Re: [RFC] Raising the UBI version

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Wed, 22 Jun 2016 16:43:49 +0200
Richard Weinberger <richard@nod.at> wrote:

> 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.

Then you still miss the version concept. Again, versions and features
are orthogonal, and you're not guaranteed that we'll never change the
on-flash format in an incompatible way...

So how about defining the following:
- /sys/class/ubi/version: user-space ABI version (should always be one)
- /sys/class/ubi/supported-on-flash-formats: either a bitfield or an
  integer representing the higher on-flash format version supported by
  the implementation (which implies that implementations have to
  support all on-flash formats up-to supported-on-flash-formats)
- /sys/class/ubi/supported-features: the features supported by the
  implementation (each bit is a specific feature or a set of features).
  The features may or may not be version dependent.
- /sys/class/ubi/ubiX/on-flash-format: the on-flash format used on the
  UBIX device
- /sys/class/ubi/ubiX/features: the features exposed by this UBIX
  device