Re: Does Linux support more than 2TB on a USB attached device?

On Wed, 3 Oct 2007 09:06:53 -0700, "Richard Sharpe" <[email protected]>
wrote:

>> We have tracked down the problem. The USB Mass Storage layer in Linux
>> does not support 16-byte CDBs, it seems. They get stamped with a
length
>> of 12 and are sent as short commands over USB, which we were issuing
an
>> ABORT to.
>
>From the perspective of ub, is there a reliable and compatible way to
>detect the need to use READ_CAPACITY(16)?

If the device returns 0xFFFFFFFF to a READ_CAPACITY(10), then you have
to issue a READ_CAPACITY(16).

We have found the problem.

drivers/usb/storage/usb.c does this:

         /*
          * Ask the SCSI layer to allocate a host structure, with extra
          * space at the end for our private us_data structure.
          */
         host = scsi_host_alloc(&usb_stor_host_template, sizeof(*us));
         if (!host) {
                 printk(KERN_WARNING USB_STORAGE
                         "Unable to allocate the scsi host\n");
                 return -ENOMEM;
         }

         us = host_to_us(host);
         memset(us, 0, sizeof(struct us_data));
         mutex_init(&(us->dev_mutex));
         init_MUTEX_LOCKED(&(us->sema));

scsi_host_alloc creates a host structure with max_cmd_len set to 12.

To allow USB Storage to accept 16-byte CDBs, the following will need to
be added:

         host->max_cmd_len = 16;  /* or some symbol */

That should allow for USB storage devices larger than 2TiB.

We are busy checking this.

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