Re: [PATCH v10 1/2] scsi: ufs: Enable power management for wlun

[Asutosh Das (asd)]

On 3/10/2021 8:27 AM, Alan Stern wrote:
> On Tue, Mar 09, 2021 at 08:04:53PM -0800, Asutosh Das (asd) wrote:
> > On 3/9/2021 7:14 PM, Alan Stern wrote:
> > > On Tue, Mar 09, 2021 at 07:04:34PM -0800, Asutosh Das (asd) wrote:
> > > > Hello
> > > > I & Can (thanks CanG) debugged this further:
> > > >
> > > > Looks like this issue can occur if the sd probe is asynchronous.
> > > >
> > > > Essentially, the sd_probe() is done asynchronously and driver_probe_device()
> > > > invokes pm_runtime_get_suppliers() before invoking sd_probe().
> > > >
> > > > But scsi_probe_and_add_lun() runs in a separate context.
> > > > So the scsi_autopm_put_device() invoked from scsi_scan_host() context
> > > > reduces the link->rpm_active to 1. And sd_probe() invokes
> > > > scsi_autopm_put_device() and starts a timer. And then driver_probe_device()
> > > > invoked from __device_attach_async_helper context reduces the
> > > > link->rpm_active to 1 thus enabling the supplier to suspend before the
> > > > consumer suspends.
> > >
> > > > I don't see a way around this. Please let me know if you
> > > > (@Alan/@Bart/@Adrian) have any thoughts on this.
> > >
> > > How about changing the SCSI core so that it does a runtime_get before
> > > starting an async probe, and the async probe routine does a
> > > runtime_put when it is finished?  In other words, don't allow a device
> > > to go into runtime suspend while it is waiting to be probed.
> > >
> > > I don't think that would be too intrusive.
> > >
> > > Alan Stern
> >
> > Hi Alan
> > Thanks for the suggestion.
> >
> > Am trying to understand:
> >
> > Do you mean something like this:
> >
> > int scsi_sysfs_add_sdev(struct scsi_device *sdev)
> > {
> > 	
> > 	scsi_autopm_get_device(sdev);
> > 	pm_runtime_get_noresume(&sdev->sdev_gendev);
> > 	[...]
> > 	scsi_autopm_put_device(sdev);
> > 	[...]
> > }
> >
> > static int sd_probe(struct device *dev)
> > {
> > 	[...]
> > 	pm_runtime_put_noidle(dev);
> > 	scsi_autopm_put_device(sdp);
> > 	[...]
> > }
> >
> > This may work (I'm limited by my imagination in scsi layer :) ).
>
> I'm not sure about this.  To be honest, I did not read the entirety of
> your last message; it had way too much detail.  THere's a time and place
> for that, but when you're brainstorming to figure out the underlying
> cause of a problem and come up with a strategy to fix it, you want to
> concentrate on the overall picture, not the details.
>
> As I understand the situation, you've get a SCSI target with multiple
> logical units, let's say A and B, and you need to make sure that A never
> goes into runtime suspend unless B is already suspended.  In other
> words, B always has to suspend before A and resume after A.
>
> To do this, you register a device link with A as the supplier and B as
> the consumer.  Then the PM core takes care of the ordering for you.
>
> But I don't understand when you set up the device link.  If the timing
> is wrong then, thanks to async SCSI probing, you may have a situation
> where A is registered before B and before the link is set up.  Then
> there's temporarily nothing to stop A from suspending before B.
>
> You also need to prevent each device from suspending before it is
> probed.  That's the easy part I was trying to address before (although
> it may not be so easy if the drivers are in loadable modules and not
> present in the kernel).
>
> You need to think through these issues before proposing actual changes.
>
> > But the pm_runtime_put_noidle() would have to be added to all registered
> > scsi_driver{}, perhaps? Or may be I can check for sdp->type?
>
> Like this; it's too early to worry about this sort of thing.
>
> Alan Stern
Hi Alan
Thanks. Understood.

I will check the details and see if I can come up with something.
I'll propose an alternate fix otherwise and drop this change altogether.

Thanks!
-asd

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