Location for a kind of GPIO bus driver

[Simon Guinot]

On Thu, Jul 25, 2013 at 12:03:24PM -0400, Jason Cooper wrote:
> On Thu, Jul 25, 2013 at 05:49:32PM +0200, Simon Guinot wrote:
> > On Wed, Jul 24, 2013 at 02:19:24PM -0400, Jason Cooper wrote:
> > > On Tue, Jul 23, 2013 at 05:18:34PM +0200, Simon Guinot wrote:
> > > > Hi,
> > > > 
> > > > I am currently working on converting the LED driver leds-netxbig to DT.
> > > > Doing so, I am also considering to move the GPIO extension bus functions
> > > > (which are currently parts of leds-netxbig) into a separate driver.
> > > > 
> > > > On the LaCie "Big Network" board family (netxbig), the LEDs are
> > > > controlled by a CPLD. In turn, the CPLD can be configured (mostly the
> > > > LEDs modes) through a kind of parallel GPIO bus, called GPIO extension
> > > > bus. Two registers (address and data) are exposed. Each of them is made
> > > > up of several GPIOs. An extra GPIO is used to notify the CPLD that the
> > > > registers have been updated. The leds-netxbig driver uses some dedicated
> > > > functions (prefixed by "gpio_ext_") to handle the GPIO bus extension.
> > > > 
> > > > On the latest "Big Network" boards, this bus is also used to enable the
> > > > PM wakeup sources: the CPLD can be asked to keep powered some devices
> > > > (as the RTC or the Ethernet PHY) while the board is down. I think it
> > > > could be nice to expose this feature to the userland. That's why I am
> > > > thinking about moving the GPIO extension bus support into a separate
> > > > driver. The problem is that I can't find a proper location for a such
> > > > driver. AFAIK, it doesn't fit with anything existing supported by Linux.
> > > > Maybe I should consider drivers/bus ? Or even drivers/misc ?
> > 
> > Hi Jason,
> > 
> > Thanks for your answer.
> > 
> > > 
> > > iiuc, the CPLD is more of gpio multiplexer/expander.  Have you seen
> > > 
> > >   66bcb58 arm: mvebu: enable gpio expander over i2c on Mirabox platform
> > > 
> > > ?  And the corresponding driver, drivers/gpio/gpio-pca953x.c?
> > 
> > All the drivers under drivers/gpio are registering some GPIO chips.
> > It is a big difference with the LaCie "GPIO extension bus". This last
> > is more an extension mechanism on the top of GPIOs already existing
> > (eventually provided by a SoC or a Super-I/O on x86 boards). No new
> > GPIOs are added. I think that the name "extension" is quite confusing.
> 
> Agreed.
> 
> > The idea behind this "bus" is to allow to configure a lot of LEDs (in
> > various modes) using only few GPIOs and a CPLD (which is already
> > available on the board for other purposes). This cheap solution allows
> > precisely to save the cost of a GPIO expander :)
> > 
> > As an example, you can see the "GPIO extension bus" configuration for
> > the Kirkwood "Big Network" boards in:
> > arch/arm/mach-kirkwood/netxbig_v2-setup.c:132.
> > 
> > > 
> > > I would try drivers/gpio.  If there are complaints, it should be trivial
> > > to move it in a new version of the series.
> > 
> > Why not, as long it is clear that the resulting driver will not provide
> > a GPIO chip but rather functions as a library.
> 
> right, but the end result of what the library would do is expose a
> series of LEDs and PM 'virtual gpios'.  So, even though it's not a gpio
> expander per-se, it's still filling that role.  Or am I still missing
> something?

The library could expose a function allowing to set the address and data
registers: gpio_ext_set_value(int addr, int value).

Actually, it is how it works inside leds-netxbig. Simply this function
would be also accessible by other drivers.

> 
> > > Is there any reason why the new capabilities in the "Big Network" boards
> > > couldn't be covered by gpio-regulator?
> > 
> > I don't have looked at the details but I think that gpio-regulator
> > could be able to configure the PM modes through the GPIO extension bus.
> > 
> > But a problem remains. The same GPIOs would be used by two different
> > drivers: leds-netxbig and gpio-regulator. This would lead to some
> > conflicts. That's why I think that at some level a driver is needed to
> > handle the GPIO extension bus.
> 
> It may be simpler to combine it into one driver and place it under
> drivers/mfd as it's certainly a multi-function device.  I don't think
> anyone outside of Lacie is going to be adopting this 'bus'...

I will definitively have a closer look at mfd. But as I understand, mfd
is more oriented platform device and resources. Here, the device don't
expose some memory regions but rather methods to configure a CPLD
through GPIOs. I am not sure how it can work with the mfd framework.

Thanks,

Simon

> 
> thx,
> 
> Jason.
> 
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