Re: [RFC PATCH] gpio: support for GPIO forwarding

[Heikki Krogerus]

On Thu, Jan 15, 2015 at 10:28:03AM +0100, Rafael J. Wysocki wrote:
> On Wednesday, January 14, 2015 08:32:12 AM Darren Hart wrote:
> > 
> > On 1/14/15 4:58 AM, Linus Walleij wrote:
> > > On Thu, Dec 18, 2014 at 9:23 AM, Heikki Krogerus
> > > <heikki.krogerus@xxxxxxxxxxxxxxx> wrote:
> > > 
> > >> This makes it possible to assign GPIOs at runtime. The
> > >> motivation for it is because of need to forward GPIOs from
> > >> one device to an other. That feature may be useful for
> > >> example with some mfd devices, but initially it is needed
> > 
> > +Samuel Ortiz for his thoughts on applicability to MFD.
> > 
> > >> because on some Intel Braswell based ACPI platforms the GPIO
> > >> resources controlling signals of the USB PHY are given to
> > >> the controller device object for whatever reason, so the
> > >> driver of that controller needs be able to pass them to the
> > >> PHY device somehow.
> > >>
> > >> So basically this is meant to allow patching of bad (bad
> > >> from Linux kernels point of view) hardware descriptions
> > >> provided by system fw in the drivers.
> > >>
> > >> Signed-off-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
> > >> ---
> > >>
> > >> Hi,
> > >>
> > >> I'm sending this first as a RFC in case you guys have some better
> > >> idea how to solve our problem. I actually already have couple
> > >> platforms where the GPIO resources are given to the "wrong" device
> > >> objects now.
> > >>
> > >> Originally we were thinking about simply handling our problem with
> > >> hacks to the PHY drivers, basically also checking if the parent has
> > >> GPIOs. That would only work if the controller is always the parent,
> > >> which it's not, but even if it was, it would be too risky. The PHY
> > >> drivers don't know which controller they are attached to, so what is
> > >> to say that the GPIOs aren't really attached to the controller.
> > >>
> > >> So the safest way to handle our problem is to deal with it in quirks
> > >> in the controller drivers. Solving it by bouncing the GPIOs did not
> > >> feel ideal there doesn't seem to be any other way. The API is copied
> > >> from clkdev (clk_register_clkdev). In the end it's really only one
> > >> function for adding the lookups and one for removing them.
> > >>
> > >> The way it's implemented is by modifying the current style of handling
> > >> the lookups a little bit. The per device lookup table is basically
> > >> reverted back to the single linked-list format since it seems that
> > >> these lookups may have to be assigned from several sources.
> > > 
> > > Oh ain't that great.
> > > 
> > > So now we start patching around the kernel because the ACPI
> > > tables are erroneous for GPIOs. I'd like some feedback from
> > > Rafael &| Darren on this patch, i.e. if you two think this is a good
> > > way of accounting for bad GPIO descriptions in ACPI tables then
> > > ACK this patch.
> > > 
> > > I guess the other option would be to fix up the ACPI DSDT
> > > itself to put resources right, correct? Is this not possible?
> > 
> > This is my gut reaction as well.
> 
> Pretty much agreed.
> 
> > Heikki, why can't we correct the firmware tables? That needs to be made clear
> > before we start adding hacks to the kernel.
> 
> We need to start working in that direction really.  Fixing problems in ACPI
> tables via kernel code is not going to scale sufficiently IMO.

Fixing the DSDT produced by the firmware was my first suggesting for
this, but it just does not seem to be happening. These products are
already out on the market (this is one of them [1]) and what I have
understood is that the firmware just is what it is. It's almost as if
there is nobody even developing it for these products anymore. Even
fixes would not go in and this is not even considered a fix. Things
work just find for them with their hacked kernel.

So the firmware and the ACPI tables it provides are not going to be
fixed. What else can we do? Can we expect the users to always use
custom DSDT, or maybe somekind of custom AML snipped (is something
like that even possible), when using these products?

> > You said, "Bad for Linux", why is this not a problem for other operating
> > systems?
> 
> Good question. :-)

I would imagine certain operating systems consider a component like
PHY as something that should always be handled by the controller
driver. It does not seem to be a problem for them even if every second
product using the same SoC happens to have different PHY. All of them
will have a custom controller driver in any case. Even though the
example product below is an Android tablet, to me it looks like the
style of writing software comes straight from those "other" operation
systems.

At least it seems clear that these guys don't understand that it's not
possible to do things like write a new driver for every product using
the same SoC in Linux.

[1] http://www.trekstor.de/detail-surftabs-en/product/surftab-xintron-i-70.html

Cheers,

-- 
heikki
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