[PATCH] nvme: Fix nvme_get/set_features() with a NULL result pointer

[Andy Lutomirski]

On Aug 25, 2016 4:20 PM, "Jens Axboe" <axboe at fb.com> wrote:
>
> On 08/25/2016 01:54 AM, Andy Lutomirski wrote:
>>
>> On Thu, Aug 25, 2016 at 12:38 AM, Christoph Hellwig <hch at lst.de> wrote:
>>>
>>> Ooops, yes.
>>>
>>> Are you looking into new nvme_set_features users?  Another thing
>>> we need to tackle is either replacing dma_addr argument with a
>>> a real kernel pointer (or just kill it until users show up)
>>
>>
>> I am, and I have a patch to do the former (and to add a length
>> argument).  But that's not -stable material.
>>
>> While I have your attention: the new use is to enable APST (power
>> saving).  In theory, it seems like I should integrate with dev_pm_qos
>> so that the standard interface for setting a latency limit will work,
>> but, on brief inspection, there are literally no drivers in the entire
>> tree that do this.  Am I missing something?  My current draft patch
>> just adds a sysfs attribute.  (It saves a *lot* of power on my laptop,
>> so supporting APST is worth doing.)
>
>
> Care to send out what you have? I'd be interested in seeing how much I
> can save on my laptop, haven't played with APST yet.

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=nvme/power

There are some todos:

 - Default to a nonzero latency (e.g. 7ms?  My SSD needs 5.5ms for max
power saving.)  To test it, write something like 7000000 to
apst_max_latency_ns.

 - Add a real changelog.

 - Optionally add a sysfs binfile or other interface to allow
uploading an entire custom table.

 - Consider *deleting* the SCSI translation layer's power saving code.
It looks almost entirely bogus to me.  It has an off-by-one in its
NPSS handling, it hardcodes power state indices which is total BS, it
ignores the distinction between operational and non-operational states
(which I think matters for non-APST usage).  It also seems likely to
be that it's never been used, since it's one of the formerly
crashy-looking set_features users.

You can inspect what it's doing with something like:

# nvme get-feature -f 0x0c -H -s 0 /dev/nvme0

if you have nvme-cli installed.

--Andy