[U-Boot] Fastboot behaviour with sparse images

[Colin Cross]

On Monday, October 12, 2015 at 6:43:37 AM UTC-7, 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? 
>

No, each blob passed to the bootloader will begin with a sparse "skip" 
chunk that will seek to the correct place to resume writing.  The 
bootloader shouldn't need to store any metadata across commands.  Just read 
in the blob from the data command, then write it out using a port of the 
Apache-licensed libsparse during the flash command.
 

> 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).


Fastboot should send an erase command before every sequence of writes. 
 Erase the whole partition on the erase command, and don't erase anything 
on the flash command.
 

>  
>
Thanks! 
> Maxime 
>
> 1: http://lists.denx.de/pipermail/u-boot/2015-August/226053.html 
>
> -- 
> Maxime Ripard, Free Electrons 
> Embedded Linux, Kernel and Android engineering 
> http://free-electrons.com 
>