On Fri, 18 Apr 2014, Matthieu CASTET wrote: > > > while playing with suspend to ram I found a strange behavior with usb > > > persistence. > > > > > > This can be easily reproduce by doing : > > > - plug a usb key > > > - start to read the usb key : "cat /dev/sdx > /dev/null" > > > - go to suspend : "echo mem > /sys/power/state" > > > - while in suspend, unplug and replug the usb key (simulate usb power > > > loss for computer that keep power) > > > - exit suspend > > > - there is read error on the usb key > > > > Does the same thing happen if you start the "cat" command after the > > suspend/resume instead of before? > No, > > "echo mem > /sys/power/state; cat /dev/sdx > /dev/null" works. I guess this works because the initial open of /dev/sdx swallows the media-changed notification. > > The device reported a "Not-ready to ready change" because the power was > > lost and then restored. The SCSI layer interprets this as meaning that > > the media was changed, even though USB keys don't have changeable media > > -- evidently your key tells the computer that it does. > > yes, I tested with differents usb key and all of them report removable > (1 > in /sys/devices/pci0000:00/*/usb*/*/*/host*/target*/*/block/sdx/removable). > > Since the SCSI layer thinks the media was changed, it has no choice but > > to fail the read request. > ok. > > I suppose the scsi layer can't ignore "Not-ready to ready change" event > at resume time ? But that event doesn't happen at resume time. It happens later, when the "cat" program issues its next read request. In any case, the SCSI people will probably argue that ignoring such events would not be safe. And if it's not safe to ignore those events during normal operation then it's not safe to ignore them at resume time. If you want to discuss the matter any further, you should post to the linux-scsi mailing list. There's not much we can do about it at the USB level -- although perhaps we could turn off the erroneous "removable" indicator. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
