> > Should I write a short summary on how to decide what a driver needs ?
>
> That would be great, I'll put that in the usb_submit_urb()
> documentation.
How to decide which mem_flags to pass to usb_submit_urb
Basically the rules are the same as for kmalloc ;-)
Now let me go to the details.
There are four different possible values: GFP_KERNEL, GFP_NOFS, GFP_NOIO
and GFP_ATOMIC. GFP_NOFS has as yet no field of application.
There are three situations you must use GFP_ATOMIC.
a) you are inside a completion handler, an interrupt, bottom half, tasklet or timer
b) you are holding a spinlock or rwlock (does not apply to semaphores)
b) current->state != TASK_RUNNING, this is the case only after you've changed it
GFP_NOIO is used in the block io path and error handling of storage devices.
Anything else uses GFP_KERNEL.
Specific uses (or rules of thumb ;-) ):
-start_xmit and timeout methods of network drivers must use GFP_ATOMIC (spinlock)
-queuecommand methods of scsi drivers must use GFP_ATOMIC (spinlock)
-If you use a kernel thread with a network driver you must use GFP_NOIO,
unless b) or c) apply
-After you have done a down() you use GFP_KERNEL, unless b) or c) apply
or your are in a storage driver's block io path
-probe and disconnect use GFP_KERNEL unless b) or c) apply
-Changing firmware on a running storage or net device uses GFP_NOIO,
unless b) or c) apply
HTH
Oliver
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