Message-ID: <MN2PR04MB69913DA014B665CFAC3AF8FFFC1D0@MN2PR04MB6991.namprd04.prod.outlook.com>
Date:   Thu, 6 Feb 2020 22:21:10 +0000
From:   Avri Altman <Avri.Altman@....com>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     Julian Calaby <julian.calaby@...il.com>,
        Avi Shchislowski <Avi.Shchislowski@....com>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: RE: [PATCH 0/5] scsi: ufs: ufs device as a temperature sensor

> 
> On Thu, Feb 06, 2020 at 07:32:12PM +0000, Avri Altman wrote:
> > Hi Julian,
> >
> > >
> > >
> > > Hi Avri,
> > >
> > > On Fri, Feb 7, 2020 at 12:41 AM Avri Altman <Avri.Altman@....com>
> wrote:
> > > >
> > > > >
> > > > > Hi Avri,
> > > > >
> > > > > On Thu, Feb 6, 2020 at 11:08 PM Avri Altman
> <Avri.Altman@....com>
> > > > > wrote:
> > > > > >
> > > > > >
> > > > > > >
> > > > > > > Hi Avi,
> > > > > > >
> > > > > > > On Thu, Feb 6, 2020 at 9:48 PM Avi Shchislowski
> > > > > > > <Avi.Shchislowski@....com> wrote:
> > > > > > > >
> > > > > > > > As it become evident that the hwmon is not a viable option to
> > > > > implement
> > > > > > > ufs thermal notification, I would appreciate some concrete
> comments
> > > of
> > > > > this
> > > > > > > series.
> > > > > > >
> > > > > > > That isn't my reading of this thread.
> > > > > > >
> > > > > > > You have two options:
> > > > > > > 1. extend drivetemp if that makes sense for this particular
> application.
> > > > > > > 2. follow the model of other devices that happen to have a built-in
> > > > > > > temperature sensor and expose the hwmon compatible attributes
> as
> > > a
> > > > > > > subdevice
> > > > > > >
> > > > > > > It appears that option 1 isn't viable, so what about option 2?
> > > > > > This will require to export the ufs device management commands,
> > > > > > Which is privet to the ufs driver.
> > > > > >
> > > > > > This is not a viable option as well, because it will allow unrestricted
> > > access
> > > > > > (Including format etc.) to the storage device.
> > > > > >
> > > > > > Sorry for not making it clearer before.
> > > > >
> > > > > I should have clarified further: I meant having the UFS device
> > > > > register a HWMON driver using this API:
> > > > > https://www.kernel.org/doc/html/latest/hwmon/hwmon-kernel-
> > > api.html
> > > > >
> > > > > *Not* writing a separate HWMON driver that uses some private
> > > interface.
> > > > Ok.
> > > > Just one last question:
> > > > The ufs spec requires to be able to react upon an exception event from
> the
> > > device.
> > > > The thermal core provides an api in the form of
> > > thermal_notify_framework().
> > > > What would be the hwmon equivalent for that?
> > >
> > > My understanding is that HWMON is just a standardised way to report
> > > hardware sensor data to userspace. There are "alarm" files that can be
> > > used to report fault conditions, so any action taken would have to be
> > > either managed by userspace or configured using thermal zones
> > > configured in the hardware's devicetree.
> > Those "alarms" are  implemented as part of the modules under
> drivers/hwmon/ isn't it?
> > We already established that this is not an option for the ufs driver.
> 
> You have established nothing. What exactly is not an option ?
> To create alarm attributes ? No one forces you to create any of those
> if you don't want to.
> 
> >
> > >
> > > thermal_notify_framework() is a way to notify the "other side" of a
> > > thermal zone to do something to reduce the temperature of that zone.
> > > E.g. spin up a fan or switch to a lower-power state to cool a CPU.
> > > Looking at your code, you're only implementing the "sensor" side of
> > > the thermal zone functionality, so your calls to
> > > thermal_notify_framework() won't do anything.
> > Right.  The thermal core allows to react to such notifications,
> > Provided that the thermal zone device has a governor defined,
> > And/or notify ops etc.
> >
> > Should the current patches implement those callbacks or not,
> > Can be discussed during their review process.
> > But the important thing is that the thermal core support it in an intuitive
> and simple way,
> > While the hwmon doesn't.
> >
> > We are indifference with respect of which subsystem to use.
> 
> Not really. Quite the opposite; you are quite obviously heavily
> opposed to a hwmon driver.
> 
> > The thermal core was our first choice because we bumped into it,
> > Looking for a way to raise thermal exceptions.
> > It provides the functionality we need, and other devices uses it,
> > Why the insistence not to use it?
> >
> 
> As mentioned before, the hwmon subsystem lets you create a bridge
> to the thermal subsystem, it creates standard attributes to report
> temperatures instead of the private ones your patch provides,
> and it would result in simpler code.
> 
> Why the insistence to _not_ use the hwmon subsystem ?
I am not. Please, guide me through it - 
We'll register a hwmon device and implement its read ops.
Since read is confined to the ufs driver, we'll still need to have the ufs-thermal module that we've created.
Next we need to respond to a thermal exception event raised by the device - 
We are expected to pass such notification onward, for whatever governor/mind to  take an applicable action,
Should such mind exists.  How do I do that?

> 
> Guenter

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