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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,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        "open list:TARGET SUBSYSTEM" <linux-scsi@vger.kernel.org>,
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        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Kiwoong Kim <kwmad.kim@samsung.com>,
        Bean Huo <beanhuo@micron.com>,
        Lee Jones <lee.jones@linaro.org>,
        Wei Yongjun <weiyongjun1@huawei.com>,
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From:   "Asutosh Das (asd)" <asutoshd@codeaurora.org>
Message-ID: <6b985880-f23a-adb3-8b7a-7ee1b56e6fa7@codeaurora.org>
Date:   Tue, 9 Mar 2021 20:04:53 -0800
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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 :) ).

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?

-asd

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