linux-kernel - Re: [PATCH/RFC] hwmon: Add support for W83667HG-B

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Message-ID: <20100703100910.06c8e17e@hyperion.delvare>
Date:	Sat, 3 Jul 2010 10:09:10 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Guenter Roeck <guenter.roeck@...csson.com>
Cc:	Jim Cromie <jim.cromie@...il.com>,
	H Hartley Sweeten <hsweeten@...ionengravers.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ethan Lawrence <e.law87@...oo.com>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lars4910@...mail.com" <lars4910@...mail.com>,
	"birdie@...monline.ru" <birdie@...monline.ru>,
	"jadcock@....net" <jadcock@....net>
Subject: Re: [PATCH/RFC] hwmon: Add support for W83667HG-B

Hi Guenter,

On Fri, 2 Jul 2010 07:54:04 -0700, Guenter Roeck wrote:
> On Fri, Jul 02, 2010 at 05:49:49AM -0400, Jean Delvare wrote:
> > On Fri, 2 Jul 2010 01:25:30 -0700, Guenter Roeck wrote:
> > > After removing the defines and trying to compile I remembered.
> > > I _knew_ there was a reason for not removing them.
> > > Guess it's too late (or early) here to do serious work.
> > > 
> > > The defines _are_ used, in:
> > > 
> > > fan_functions(fan_max_output, FAN_MAX_OUTPUT)
> > > fan_functions(fan_step_output, FAN_STEP_OUTPUT)
> > > 
> > > which expands to W83627EHF_REG_FAN_MAX_OUTPUT and W83627EHF_REG_FAN_STEP_OUTPUT.
> > > 
> > > Tricky ... and that was also the reason why I retained the original 
> > > global variables.
> > 
> > Tricky indeed. We normally don't accept code like this in the kernel.
> > 
> > > I'll move the pointers into per-device code as you suggested, but I'll
> > > have to think about how to do that w/o having to change a lot of code.
> > 
> > If code changes are desirable, let's just do them. You can do that in a
> > preliminary patch, and then your patch adding support for the
> > W83667HG-B goes on top of it.
> > 
> Without the support for -B the changes are not really needed, so that patch
> would not make much sense without it.

It is still a good practice to split patches in this case. This makes
review and bisection easier. Simply, we don't get to apply the first
patch if we don't also apply the second.

> Have you looked at v2 of the patch ?

Not yet, will do now.

> > > As for the 0xff - that pretty much applies to all chips supported by this driver. 
> > > I guess it is supposed to mean "not supported", and as a result the code will
> > > write to a non-existing register. I don't really want to touch that.
> > 
> > I want you to touch that. Writing to non-existing registers is a bad
> > idea. You never know what actually happens when you do that.
>
> Good point.

BTW, as I wasn't clear enough about this originally: I didn't mean this
to fix to be included into your W83667HG-B support patch. It should be
a separate patch, so that it can be easily backported.

> Clean fix would be not to provide the unsupported attributes. Simple workaround
> would be to return an error if a write is attempted on a non-supported attribute.
> I am sure it would be better to not provide the attribute, but would you accept
> the workaround ?

I will accept whatever you are willing to provide. As you're the one
spending the time, I'm not the one deciding how much you spend.

> > > The size difference (3 entries vs. 4) doesn't matter, since the chips are both 
> > > configured to have only three pwm fan controllers (even though the W83667HG
> > > is supposed to have four per its datasheet). So the 4th element of the arrays
> > > will not be accessed by the code if W83667HG(-B) is detected.
> > 
> > OK.
>
> On a side note, any idea why the 4th pwm is disabled for the W83667HG ?

The W83667HG chip has only 3 FANOUT pins (98, 125 and 127), so it is
expected that the driver exposes only 3 pwm outputs for this chip. I
don't understand why you think it is a problem?

Pin 98 can be used for an alternative function, so the corresponding
pwm output should not even be unconditionally instantiated.

Now the chip is quite complex in that it apparently has 4 fan control
units, and which one controls which fan output can be decided by
register values. I have no idea why they made it so complex at the
hardware level, but this certainly explains why the driver is a little
messy in this respect. We might have to move 667HG and later support to
a separate driver at some point, I don't know.

Now I'm sure you understand why I never took the time to look into
adding W83667HG-B support ;)

-- 
Jean Delvare
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