[U-Boot] Fastboot behaviour with sparse images

[Maxime Ripard <maxime.ripard@free-electrons.com>]

On Thu, Oct 22, 2015 at 07:22:54AM -0400, Tom Rini wrote:
> On Thu, Oct 22, 2015 at 09:57:05AM +0200, Maxime Ripard wrote:
> > Hi Tom,
> > 
> > On Wed, Oct 21, 2015 at 10:09:28AM -0400, Tom Rini wrote:
> > > On Mon, Oct 12, 2015 at 03:43:01PM +0200, Maxime Ripard wrote:
> > > > Hi,
> > > > 
> > > > I'm currently writing the support in U-Boot for NAND-backed devices
> > > > using fastboot [1], and that work derived a bit to supporting the
> > > > sparse images.
> > > > 
> > > > For "regular" images that are being stored, we expect a pair of
> > > > download and flash commands. Simple.
> > > > 
> > > > Things start to get a bit more complex with sparse images that have
> > > > been split because of a max-download-size lower than the actual image
> > > > size.
> > > > 
> > > > Here, from what I could gather from various random blog posts, the
> > > > fastboot client implementation and dumping a few USB sessions, the
> > > > client simply creates several download / flash pairs, always on the
> > > > same partition, without any way to distinct that from several
> > > > subsequent writes issued by the user.
> > > > 
> > > > So, I'm guessing that the expectation is that the bootloader
> > > > implementation should store the last offset it wrote to, and simple
> > > > resume from there if the partition names in the flash commands are the
> > > > same, which would prevent two subsequent write on the same partition
> > > > by any client. Am I right?
> > > > 
> > > > A related question is when should we erase the NAND partition? Only
> > > > when doing fastboot erase, or also when doing fastboot write (which,
> > > > combined with the issue raised above, would also mean that we don't
> > > > want to do an erase on the whole partition everytime there's a flash
> > > > command on it).
> > > 
> > > I think for this last question, some experimentation with the existing
> > > tools might be required.  As there's no required explicit erase for MMC,
> > > I think it might make sense to say you erase nand up front and then
> > > write as anything else starts getting really tricky and we're just
> > > second-guessing the user.
> > 
> > Actually, the only FS the fastboot tool seems to be doing it for the
> > moment are ext4 and F2FS. It can probably be extended to UBI and raw
> > partitions, but that won't fix the tools that are bundled by the
> > distros at the moment.
> > 
> > So I guess we can always erase it now using the session counter: if we
> > are writing the first chunk, erase the whole partition, if we're not,
> > then simply flash it at the previous offset.
> > 
> > How does it sound?
> 
> Sounds workable but testing with the existing tools will be the key and
> the hard part here :(

Well, if we always erase when we write, the worst case scenario would
be one erase too many. It doesn't sound that bad, or hard.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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