Re: Microchip USB2642 Hub not resuming from USB autosuspend

[Alan Stern <stern@rowland.harvard.edu>]

[CC: list drastically trimmed]

On Wed, Jun 24, 2020 at 09:22:42AM +0200, Martin Kepplinger wrote:
> On 23.06.20 16:54, Alan Stern wrote:
> > I don't understand this.  As far as I know, runtime-PM support in the SCSI 
> > and block layers has been complete for many years.  If you have to do 
> > anything extra (like applying the patch in the email you mentioned) then 
> > something is broken.  The device should be able to go into runtime suspend 
> > just fine with the current code -- even when a file system is mounted.
> > 
> 
> The scsi and usb layers have good implementations for runtime pm for a
> long time indeed. The scsi drivers though vary: for example sr indeed
> suspends when unused, when mounted and open too.
> 
> look at the sd driver in comparison though: From what I see, it "uses"
> autopm (runtime pm) as it would let the device suspend, but never
> resumes from it (except when removing and probing again). My suspicion
> is that it's not really used for that reason.
> 
> I might be wrong of course - I usually don't look at scsi code and I'm
> so thankful for feedback. That's what I read though and also what my
> tests verify. Hence the patch to the sd driver that makes enabling
> runtime pm result in a usable device (granted, not yet saving as much
> power as it really could, but that can be added later).

I haven't looked at this code or tested it for a long time, so maybe it 
isn't working properly.

Still, here's the general idea of how it's _intended_ to work: The 
runtime PM control for sd lies not in the sd driver itself but in the 
block layer core.  If a SCSI drive is suspended when a block request is 
issued, the block layer initiates a runtime resume.  See the kerneldoc for 
blk_pm_runtime_init() and the following functions in block/blk-pm.c.

If you need a more detailed explanation, please ask.

Alan Stern