[RFC PATCH 1/4] dt-binding: i2c: add generic properties for GPIO bus recovery

[Russell King - ARM Linux admin]

On Thu, Jul 30, 2020 at 09:00:36AM +0000, Codrin.Ciubotariu at microchip.com wrote:
> On 27.07.2020 13:50, Russell King - ARM Linux admin wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > 
> > On Mon, Jul 27, 2020 at 10:44:57AM +0000, Codrin.Ciubotariu at microchip.com wrote:
> >> On 24.07.2020 23:52, Russell King - ARM Linux admin wrote:
> >>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> >>>
> >>> On Fri, Jul 24, 2020 at 09:39:13PM +0200, Wolfram Sang wrote:
> >>>> On Sun, Jul 05, 2020 at 11:19:18PM +0200, Wolfram Sang wrote:
> >>>>>
> >>>>>> +- pinctrl
> >>>>>> + add extra pinctrl to configure SCL/SDA pins to GPIO function for bus
> >>>>>> + recovery, call it "gpio" or "recovery" state
> >>>>>
> >>>>> I think we should stick with "gpio" only. That is what at91 and imx have
> >>>>> in their bindings. pxa uses "recovery" as a pinctrl state name but I
> >>>>> can't find any further use or documentation of that. PXA is not fully
> >>>>> converted to the best of my knowledge, so maybe it is no problem for PXA
> >>>>> to switch to "gpio", too? We should ask Russell King (cced).
> >>>
> >>> Fully converted to what?  The generic handling where the i2c core layer
> >>> handles everything to do with recovery, including the switch between
> >>> modes?
> >>>
> >>> i2c-pxa _intentionally_ carefully handles the switch between i2c mode and
> >>> GPIO mode, and I don't see a generic driver doing that to avoid causing
> >>> any additional glitches on the bus.  Given the use case that this recovery
> >>> is targetted at, avoiding glitches is very important to keep.
> >>
> >> Why is it not possbile to handle glitches in a generic way? I guess it
> >> depends on the pinctl, but we could treat a worst-case scenario to
> >> assure the switch between states is done properly.
> > 
> > Please look at how i2c-pxa switches between the two, and decide whether
> > the generic implementation can do the same.
> 
> The handling of glitches from initialization looks generic to me. I see 
> that there are specific clear/reset routines that are in the 
> (un)prepare_recovery() callbacks, but these callbacks are not replaced 
> by the generic i2c recovery and will still be used if given by the 
> driver. The only thing the generic recovery does is to switch the pinmux 
> state. We can discuss whether we want to change the pinmux state first 
> or call the (un)preapre_recovery().

Right, the key point i2c-pxa does is that on prepare:
- read the current state of the SCL and SDA lines and set the GPIO to
  reflect those values.
- then switch the pinmux state.

That must be preserved, otherwise if SCL is being held low by the I2C
master, and we switch to GPIO mode, SCL will be released.  So the
driver needs to be involved before the pinmux state is changed.

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