Re: [PATCH] libata: rewrite SCSI host scheme to be one per ATA host

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Jens Axboe wrote:
On Wed, Apr 22 2009, Jeff Garzik wrote:
Jeff Garzik wrote:
Currently, libata creates a Scsi_Host per port.  This was originally
done to leverage SCSI's infrastructure to arbitrate among master/slave
devices, but is not needed for most modern SATA controllers.   And I
_think_ it is not needed for master/slave if done properly, either.
BTW note the above, with regards to the libata SCSI->block conversion. libata currently relies on SCSI for some amount of generic device arbitration, in several situations (see ->qc_defer, SCSI_MLQUEUE_.*_BUSY). libata expects SCSI to be intelligent and not starve devices, etc.

Defer looks like internal policy, I don't see that functioning any
different in the block layer. SCSI_MLQUEUE_*_BUSY in SCSI is primarily
using the block layer functionality of BLKPREP_DEFER to begin with, so I
think we're pretty close to providing all that already.

It's not quite that simple. I am referring mainly to arbitration across multiple request_queue's. SCSI has useful code in place to deal with target-busy and host-busy conditions, both of which could potentially be blocking and unblocking multiple request queues.

mlqueue is much more than just a wrapper over block requeueing functions. Read scsi_next_command() and scsi_run_queue(), and grep for starved_list, host_{busy,blocked}, target_{busy,blocked}, device_{busy,blocked}.

In our master/slave case, we must choose between queue A and queue B, making sure to starve neither. For simplex DMA, we potentially have queues A, B, C and D serving requests across the "bus bottleneck," and must ensure no starvation of A, B, C or D.


Although I have no code to back this up, my gut feeling is that a "request queue group" object, with associated functions, that would be the appropriate place for cross-queue or "host-wide" (as in, struct Scsi_Host or struct ata_host) functionality.

Whatever the solution, libata definitely makes use of SCSI's cross-request_queue arbitration, so any move to block will require similar functionality.

	Jeff



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