On Fri, 30 Mar 2012, Norman Diamond wrote: > > It is a separate value from the product ID. It appears in the output from "lsusb -v" or in /sys/kernel/debug/usb/devices (where it is labelled "Rev="). > > bcdDevice is 1.00 That's the same as a bridge that was reported non-working. Which means the OS has no way to tell them apart. > >> It is hard to imagine a USB bridge adding 1 to the number of blocks. Did that really happen? > > > > No. What happened was that the bridge reported the number of blocks, but SCSI's READ CAPACITY command requires the device to report the highest available block number (which is one smaller since blocks are numbered starting from 0). > > I guess you mean that some USB bridges screw up the result of the READ CAPACITY command, so you have to subtract 1 in those cases. This particular bridge isn't one of them, and the subtraction of 1 was wrong in this case. True, but the computer has no way to know. However you can tell usb-storage not to manipulate the capacity of your drive by specifying a module parameter: modprobe usb-storage quirks=067b:2507: > hdparm reports the bugged capacity after the incorrect subtraction of 1. HDIO_GETGEO_BIG also gets the bugged capacity. I don't see an option to sdparm to get the device capacity (real SCSI devices probably have it in a VPD page but this USB bridge doesn't provide any VPD pages). I don't know if the information would be in a VPD page or not. There's not much point in putting it there, since it's already available via READ CAPACITY. > sg_dd with correct parameters successfully read the actual last block! That must be because sg_dd accessed /dev/sgsomething instead of /dev/sdmumble. > > My program doesn't know if a USB bridge is defective and/or if a driver defectively manipulated a block count. So I have to open a /dev/sgsomething, try to read 1 sector past the last sector that we think is valid. If it fails then I have to find out if the error was due to nonexistence or other reason. If it succeeds then I have to execute more painful hacks that I have to add. It's not as simple as that. With many bridges, if you test to see whether the block count is too small by trying to read "the block beyond the end", you will hang the bridge -- requiring an unplug to fix. There is no good solution to this problem. 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
