Re: [PATCHv3] UBI: new module ubiblk: block layer on top of UBI

[Artem Bityutskiy <dedekind1@gmail.com>]

On Fri, 2011-09-09 at 16:25 +0200, Arnd Bergmann wrote:
> On Friday 09 September 2011, Artem Bityutskiy wrote:
> > On Thu, 2011-09-08 at 17:26 +0200, Arnd Bergmann wrote:
> > > On Tuesday 06 September 2011, Artem Bityutskiy wrote:
> > > > Not sure about the bus approach - David, could you take a look at it
> > > > please? If we can handle errors there - then we could indeed re-use the
> > > > UBI control device. We could even re-use the ioctl data structures for
> > > > UBI volumes creation/removal - we have plenty of space there reserved
> > > > for future extensions.
> > > 
> > > I would generally recommend using new ioctl commands. ioctl numbers
> > > are cheap, but complexity in data structures is not, because every
> > > user who wants to deal with the data structures has to understand
> > > them. Also, changing the ABI is always tricky since you have to
> > > provide backward and forwards compatibility with existing kernels
> > > and with existing user space.
> > 
> > Hmm, what do we do if ubiblk module is not loaded, and UBI would have
> > to return an error (because the block device cannot be created), how
> > will UBI know that ubiblk is not there? Any direct call to ubiblk from
> > UBI would be a direct dependency and would require ubiblk to be always
> > loaded, which is bad.
> 
> No, the idea of this approach is that the main ubi driver creates
> a device, which can always succeed. It's just that there won't
> be a block device node created, because that is part of what
> the ubiblk driver does.
> 
> Compare this to how scsi works:
> 
> A scsi host driver scans the host controller and adds scsi devices
> internal to the kernel, each of them have a specific type (disk,
> tape, ...). If the scsi disk driver is loaded, it will create
> a blockdev for each disk device. It doesn't matter in which order
> the drivers are loaded though.
> 
> In case of ubiblk, it's similar, except that there is no way for
> the ubi layer to know if some partition should be a block device or
> not, so it relies on user space to tell it.

There is another difference, though. Disk types are persistent. UBI
volumes types are not necessarily persistent:

I may create a "just UBI volume", flash something there, may-be re-size,
do other things.

Then I want to "turn" it into a "ubiblk" volume without destroying it.
So I call the corresponding ioctl. And my requirements are:

1) if ubiblk is not loaded, I want to get -ENODEV or something
2) if ubiblk fails, I want to get -ESOMETHING as well.
3) I want to be blocked while ubiblk is creating the block device.

Similarly, I may want to ask ubiblk to "release" the volume and turn it
to "just a volume" again.

So I am not sure the analogy is very close, but thanks a lot for
providing it - it makes things clearer.

-- 
Best Regards,
Artem Bityutskiy