[Bug 12207] block reads/writes > 122880 bytes to USB tape drive gives EBUSY

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http://bugzilla.kernel.org/show_bug.cgi?id=12207





--- Comment #32 from Alan Stern <stern@rowland.harvard.edu>  2009-04-11 14:26:00 ---
> Yes, SCSI uses the (Linux) block layer and the attributes do exist within 
> the kernel. They just are not visible in sysfs for character devices. Last 
> night I read the code to find out why. Your analysis is correct but it 
> only shows that the kobject exists. The kobject is added to sysfs using 
> kobject_add() in blk_register_queue() called by add_disk(), which is only 
> called by disk drivers.

You are right.

And this makes me feel better -- I had thought that usb-storage's 
max_sectors attribute was redundant.  Now I know that for some 
SCSI-over-USB devices, the information is not accessible anywhere else.

> Whether character devices should add the kobject somewhere in sysfs is 
> getting beyond the original topic in this bugzilla entry. This is a more 
> general problem because there is no /sys/char directory. My current 
> opinion is that it would be nice to see these attributes somewhere, but 
> probably not worth the trouble.

Until somebody asks for them, we're okay.  However it shouldn't be very
hard to write a function like blk_register_queue() that accepts a
non-disk device as parent.  (And likewise for blk_unregister_queue, of
course.)

Alan Stern

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