linux-kernel - Re: [PATCH] scsi: sd: add runtime pm to open / release

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Date:   Mon, 29 Jun 2020 11:42:59 +0200
From:   Martin Kepplinger <martin.kepplinger@...i.sm>
To:     Alan Stern <stern@...land.harvard.edu>,
        Bart Van Assche <bvanassche@....org>
Cc:     jejb@...ux.ibm.com, Can Guo <cang@...eaurora.org>,
        martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...i.sm
Subject: Re: [PATCH] scsi: sd: add runtime pm to open / release



On 26.06.20 17:44, Alan Stern wrote:
> On Fri, Jun 26, 2020 at 08:07:51AM -0700, Bart Van Assche wrote:
>> On 2020-06-25 01:16, Martin Kepplinger wrote:
>>> here's roughly what happens when enabling runtime PM in sysfs (again,
>>> because sd_probe() calls autopm_put() and thus allows it:
>>>
>>> [   27.384446] sd 0:0:0:0: scsi_runtime_suspend
>>> [   27.432282] blk_pre_runtime_suspend
>>> [   27.435783] sd_suspend_common
>>> [   27.438782] blk_post_runtime_suspend
>>> [   27.442427] scsi target0:0:0: scsi_runtime_suspend
>>> [   27.447303] scsi host0: scsi_runtime_suspend
>>>
>>> then I "mount /dev/sda1 /mnt" and none of the resume() functions get
>>> called. To me it looks like the sd driver should initiate resuming, and
>>> that's not implemented.
>>>
>>> what am I doing wrong or overlooking? how exactly does (or should) the
>>> block layer initiate resume here?
>>
>> As far as I know runtime power management support in the sd driver is working
>> fine and is being used intensively by the UFS driver. The following commit was
>> submitted to fix a bug encountered by an UFS developer: 05d18ae1cc8a ("scsi:
>> pm: Balance pm_only counter of request queue during system resume") # v5.7.
> 
> I just looked at that commit for the first time.
> 
> Instead of making the SCSI driver do the work of deciding what routine to 
> call, why not redefine blk_set_runtime_active(q) to simply call 
> blk_post_runtime_resume(q, 0)?  Or vice versa: if err == 0 have 
> blk_post_runtime_resume call blk_set_runtime_active?
> 
> After all, the two routines do almost the same thing -- and the bug 
> addressed by this commit was caused by the difference in their behaviors.
> 
> If the device was already runtime-active during the system suspend, doing 
> an extra clear of the pm_only counter won't hurt anything.
> 
>> I'm not sure which bug is causing trouble on your setup but I think it's likely
>> that the root cause is somewhere else than in the block layer, the SCSI core
>> or the SCSI sd driver.
>>
>> Bart.
> 
> Martin's best approach would be to add some debugging code to find out why 
> blk_queue_enter() isn't calling bkl_pm_request_resume(), or why that call 
> doesn't lead to pm_request_resume().
> 
> Alan Stern
> 

Hi Alan,

blk_queue_enter() always - especially when sd is runtime suspended and I
try to mount as above - sets success to be true for me, so never
continues down to bkl_pm_request_resume(). All I see is "PM: Removing
info for No Bus:sda1".

thanks,
                                martin