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Many SCSI HBAs support a hostwide tagset, whereby each command submitted to
the HW from all submission queues must have a unique tag identifier.
Normally this unique tag will be in the range [0, max queue], where "max
queue" is the depth of each of the submission queues.
Add support for this hostwide tag feature, via module parameter
"host_max_queue". A non-zero value means that the feature is enabled. In
this case, the submission queues are not exposed to upper layer, i.e. from
blk-mq prespective, the device has a single hw queue. There are 2 reasons
for this:
a. It is assumed that the host can support nr_hw_queues * can_queue
commands, but this is not true for hostwide tags
b. For nr_hw_queues != 0, the request tag is not unique over all HW
queues, and some HBA drivers want to use this tag for the hostwide tag
However, like many SCSI HBA drivers today - megaraid sas being an example -
the full set of HW submission queues are still used in the LLDD driver. So
instead of using a complicated "reply_map" to create a per-CPU submission
queue mapping like megaraid_sas (as it depends on a PCI device + MSIs) -
use a simple algorithm:
hwq = cpu % queue count
If the host_max_queue param is set non-zero, then the max queue depth is
fixed at this value also.
If and when hostwide shared tags are supported in blk-mq/scsi mid-layer,
then the policy to set nr_hw_queues = 0 for hostwide tags can be revised.
Link: https://lore.kernel.org/r/1594297400-24756-3-git-send-email-john.garry@huawei.com
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.