pinctrl: group/pin identity mapping

[gregor.boirie@parrot.com (Grégor Boirie)]

Hi,

Many thanks for your answers.

One more point which may be of interest. We have a few IPs which have
the ability to multiplex their own outputs to chip pins/pads...
Considering this, I'd like to avoid misconfigurations between drivers
(for IPs outputs) and platform pinctrl level while keeping source
verbosity to a minimum.

It seems to me the simplest way to achieve this, is to enforce a direct
group/pin mapping at pinctrl level and perform complex multiplexing
inside the IP/driver (since it is needed any way).
As far as I understand, this is the most flexible approach for us and
allow to address every single setup future boards would require without
breaking existing platform/pinctrl implementation.

Thanks again for having shared your views. Regards,
Gr?gor

On 06/01/2012 05:05 PM, Stephen Warren wrote:
> On 06/01/2012 04:06 AM, Gr?gor Boirie wrote:
>> Hi All,
>>
>> I'm trying to implement pinctrl support for our new cortex based SoC.
>> Our driver should provide support for pinctrl/pinmux/pinconf combination
>> without device tree handling at the moment.
>> However, I'm afraid I misunderstand how groups relate to muxed pins and
>> I'd like to hear your suggestions.
>>
>> Here's the HW: our I/O pins subsystem allows us to multiplex every
>> controllable pins using up to 4 mutually exclusive functions, i.e. there
>> is no notion of pin GROUP multiplexing at the HW level (one register per
>> pin is available to setup multiplexing and other electronic properties
>> like drive strength, pull up/down...).
>>
>> However, it seems the pinctrl subsystem requires the driver to provide
>> pin groups for pinmux implementation.
>> So my question is: having no notion of HW pin groups whatsoever, should I:
>>
>> 1) implement a software/logical group that would arbitrarily gather
>> multiple HW pins in a platform dependent manner ?
>>
>> 2) perform an identity mapping between HW pin and software/logical group
>> with up to 4 groups/HW pins per pinmux function ?
>>
>> 3) anything else ? any way to bypass the pinctrl group logic to directly
>> assign pins rather than groups to pinmux functions ?
> 
> Here's my take.
> 
> pinctrl originally (early during design) only supported per-pin muxing.
> This didn't work for Tegra (since it really has register fields that
> affect muxing of multiple pins at once; groups), so I requested allowing
> per-group muxing as well. We ended up only allowing per-group muxing and
> dropped per-pin muxing:-( So, /my/ intention was to only use groups for
> true groups in HW.
> 
> However, many others have taken this group muxing capability and used it
> for a different purpose; to define SW-only groups that end up affecting
> HW registers/fields and hence multiple pins at once, e.g. a group for
> for each of the possible mux locations for e.g. HW module UART A , which
> contain s of the pins used for that mux option.
> 
> So, you have two choices:
> 
> a) (Like Tegra30): Define a group for each pin, that contains just that
> one pin. This will allow you to pick the mux option for each pin
> individually (since there's a group per pin) in the pinctrl mapping
> table. In my (personal) opinion (which is evidently rarely shared), this
> is the correct approach, since the pinctrl driver directly models the HW
> capabilities. Also, if/when pinctrl re-gains the capability to request
> mux options per-pin in addition to per-group, the conversion will likely
> be quite trivial if you want to do so.
> 
> b) For each HW module, for each set of pins it makes sense to use for
> that HW module (i.e. all the different sets of pins it can be mux'd to),
> create a group for those pins. Many pins will be in multiple different
> semi-arbitrarily overlapping groups. I personally call these "virtual
> groups" since they don't correspond 1:1 with HW registers/fields. Create
> a function for each HW module. Allow each HW module's function to be
> mux'd onto each of the groups you created for it. There are probably
> quite a few existing examples of this in drivers/pinctrl.
> 
> Hope this helps!


-- 
Gr?gor Boirie
Software engineer
R&D / OS platform
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