Re: REQUEST_SENSE and ide-cd.c

[Chuck Ebbert <cebbert@redhat.com>]

On 07/11/2007 04:30 AM, Piotr Muszynski wrote:

[cc: linux-usb, linux-ide]

> I am adding transparent ATAPI capability to USB gadget Mass Storage
> driver. The idea is to pass USB traffic to block device queue as packet
> requests. At the end of the queue, the requests are handled by ide-cd.c
> driver.
> 
> It breaks when the ide-cd.c driver unconditionally generates
> REQUEST_SENSE for requests that ended in unit attention condition.
> 
> By clearing the drive's unit attention condition, this additional
> REQUEST_SENSE confuses the host, which fires it's own REQUEST_SENSE
> packet, to which the drive replies with NO SENSE.
> 
> I can see three solutions:
> 
> 1. Intercept the sense data returned by ide-cd.c and emulate unit
> attention condition in file_storage.c driver;
> 
> 2. Introduce a new request flag causing ide-cd.c to skip calling
> cdrom_queue_request_sense() for flagged requests, like below:
> 
>     cdrom_decode_status() 2.6.12:
>     -    if (stat & ERR_STAT) {
>     +    if (stat & ERR_STAT && !(rq->flags & REQ_NO_AUTOSENSE)) {
>              spin_lock_irqsave(&ide_lock, flags);
>              blkdev_dequeue_request(rq);
>              HWGROUP(drive)->rq = NULL;
>              spin_unlock_irqrestore(&ide_lock, flags);
>              cdrom_queue_request_sense(drive, rq->sense, rq);
>          } else
>              cdrom_end_request(drive, 0);
> 
> 3. Acknowledge that ide-cd.c was not meant to work as in (2) and search
> for another mechanism. Where?
> 
> (1) would unnecessarily duplicate the drive's state. I'd rather do (3).
> 
> So far, the (2) works well, but how bad is it?
> I'd greatly appreciate any critical feedback.