Re: [lm-sensors] [PATCH 3/4] hwmon: (pmbus) Improve fan detection

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

On Sun, Jul 03, 2011 at 02:25:03PM -0400, Jean Delvare wrote:
> On Sun, 3 Jul 2011 09:27:46 -0700, Guenter Roeck wrote:
> > Hi Jean,
> > 
> > On Sun, Jul 03, 2011 at 04:14:32AM -0400, Jean Delvare wrote:
> > > Hi Guenter,
> > > 
> > > On Thu, 30 Jun 2011 10:29:52 -0700, Guenter Roeck wrote:
> > > > Some PMBus devices return no error when reading fan speed registers, but don't
> > > > really support fans. Strengthen fan detection by also checking if fan
> > > > configuration registers exist.
> > > > 
> > > > Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
> > > > ---
> > > >  drivers/hwmon/pmbus.c |    6 ++++--
> > > >  1 files changed, 4 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/hwmon/pmbus.c b/drivers/hwmon/pmbus.c
> > > > index 98e2e28..b0ea00b 100644
> > > > --- a/drivers/hwmon/pmbus.c
> > > > +++ b/drivers/hwmon/pmbus.c
> > > > @@ -47,12 +47,14 @@ static void pmbus_find_sensor_groups(struct i2c_client *client,
> > > >  	if (info->func[0]
> > > >  	    && pmbus_check_byte_register(client, 0, PMBUS_STATUS_INPUT))
> > > >  		info->func[0] |= PMBUS_HAVE_STATUS_INPUT;
> > > > -	if (pmbus_check_word_register(client, 0, PMBUS_READ_FAN_SPEED_1)) {
> > > > +	if (pmbus_check_byte_register(client, 0, PMBUS_FAN_CONFIG_12) &&
> > > > +	    pmbus_check_word_register(client, 0, PMBUS_READ_FAN_SPEED_1)) {
> > > >  		info->func[0] |= PMBUS_HAVE_FAN12;
> > > >  		if (pmbus_check_byte_register(client, 0, PMBUS_STATUS_FAN_12))
> > > >  			info->func[0] |= PMBUS_HAVE_STATUS_FAN12;
> > > >  	}
> > > > -	if (pmbus_check_word_register(client, 0, PMBUS_READ_FAN_SPEED_3)) {
> > > > +	if (pmbus_check_byte_register(client, 0, PMBUS_FAN_CONFIG_34) &&
> > > > +	    pmbus_check_word_register(client, 0, PMBUS_READ_FAN_SPEED_3)) {
> > > >  		info->func[0] |= PMBUS_HAVE_FAN34;
> > > >  		if (pmbus_check_byte_register(client, 0, PMBUS_STATUS_FAN_34))
> > > >  			info->func[0] |= PMBUS_HAVE_STATUS_FAN34;
> > > 
> > > Then wouldn't it be sufficient to only test PMBUS_FAN_CONFIG_12 and
> > > PMBUS_FAN_CONFIG_34, respectively? This would make the function
> > > somewhat faster.
> > 
> > I'd rather keep both. Devices should not return a valid value or should at least set 
> > the command error flag in the status register if they don't support fans and the fan speed
> > register is read. If I can't trust that (and I can't because Intersil devices do just that),
> > I can not trust the configuration register either. 
> 
> I think that what worries me is the asymmetry. Is it not possible for a
> device to have FAN_SPEED_2 and not FAN_SPEED_1? If not then your code
> is fine, I agree. But if it is possible, then we need smarter code.
> 
With PMBus, everything is possible ;). It is pretty much impossible to cover all
possible variants. Who would have thought that the fan speed register would return
a value even though the device doesn't support fans ?

My approach is to get my hand on as many PMBus devices as possible and improve the code
whenever I find something new. So far the manufacturers have almost never failed
to surprise me - it is almost as if there is a competition to find as many
interpretations of the standard as possible.

I have some work to do to improve fan support anyway, so I'll keep this in mind.
I think for the time being we are good.

> But also note that I don't know a thing about PMBus, so there may be
> nothing to worry about.
> 
Not even sure if it is _possible_ to completely understand PMBus ...

Thanks,
Guenter

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