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Subject: Re: [RFC] Raising the UBI version
To: Boris Brezillon <boris.brezillon@free-electrons.com>
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 Artem Bityutskiy <dedekind1@gmail.com>, Alexander Kaplan
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 Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
From: Richard Weinberger <richard@nod.at>
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Date: Wed, 22 Jun 2016 16:21:36 +0200
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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.

> 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 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;
        }

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.

Thanks,
//richard