[U-Boot] Fastboot behaviour with sparse images

[Maxime Ripard]

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?
Maxime

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