On Wed, 2010-11-24 at 09:49 -0500, Alan Stern wrote: > On Wed, 24 Nov 2010, James Bottomley wrote: > > > On Wed, 2010-11-24 at 01:02 -0800, Luben Tuikov wrote: > > > I doubt this as very unlikely. Has anyone actually seen a device that > > > sends mode parameter data with faux Caching mode page or corrupted > > > data that is in fact interpreted as a Caching mode page? Is such a > > > device fully operational sans the faux Caching mode page, or does it > > > just not work? Is it common to have devices having a faux Caching mode > > > page or corrupted mode parameter data resulting in a Caching mode page > > > with random data? > > > > > > Undoubtedly, as the usb-storage maintainer, you must have variety of > > > devices, some broken some not. Could you apply this patch to your tree > > > and test some of the devices you have? My tests indicate a stable > > > behavior. > > > > The basic problem isn't devices lying ... the worst we'll do is current > > behaviour (not SYNC when we should). The problem is devices that get > > confused (or worse simply crash the firmware). The best way to avoid > > the crashing firmware problem ... if we can assume that modern USB > > devices are better is to key off the SCSI version. Unfortunately, in > > spite of several attempts, we've never managed to stop usbstorage lying > > about this: > > > > /* Some devices report a SCSI revision level above 2 but are > > * unable to handle the REPORT LUNS command (for which > > * support is mandatory at level 3). Since we already have > > * a Get-Max-LUN request, we won't lose much by setting the > > * revision level down to 2. The only devices that would be > > * affected are those with sparse LUNs. */ > > if (sdev->scsi_level > SCSI_2) > > sdev->sdev_target->scsi_level = > > sdev->scsi_level = SCSI_2; > > > > Untangling all of this would be rather complex, I fear. > > Quite likely. > > > The final question is is it worth it? Since USB devices are supposed to > > be hot unpluggable, surely a USB device with a write back cache would be > > a disaster: no-one will SYNC the cache on a surprise unplug anyway ... > > therefore there shouldn't really be any of them surviving in the wild > > (famous last words, I suppose). > > Well, hot unpluggable doesn't mean it's okay to unplug the device at > any time. For example, under Windows you're not supposed to unplug a > USB drive without first going through the "Safely remove hardware" > applet. And of course, you can easily guess what command that applet > sends to the device... > > On the whole, I'm with Luben on this. The likelihood of introducing > bad behavior because of devices sending incorrect cache-page > information seems very small. Yes, just assure me that sending the mode page request won't kill anything and I'm fine with applying it. As I said, as long as we get a reply, whatever it is it can't make use behave any worse than we do now. James -- 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
