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Subject: Re: [PATCH v10 1/2] scsi: ufs: Enable power management for wlun
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Bart Van Assche <bvanassche@acm.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>, cang@codeaurora.org,
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From:   "Asutosh Das (asd)" <asutoshd@codeaurora.org>
Message-ID: <a89ad647-6c0c-b45e-cff3-a205bed034cf@codeaurora.org>
Date:   Wed, 10 Mar 2021 08:39:51 -0800
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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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