On Fri, 2011-09-30 at 23:26 +0530, Amit Sahrawat wrote:

Adding linux-usb - to get more insight's into the problem.

On Fri, Sep 30, 2011 at 11:23 PM, Amit Sahrawat
[off-list ref] wrote:

quoted

On Fri, Sep 30, 2011 at 5:48 PM, James Bottomley
[off-list ref] wrote:

quoted

On Fri, 2011-09-30 at 12:26 +0530, Amit Sahrawat wrote:

quoted

Now, for the USB HDD which do have write cache - sginfo is showing
them to Write Cache Enabled as false.
Why do the result of hdparm identification and sginfo varies- (I know
they have different interface to work with and hdparm takes care of
that by using SG_IO interface from it's code)? hdparm showed me
correct results - that lead me to digging in the kernel code and
checking the performance for USB HDD with Write cache enabled/disabled
- which also showed that QUEUE ordering chosen for USB HDD is not
correct.

Well, what all this means is the SATL in the USB device is implemented
wrongly.  Since USB devices only preset SCSI interfaces, that's what we
have to believe.

hdparm when used correctly sends an ATA inquiry command wrapped in an
ATA_12 or ATA_16 SCSI command.  A large number of legacy SATLs are known
to crash on these commands.

Are you sure the ATA command is reporting correctly?  A write back cache
is a remarkably silly thing to enable for a USB device because they're
highly likely to be surprise ejected which powers the device down.

In addition to the problem reported - there is one more thing I have
noticed with USB HDD - they should be shown as 'removable' but the
removable is marked only for USB PEN Drives. This seems to be a bit of
confusing, any mass storage media connected on USB port should be
recognized as removable.

I don't really think so.  Removable to sd means that the drive can be
removed from the housing, not that the connector cable is hot plug so
the whole disk can disappear.  I think your disk is the latter, and
therefore removable probably isn't what you want (otherwise the sd
driver will start probing for medium change and other things your device
won't understand).

quoted

So, for handling the issue, I would consider adding the handling in
slave_configure()(usb/storage/scsiglue) which marks the HDD/pen drives
as removable also signifying them to be USB based.
Then, as part of sd_revalidation – how about adding the ATA_IDENTIFY
command part if the device is USB HDD? As far as the result of
ATA_IDENTIFY is concerned – they return proper ‘256’ bytes - response
and the Words – 82, 85 used for feature supported and enabled/disabled
returns proper values for the USB HDD’s I have seen. In case of USB
pen drives – they return failure – I did not see any crash – maybe I
don’t have one of the legacy SATL based disk.
Since, I am new to this – I will check more on this to get a viable
solution. Please add your opinion. Can you share the name of the
device which causes crash with these ATA commands, If I am able to get
one I can try on that also.

How?  There are many USB devices whose SATL crashes and burns on ATA_12
or ATA_16, so this would add fragility to the detection path. The
detection path has to be cast iron because it has to work on a vast
range of devices.  The only way this could really work is to add it in
the usb storage driver and then replace the bad mode pages.

James