Re: [PATCH] nvme: determine the number of IO queues

From: Minwoo Im
Date: Thu Apr 18 2019 - 08:13:42 EST

On 4/18/19 3:21 PM, Aaron Ma wrote:
On 4/18/19 1:33 AM, Maxim Levitsky wrote:
On Wed, 2019-04-17 at 20:32 +0300, Maxim Levitsky wrote:
On Wed, 2019-04-17 at 22:12 +0800, Aaron Ma wrote:
Some controllers support limited IO queues, when over set
the number, it will return invalid field error.
Then NVME will be removed by driver.

Find the max number of IO queues that controller supports.
When it still got invalid result, set 1 IO queue at least to
bring NVME online.
To be honest a spec compliant device should not need this.
The spec states:

"Number of I/O Completion Queues Requested (NCQR): Indicates the number of I/O
Queues requested by software. This number does not include the Admin
Queue. A
minimum of one queue shall be requested, reflecting that the minimum support
for one I/O
Completion Queue. This is a 0âs based value. The maximum value that may be
specified is 65,534
(i.e., 65,535 I/O Completion Queues). If the value specified is 65,535, the
controller should return
an error of Invalid Field in Command."

This implies that you can ask for any value and the controller must not
with an error, but rather indicate how many queues it supports.

Maybe its better to add a quirk for the broken device, which needs this?

Adding quirk only makes the code more complicated.
This patch doesn't change the default behavior.
Only handle the NVME error code.

Yes the IO queues number is 0's based, but driver would return error and
remove the nvme device as dead.

IMHO, if a controller indicates an error with this set_feature command, then
we need to figure out why the controller was returning the error to host.

If you really want to use at least a single queue to see an alive I/O queue,
controller should not return the error because as you mentioned above,
NCQA, NSQA will be returned as 0-based. If an error is there, that could
mean that controller may not able to provide even a single queue for I/O.

Minwoo Im

So set it as 1 at least the NVME can be probed properly.


I forgot to add the relevant paragraph:

"Note: The value allocated may be smaller or larger than the number of queues
requested, often in virtualized
implementations. The controller may not have as many queues to allocate as are
requested. Alternatively,
the controller may have an allocation unit of queues (e.g. power of two) and may
supply more queues to
host software to satisfy its allocation unit."

Best regards,
Maxim Levitsky

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