linux-kernel - Re: [PATCH RFC] scsi: ufs: Disable blk-mq for now

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Date:   Fri, 14 Sep 2018 09:17:14 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     "Martin K . Petersen" <martin.petersen@...cle.com>,
        "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Bart Van Assche <Bart.VanAssche@....com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Stanislav Nijnikov <stanislav.nijnikov@....com>,
        Evan Green <evgreen@...omium.org>,
        Vinayak Holikatti <vinholikatti@...il.com>,
        Janek Kotas <jank@...ence.com>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Asutosh Das <asutoshd@...eaurora.org>,
        Subhash Jadavani <subhashj@...eaurora.org>,
        Sayali Lokhande <sayalil@...eaurora.org>,
        Li Wei <liwei213@...wei.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Alex Lemberg <Alex.Lemberg@...disk.com>
Subject: Re: [PATCH RFC] scsi: ufs: Disable blk-mq for now

On 14/09/18 04:52, Ming Lei wrote:
> On Thu, Sep 13, 2018 at 03:15:39PM +0300, Adrian Hunter wrote:
>> On 13/09/18 15:05, Ming Lei wrote:
>>> On Thu, Sep 13, 2018 at 02:28:48PM +0300, Adrian Hunter wrote:
>>>> blk-mq does not support runtime pm, so disable blk-mq support for now.
>>>
>>> So could you describe a bit what the issue you are trying to fix?
>>
>> UFS is a low-power solution, so we must be able to runtime suspend.
>>
>>>
>>> This is host level runtime PM you are trying to address, and if blk-mq
>>> runtime isn't enabled, I guess the host won't be runtime suspended at all
>>> because some of its descendant are always active.
>>>
>>> So seems we need to do nothing for preventing the host controller from
>>> entering runtime suspend.
>>
>> We don't want to prevent the host controller from runtime suspending, quite
>> the opposite.
> 
> OK, got it.
> 
> However, in previous discussion, it is strongly objected to use
> per-driver/device .use_blk_mq switch, so not sure if this way can
> be accepted.

It is only needed for 4.19 so far.  Otherwise just revert d5038a13eca7
("scsi: core: switch to scsi-mq by default")

> 
> BTW, I just posted the runtime PM enablement patches[1] for blk-mq,
> and I verified that it works fine and passed blktests & xfstest & my
> other sanity tests, could you try it on UFS?
> 
> [1] https://marc.info/?l=linux-block&m=153684095523409&w=2

I will give it a go.

Obviously, if those patches go in, we wouldn't need to disable blk-mq
anymore, but that isn't until 4.20 at least.