That's not what I meant.  Suppose the driver wants to carry out special
requests A and B before carrying out request R, which is initially at
the head of the queue.  The driver inserts A at the front, calls
blk_start_queue(), and inserts B at the front when A completes.  
What's to prevent the block layer from sending R to the driver while A

I see.  You're suggesting we add a new "power_mode" or "queue_idle"  
callback to the request_queue struct, and make the block layer invoke
this callback whenever a request completes and there are no other
requests pending or in flight.  Right?  And similarly, invoke the
callback (with a different argument) when the first request gets added
to an otherwise empty queue.

That would suit my needs.

Alan Stern

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Nothing, you will have to maintain that state and defer when
appropriate. Which should happen automatically, since you would not be

Yep, that is what I'm suggesting.

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Jens Axboe

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