Hi Colin, On Tue, Oct 27, 2015 at 02:00:24PM -0700, Colin Cross wrote: > 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.
Oh, so that's how it works. Great. I guess however that you still need to scan out the area you skip for bad blocks to account them in the offset calculation as well then (when you're using NAND). > > 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. Ack. Thanks! Maxime -- Maxime Ripard, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com
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