Re: [usb-storage] USB storage devices and SAT

[Boaz Harrosh <bharrosh@panasas.com>]

Matthew Dharm wrote:
> On Mon, Aug 04, 2008 at 03:10:04AM +0200, Douglas Gilbert wrote:
>> Alan Stern has already noted to another smartmontools developer
>> that such a change is likely to break some USB storage devices.
>> Perhaps the maximum sense buffer size could be optionally
>> specified per usb storage device. Alternatively the usb mass
>> storage logic could make some dynamic decisions itself.
> 
> To clarify: A great many devices choke (fatally) if asked for sense data
> other than 18 bytes.  Since the first TEST_UNIT_READY will likely require
> sense data, almost every device sees REQUEST_SENSE.
> 
> Personally, I hate having to make dynamic decisions in usb-storage.  The
> more we try to do there, the more likely we are to get it wrong.
> 
> If you've got an app that is sending a command, and you KNOW that command
> should produce >18 bytes of sense data, then there should be a way to
> specify to the SCSI core (and thus get passed to usb-storage) that sense
> data of >18 bytes should be requested.
> 

What?!! The number 18 comes totally from USB land at:
drivers/usb/storage/transport.c:601 via call to scsi_eh_prep_cmnd
Otherwise the entire kernel including scsi-ml and all user applications
request 96 bytes of sense, always.

So there is nothing the SCSI core or application can do about it. 
It is USB fix time I'm afraid.

> Matt
> 

The SAT layer will have to override transport.c/usb_stor_invoke_transport
somehow and make do for bigger REQUEST_SENSE. Or a sense_size param per host,
or per command.

Boaz