Re: [usb-storage] Re: Make UAS work on HS for devices with and without command tagging support

[Sarah Sharp <sarah.a.sharp@xxxxxxxxxxxxxxx>]

On Fri, Dec 16, 2011 at 09:42:41PM +0100, Sebastian Andrzej Siewior wrote:
> * Matthew Wilcox | 2011-12-16 15:31:46 [-0500]:
> 
> >On Fri, Dec 16, 2011 at 09:12:36PM +0100, Sebastian Andrzej Siewior wrote:
> >> * Sebastian Andrzej Siewior | 2011-12-16 15:47:24 [+0100]:
> >> 
> >> >>If you want to take a stab at redoing your patch #2 to use only one
> >> >>status URB for USB 2.0 devices, I would appreciate it.  Then I can build
> >> >>the abort/reset synchronization on top of it.
> >> >Okay.
> >> 
> >> Just once things started to become easy.... So while I tried to have
> >> only one status urb which I always re-submit (as Matthew/ You suggested)
> >> I run into the problem that I don't have struct scsi_device yet. So I
> >> just created a device with two luns to see if this struct happens always
> >> to be same. Ofcourse it is not. 
> >
> >Can you not send one status URB per LUN (instead of one per command)?
> 
> The thing is by the time a status URB completes I have only the *TAG*
> number from the device which tells to which command it belongs. Sending
> one status per LUN does not help because once a status URB with TAG 1
> arrived I have no idea to which device/LUN it does belong. ->context
> does not help here at all.

Oh, right, now I understand.  With USB 2.0, we might have one status URB
per LUN, but they're being queued to the same endpoint ring.  Since the
device can re-order the commands any way it likes, we can't rely on the
scsi_device that's stored on the urb->context to be correct.

Ok, I think we just need to divide the tag address space equally between
devices.  The devinfo can keep track of the pointers to the scsi_devices
for each LUN, and what tag range each LUN has.  So if you have 255 tags
available, and two LUNs, you can give one of them tag 1 to 122, and the
other tags 123 to 254.

Sarah Sharp
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