idea
>> but now it hits us hard.
> 
> Yes, I understand that, but this also means /sys/class/ubi/version is
> just a dummy file which only purpose is to make libubi happy :).

That's the plan.

> If this is the case, then I think we should have another file encoding
> the supported on-flash formats...

This is what /sys/class/ubi/features was supposed to do.

Thanks,
//richard