Message-ID: <20080714095914.0644ac5d@hyperion.delvare>
Date:	Mon, 14 Jul 2008 09:59:14 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	"David Hubbard" <david.c.hubbard@...il.com>
Cc:	"Hans de Goede" <j.w.r.degoede@....nl>, linuxppc-dev@...abs.org,
	"Samuel Ortiz" <samuel@...tiz.org>, linux-kernel@...r.kernel.org,
	"Milton Miller" <miltonm@....com>, lm-sensors@...sensors.org
Subject: Re: [RFC] (almost) booting allyesconfig -- please don't poke
 super-io without request_region

On Sun, 13 Jul 2008 15:26:56 -0600, David Hubbard wrote:
> Hi Hans,
> 
> >> I propose writing a subsystem driver. (Is that properly called "The
> >> SuperIO Bus Driver"?) If no one thinks it's a really bad idea I will
> >> put together some code and submit it for review, and maintain it.
> >>
> >> Some hwmon chips have odd / unique probe sequences. IMHO this is where
> >> the design needs to be inspected. One of those is the w83627ehf, which
> >> Jean and Hans are familiar enough with to check my work.
> >>
> >> Thoughts?
> >
> > I'm afraid that making this a "bus" will be a bit overkill. Jim's patches
> > are quite simple and seem todo the job.
> >
> > Also keep in mind that most users will be platform devices which just want
> > to use the superio registers to find out the baseaddress of their logical
> > device, a whole bus seems overkill for this, would the hwmon driver then
> > need to be a superio_driver (as well as an platform_driver) or can the bus
> > be queried / used
> > without having to be a bustype-driver?
> 
> I think that's a valid point. I am willing to keep it small, but I
> would prefer to follow the pattern set in other subsystems. It may be
> my lack of experience in designing a subsystem showing here! What are
> some alternative ways to implement it?
> 
> In other words, Jim's patches are a good start, but maybe I am
> misunderstanding them. I see it as the superio-locks module, a driver
> that other drivers would depend on / auto-load. Is that something
> other subsystems also do?

Well, there are two approaches to the problem. The first approach
(which I think Jim took in his patches? I don't really remember) is to
simply solve the problem of concurrent I/O access to the Super-I/O
configuration ports (typically 0x2e/0x2f or 0x4e/0x4f). That would be a
simple driver requesting the ports in question and exporting an API for
other drivers to access them in a safe way.

The second approach is to make it a whole subsystem, as David is
suggesting. The Super-I/O driver would then not only request the I/O
ports, it would also identify which Super-I/O is there, and it would
create devices (in the Linux device driver model sense of the term) for
every logical device we are interested in (amongst which the hwmon
ones.) The hwmon drivers would have to be converted from platform
drivers to superio drivers.

Each approach has its advantages. The first one is rather simple and
also very generic in nature. It could be reused for other purposes. The
second one offers automatic loading of hwmon drivers, which would be
nice to have.

There's probably a middle way which would keep the simplicity of the
first approach while still allowing for driver auto-loading, without
changing the bus type of all drivers. It would probably take some
research though.

Me, I don't really care which path we choose, given that I am not the
one who will take care of it. All I have to say is that this has been
discussed several times over the past 2 years and nothing came out of
it (as far as the mainline kernel is concerned, at least) so whatever
is done now, what really matters is that it makes it into the kernel
quickly. We have some drivers missing features because of this (for
example real-time reading of VID pin values.)

This should also not prevent us from fixing the drivers now so that
they temporarily request the Super-I/O ports they are using, as Milton
suggested. Not only we don't know when we will have something better to
offer, but it might also help us find conflicts between drivers, so
that we know which drivers should make use of the future superio
driver. This could also reveal conflicts with PNP BIOS reservations,
etc. Milton, will you write a patch?

-- 
Jean Delvare
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