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

Jeff Garzik
2009-04-23T10:39:39Z

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