[U-Boot] Fastboot behaviour with sparse images

[Tom Rini]

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.

-- 
Tom
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