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).
Thanks! Maxime
1: http://lists.denx.de/pipermail/u-boot/2015-August/226053.html