Re: Silead DMI driver

[Darren Hart <dvhart@infradead.org>]

On Wed, May 03, 2017 at 10:19:53AM +0200, Jean Delvare wrote:
> Hi Darren,
> 
> On Thu, 27 Apr 2017 14:13:22 -0700, Darren Hart wrote:
> > On Tue, Apr 25, 2017 at 12:19:39PM +0200, Jean Delvare wrote:
> > > Thanks for the pointer Andy. OK, I can understand the argument of
> > > Dmitry that platform-specific quirks do not belong to the device
> > > drivers, even though in practice I'm not sure the cost of having a
> > > separate platform driver for the purpose is always worth it - depends
> > > on how "popular" the device is, I suppose.
> > > 
> > > But still, this can't justify non-modular platform code that will run
> > > on every X86 system out there. Any piece of platform-specific quirks,
> > > we should be able to build as a module, otherwise it simply doesn't
> > > scale. PCI quirks and such are enough pain already without inventing
> > > more flavors of bloat :-(
> > 
> > Jean makes a good point. I would insist on this if the Kconfig default was y or
> > if distros were likely to enable this by default. However, this is for the still
> > very rare x86 tablet and is likely to only be enabled for those systems which
> > require it.
> 
> The problem is that there is no clear line of what hardware openSUSE
> kernels should support or not. I don't know if other general purpose
> Linux distributions have made any statement on the matter, but we have
> not. As a matter of fact, the person in charge of deciding what to do
> with new kernel drivers did enable both Silead touchscreen options in
> our standard i386 and x86_64 kernels.

I'd assert that wasn't the right call. Happy to discuss with them if they are
interested.

> 
> There are also people working on getting openSUSE to run on odd
> hardware such as the GPD Win, and we have ARM kernels running on
> embedded platforms. As X86-based embedded platforms are getting
> popular, what sense would it make to support everything from X86
> servers to embedded ARM but rule out X86 tablets? Especially when
> hybrid laptop/tablet designs are becoming more popular as well. The
> same touchscreen device used for pure tablets may be used for hybrid
> models tomorrow.

I suppose it depends what you mean by support. If you mean have one kernel that
runs on everything from embedded dev boards to HPC, I think you'll find that to
be an untenable goal. In my time maintaining the Yocto Project BSPs for Intel,
it is still the case that embedded systems prefer to run more tailored kernels.
We tried to reduce that change as much as possible on IA, by at least building
from the same source branch, but ultimately, people want to change configuration
options on embedded systems.

> 
> This is one of the reasons why modular drivers are a must. If we enable
> them when we shouldn't have, the cost stays low. Of course it would be
> better if we could just enable what we really need, but in practice
> this is terribly hard to achieve. This is also the reason why I am
> actively promoting the mentioning of intended target hardware on new
> drivers, to limit the risks of us enabling drivers on kernels that
> don't need it.
> 

This works great for Desktops and Datacenters, but it doesn't hold up very long
in discussions with the more embedded and special purpose device space. 

> > (...)
> > While the default for Kconfig is n if not explicitly set to m or y, perhaps
> > "default n" should be added to TOUCHSCREEN_SILEAD and SILEAD_DMI to make it more
> > explicit. I don't know if there is an official preference on "default n"
> > statements or not.
> 
> I wondered before as well. But given that n is the default default, and
> an explicit default is not listed by "make config" nor "make
> menuconfig", I can't see the value of "default n". It makes no
> difference for the user. In fact my never-ending project list has a
> point "Kconfig: kill all default n and add a test to checkpatch so they
> don't come back". Chances that I get to that one before retirement are
> low though.

:-) Agreed on all points

> 
> I am not convinced that a default value for a device driver makes any
> sense in the first place. If you don't need support for the device, you
> don't enable the driver, period. I can't see how a default answer can
> be relevant. In this specific case, the default answer should be n if
> not building a kernel for tablets. But would become y if building a
> kernel for tablets. What reason would there be to favor one scenario
> over the other?

The liklihood of use is the most compelling argument. e1000 is very likely to be
used relative to silead-touchscreen, for example. If it's highly specialized, it
should typically be default n (or not default y|m as discussed above). Of course
people like you ultimately are on the hook to make that call.

> 
> I believe default values are only useful for driver feature options,
> general kernel configuration options, or automatic selection of hidden
> options.

I can see that view, especiacally coming from your position.

-- 
Darren Hart
VMware Open Source Technology Center