On 1/28/19 3:03 PM, Christoph Hellwig wrote:
On Thu, Jan 24, 2019 at 08:52:20AM +0100, Hannes Reinecke wrote:
Hi all,
blk-mq has the concept of 'private' tags to handle driver-internal commands.
Also quite some SCSI HBAs use internal commands for configuration, event
handling etc.
But sadly no interface to use the 'private' tags from the block layer
exists, so quite some drivers like megaraid_sas, aacraid, or hpsa have to
implement their own management of internal commands.
Well, it would be rather trivial to expose, just waiting for a user.
Please just go ahead and add the interface together with your first
user.
The biggest blocker so far has been that the legacy request code has no
concept of reserved requests and adding them for blk-mq only would lead
to diverging code paths. With the legacy code gone now we are free to
move ahead.
But that is precisely the point: which interface?
The most straigthforward approach would be something like
'blk_mq_get_private_request()' (ie similar to the existing
'blk_mq_get_request()'). But that requires a working request queue and
will expose a 'struct request', (and by implication a struct scsi_cmnd).
For most users of private tags/commands we need to be the queue working
_prior_ to setting up the 'normal' request queues, as the commands are
used to fetch information from the hardware required to setup the queues.
And typically the 'private' commands are driver-specific, ie the payload
_after_ the scsi command, so the allocation for the request and the scsi
command is pretty much pointless here.
The alternative approach would be to define an interface into the block
layer to get the 'raw' tag directly from the tagset, but that is quite
some surgery in the block layer which I won't attempt without further
confirmation that this is the way we will be going.
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
hare@xxxxxxx +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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