On Wed, Jun 19, 2019 at 05:05:49PM -0400, Alan Stern wrote: > On Wed, 19 Jun 2019, shuah wrote: > > > I missed a lot of the thread info. and went looking for it and found the > > following summary of the problem: > > > > ================== > > The issue which prompted the commit this thread is about arose in a > > situation where the block layer set up a scatterlist containing buffer > > sizes something like: > > > > 4096 4096 1536 1024 > > > > and the maximum packet size was 1024. The situation was a little > > unusual, because it involved vhci-hcd (a virtual HCD). This doesn't > > matter much in normal practice because: > > > > Block devices normally have a block size of 512 bytes or more. > > Smaller values are very uncommon. So scatterlist element sizes > > are always divisible by 512. > > > > xHCI is the only USB host controller type with a maximum packet > > size larger than 512, and xHCI hardware can do full > > scatter-gather so it doesn't care what the buffer sizes are. > > > > So another approach would be to fix vhci-hcd and then trust that the > > problem won't arise again, for the reasons above. We would be okay so > > long as nobody tried to use a USB-SCSI device with a block size of 256 > > bytes or less. > > =================== > > > > Out of the summary, the following gives me pause: > > > > "xHCI hardware can do full scatter-gather so it doesn't care what the > > buffer sizes are." > > > > vhci-hcd won't be able to count on hardware being able to do full > > scatter-gather. It has to deal with a variety of hardware with > > varying speeds. > > Sure. But you can test whether the server's HCD is able to handle > scatter-gather transfers, and if it is then you can say that the > client-side vhci-hcd is able to handle them as well. Then all you > would have to do is preserve the scatterlist information describing the > transfer when you go between the client and the server. > > The point is to make sure that the client-side vhci-hcd doesn't claim > to be _less_ capable than the server-side actual HCD. That's what > leads to the problem described above. > > > "We would be okay so long as nobody tried to use a USB-SCSI device with > > a block size of 256 bytes or less." > > > > At least a USB Storage device, I test with says 512 block size. Can we > > count on not seeing a device with block size <= 256 bytes? > > Yes, we can. In fact, the SCSI core doesn't handle devices with block > size < 512. > > > In any case, I am looking into adding SG support vhci-hci at the moment. > > > > Looks like the following is the repo, I should be working with? > > > > git://git.infradead.org/users/hch/misc.git > > It doesn't matter. Your work should end up being independent of > Christoph's, so you can base it on any repo. I implemented SG support of vhci. I will send it as a patch. Please look at it and let me know if you have a feedback. Regards Suwan Kim
