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

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Date:   Tue, 9 Mar 2021 22:14:38 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Asutosh Das \(asd\)" <asutoshd@...eaurora.org>
Cc:     Bart Van Assche <bvanassche@....org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>, cang@...eaurora.org,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "open list:TARGET SUBSYSTEM" <linux-scsi@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Stanley Chu <stanley.chu@...iatek.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Kiwoong Kim <kwmad.kim@...sung.com>,
        Bean Huo <beanhuo@...ron.com>,
        Lee Jones <lee.jones@...aro.org>,
        Wei Yongjun <weiyongjun1@...wei.com>,
        Dinghao Liu <dinghao.liu@....edu.cn>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Tomas Winkler <tomas.winkler@...el.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Satya Tangirala <satyat@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/SAMSUNG S3C, S5P AND EXYNOS ARM ARCHITECTURES" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/SAMSUNG S3C, S5P AND EXYNOS ARM ARCHITECTURES" 
        <linux-samsung-soc@...r.kernel.org>,
        "moderated list:UNIVERSAL FLASH STORAGE HOST CONTROLLER DRIVER..." 
        <linux-mediatek@...ts.infradead.org>,
        Linux-PM mailing list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v10 1/2] scsi: ufs: Enable power management for wlun

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/@...t/@...ian) 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