Re: [PATCH 10/53] Input: atmel_mxt_ts - Add memory access interfacevia sysfs

[Mark Brown]

On Tue, Jun 11, 2013 at 01:16:31PM +0100, Nick Dyer wrote:

> Without being able to name all of the registers (which would require a
> large amount of architecture to keep up-to-date and would probably turn
> into an unmaintainable mess), you can only split up the register map into
> separate parts. You'd end up with binary attributes like this (refer to the
> document I linked for explanation):

...

> Is that a nicer design from your point of view? I don't personally think
> that is really gains anything functional other than making the API more
> complex and increasing the number of read/write operations. I guess you

Yes, to be honest.  I'd hope it wouldn't be increasing the number of
read/write operations...

> would stop bugs in user space code from accidentally writing into the wrong
> object.

That seems like a useful thing, and it does allow the driver to
relatively easily understand what any of the attributes is doing and
take it over if it needs to.

> > Well, there's also the potential issues with standard application layer
> > code either not being able to do things that the hardware supports or
> > getting into a fight with the magic custom stuff.

> I think it's better to present a clean API cut at the right level. If you
> look at the drivers in various Android trees for these maXTouch chips,
> there's an awful lot of phone specific code which is not very general and
> it would be impossible to mainline without having a 20,000 line driver full
> of #ifdefs. Surely it's better for that to go into a userspace daemon if
> possible?

Yes, having some of the stuff that understands the contents of the
controls in userspace is sensible.  However the kernel does offer an API
for controlling devices it supports and it seems reasonable to expect
that to work too - callibration and whatnot is a bit different but core
functionality should just work from the kernel.

> >> On suspend the driver puts chip into "deep sleep" where touch acquisition
> >> is halted and minimal power consumed. But it will still come out of its
> >> sleep state temporarily to service I2C comms if necessary (although one
> >> particular family requires that I2C retry for this case).

> > So these errors are just part of waking the chip up - in that case
> > shouldn't the driver be waking the chip up automatically?

> In the reference design for that particular model of chip (mXT1386), there
> is a WAKEUP pin cross-wired to an I2C line. The only way of getting it to
> wake up when it is asleep is by trying to perform an I2C operation, which
> will fail, and then retrying before it times out and goes back to sleep
> again. There isn't any other way of waking it up.

That still sounds like something the driver can handle (for example, by
eating the first error silently if it knows the chip is powered down)
and of course a system integrator may choose not to copy the reference
design in this respect, it does seem a bit odd after all.

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