Re: [PATCH] Dell laptop backlight driver

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Matt Domsch wrote:
On Sun, Oct 28, 2007 at 07:06:23PM +0100, [email protected] wrote:
Matt Domsch wrote:
On Sun, Oct 28, 2007 at 05:12:37PM +0100, [email protected] wrote:
Hello,
this driver implements backlight control on Dell laptops
which use SMI  for changing brightness levels.

The driver is INCOMPLETE since it is unable to probe some required parameters
in order to perform backlight control. Such parameters are found in a Dell
proprietary DMI table which should be parsed. For now external tools may be
used to find these parameters by hand. So if you intend to try this out,
FIRST write your laptop model parameters correctly inside the source code
as explained in Documentation/dell-laptop.txt.

Parts of this driver may also be used to provide additional functionalities
similarly to the drivers/misc/*-laptop.c drivers.
Why is this better done in the kernel rather than in userspace with
libsmbios as you've noted?

I had to do a kernelspace driver for controlling the backlight. This
was part of a college project assignment. In order for it to be valid
for an operating system course, it had to be in kernelspace (not
Unix programming) :).

As i mentioned that can be done in userspace and it IS sensible to do
so. I know that the code which was already written for Dell implied
a userspace approach, but i simply had no choice.

I also don't expect this driver to become mainstream, but since i have
written it, other people might want to have a look at it.

OK, that's fair.

Finally i really don't think there's any sensible way of implementing
Linux LCD Backlight Abstraction relying on a userspace application.
That would mean the kernel calling userspace code to change the
brightness, then this latter code would again call the kernel to trigger
a SMI and so on. That's just not a good design. I think a userspace
solution implies choosing NOT to implement the LCD Abstraction. Causing
Dell laptops to be treated differently from other machines (as they are
not compliant with Linux's own interface).

I've had this discussion on lkml before. Short story is, until Dell
laptops implement the ACPI methods that the backlight API expects, the
only sane way to do it is to let libsmbios handle it.  Moving the
fairly large and complex libsmbios into the kernel isn't a good idea
(imho).




I agree. The problem is that Dell's DMI tables are not defined in the SMBIOS standard. Thus getting the parameters required for the backlight
control (io port, io code, location) needs to be dealt with specifically.
Plus DMI is not as comprehensively supported as ACPI is under Linux.

I think reading Dell's tables to find out backlight parameters in
kernelspace is possible. Unfortunately it would require a significant
amount of brand new code to be written. And i'm not sure it is worth
the effort. Libsmbios is big and complex because it is written in
proficient C++ with lots of OO abstraction. It is also portable. But
the task accomplished is simple in principle.

jacopo

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