Re: [Nouveau] hwmon API update

[Guenter Roeck <guenter.roeck@ericsson.com>]

On Thu, 2011-03-03 at 15:48 -0500, Lucas Stach wrote:
> Am Donnerstag, den 03.03.2011, 18:29 +0100 schrieb Martin Peres:
> > Le 03/03/2011 16:22, Guenter Roeck a écrit :
> > > On Thu, Mar 03, 2011 at 04:36:09AM -0500, Dave Airlie wrote:
> > >> On Mon, Feb 14, 2011 at 8:08 AM, Jean Delvare<khali@linux-fr.org>  wrote:
> > >>> On Sun, 13 Feb 2011 09:16:40 -0800, Guenter Roeck wrote:
> > >>>> On Sun, Feb 13, 2011 at 07:18:44AM -0500, Martin Peres wrote:
> > >>>>> Hi,
> > >>>>>
> > >>>>> I am working on power management on the nouveau driver and I need a way
> > >>>>> to get data out of and send commands to the i2c drivers from the kernel
> > >>>>> space.
> > >> Martin,
> > >>
> > >> you probably should have cc'ed Matthew since it was his patch you based this on,
> > >> and I think he can provide a good explaination.
> > >>
> > >> to clarify some points,
> > >>
> > >> radeon does probably want something exactly like this, we just haven't gotten to
> > >> it completely yet, I'd rather not have two drivers in the kernel for
> > >> exact same hardware,
> > >> and I believe sharing the hwmon code to do what we want is a good plan since you
> > >> don't go around reinventing wheels, but if hwmon/i2c maintainers have
> > >> no interest
> > >> it leaves with little choice but to implement about 5-10 i2c drivers
> > >> again in drm codebase.
> > >>
> > >> Maybe hwmon/i2c maintainers could suggest a cleaner way to implement
> > >> what we want,
> > >> which I think I can summarize as
> > >>
> > >> a) access to monitored values in-kernel
> > >> b) no userspace access to the same values except via sanitised via the driver.
> > >>
> > > This is not a matter of "no interest". Interest is there, but if one demands
> > > too much one may get nothing.
> > >
> > > Request for b) so far was "no userspace access", period. This is unacceptable
> > > since providing userspace access to monitored values is the whole point of hwmon.
> 
> And that is what we want to do. But it would be nice if the graphics
> drivers could provide a _single_ interface to userspace. Not all boards
> have i2c hardware monitoring chips and with a single interface we could
> fall back to the internal gpu sensor, transparently for the user.
> 
> > >
> > > I could imagine an API that covers both a) and b), as long as b) focuses
> > > on the "sanitize" aspect and doesn't try to limit userspace access to attributes.
> > >
> > > Guenter
> > b) was introduced by Dave, I never asked for it because I don't mind 
> > duplicating sensor data (one hwmon device named nouveau and one for the 
> > raw access to the i2c chip).
> 
> Sorry for the confusion Martin, I brought up the point of limiting
> userspace access and did not cc the nouveau mailing list. I think it is
> bad behaviour to expose values to userspace which are totally off the
> real values. This confuses users and should be avoided, especially since
> we can provide sanitized values.
> 
> Why should we push the logic and api for sanitizing the values to many
> hwmon drivers if we could easily do this at a single point in the
> graphics driver, if we provide the userspace interface ourself and use
> the hwmon driver only to instrument the monitoring chip?

I don't think the functionality (nor the internal API) should have to be
implemented in individual drivers. Instead, there should be a new API
between hwmon drivers and the hwmon core. Using this API, the hwmon core
would read/write raw values from/to hwmon drivers and make those values
available to userland and to other drivers (such as the nouveau driver).
The hwmon core would then also perform sanitizing if necessary, ie call
registered sanitizing functions.

With this approach, hwmon would report sanitized values, graphics
drivers would not have to support/generate hwmon sysfs ABI attributes,
and hwmon driver structure would be much simpler, since drivers could
concentrate on getting data from and to chips instead of having to deal
with sysfs attributes. That would be a win for everyone.

Guenter



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