[PATCH] pinctrl: document the "GPIO mode" pitfall

[Laurent Pinchart]

Hi Linus,

On Friday 26 April 2013 10:49:12 Linus Walleij wrote:
> On Fri, Apr 26, 2013 at 1:15 AM, Laurent Pinchart wrote:
> > On Thursday 25 April 2013 23:39:18 Linus Walleij wrote:
> >> On Tue, Apr 23, 2013 at 3:33 PM, Laurent Pinchart wrote:
> >> > Could you clarify the exact scope of the two configuration parameters ?
> >> 
> >> PIN_CONFIG_OUTPUT is left a bit unspecified, but here the idea was a
> >> passive drive, like just connecting the pin to VDD or GND without any
> >> driver stage at all.
> > 
> > Isn't that a driver stage ? :-)
> 
> OK something that is not a totempole type drive ...
> push/pull surely implies a totempole type design.
> 
> > What is unclear to me is the interaction between OUTPUT and DRIVE_*.
> > That's the part I would like to see clarified.
> 
> I sent some patch now, check it ... hm reported-by still doesn't add
> you to CC :-/ better patch git-send-email...
> 
> > Does DRIVE_* imply that the pin is driven by the selected function, and
> > OUTPUT imply that the pin is driven to a fixed level ?
> 
> That is unclear, but I suggest DRIVE* implies that everything on the pin
> is driven according to that configuration. (Else it is getting ignored...)
> 
> OUTPUT would be used when you don't know the particulars or when
> the driving cannot be controlled in a fine-grained manner like with the
> DRIVE* configs.
> 
> Does this make sense?

Kinda, but it's still unclear to me. While the options are (briefly) 
documented, how they interact isn't.
 
> > If so, how do you configure the drive type of a pin that will be used
> > through the GPIO API ?
> 
> It is possible to use the pinctrl and GPIO APIs orthogonally.
> 
> For example the pinctrl can use hogs to reconfigure the pins during sleep
> without intervention from the GPIO API.
> 
> So they will be fingering on the same pins registers. Maybe even the same
> register (if access can be protected properly) from different APIs.
>
> > What about cases where I want to drive the pin to a fixed level in a non
> > low-power output mode (for instance because I need more current that what
> > the low-power output mode provides) ?
> 
> Just use pinconfig for that?

How would you do so ? Only OUTPUT allows setting the output level explictly, 
the other DRIVE_* options don't specify the output level.

> Is the usecase something like a power-supplying GPIO pin and then sometimes
> you want to provide more power from it?
> 
> Then use pinconfig to shunt in the driver stages, and GPIO API to
> enable/disable it.

-- 
Regards,

Laurent Pinchart