Date:   Mon, 30 Jan 2017 22:35:21 +0000
From:   Peter Griffin <peter.griffin@...aro.org>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com,
        linux-serial@...r.kernel.org, robh+dt@...nel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kernel@...inux.com
Subject: Re: [STLinux Kernel] [PATCH 3/8] serial: st-asc: Read in all Pinctrl
 states

Hi Lee,

On Mon, 30 Jan 2017, Lee Jones wrote:

> On Mon, 30 Jan 2017, Peter Griffin wrote:
> > On Mon, 30 Jan 2017, Lee Jones wrote:
> > > On Mon, 30 Jan 2017, Peter Griffin wrote:
> > > > On Fri, 27 Jan 2017, Lee Jones wrote:
> > > > > On Wed, 25 Jan 2017, Peter Griffin wrote:
> > > > > > On Tue, 24 Jan 2017, Lee Jones wrote:
> > > > > > 
> > > > > > > There are now 2 possible separate/different Pinctrl states which can
> > > > > > > be provided from platform data.  One which encompasses the lines
> > > > > > > required for HW flow-control (CTS/RTS) and another which does not
> > > > > > > specify these lines, such that they can be used via GPIO mechanisms
> > > > > > > for manually toggling (i.e. from a request by `stty`).
> > > > > > > 
> > > > > > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
> > > > > > > ---
> > > > > > >  drivers/tty/serial/st-asc.c | 28 ++++++++++++++++++++++++++++
> > > > > > >  1 file changed, 28 insertions(+)
> > > > > > > 
> > > > > > > diff --git a/drivers/tty/serial/st-asc.c b/drivers/tty/serial/st-asc.c
> > > > > > > index 397df50..03801ed 100644
> > > > > > > --- a/drivers/tty/serial/st-asc.c
> > > > > > > +++ b/drivers/tty/serial/st-asc.c
> 
> [...]
> 
> > > > > > > +		pinctrl_lookup_state(ascport->pinctrl, "manual-rts");
> > > > > > > +	if (IS_ERR(ascport->states[MANUAL_RTS]))
> > > > > > > +		ascport->states[MANUAL_RTS] = NULL;
> > > > > > > +
> > > > > > 
> > > > > > The different pinctrl states looks like a neat solution to the problem.
> > > > > > 
> > > > > > My only concern here is that 'default' state is implying a hw-flow-control
> > > > > > pinmux config, and manual-rts is implying what is the current upstream
> > > > > > 'default' pinmux config.
> > > > > > 
> > > > > > Which maybe ok if you update all uarts, but currently only serial0
> > > > > > is updated. So the other uarts current 'default' is actually the same as serial0
> > > > > > 'manual-rts' grouping, which conceptually is odd.
> > > > > > 
> > > > > > Would it not be better to make 'manual-rts' the default state? As that aligns
> > > > > > to what is currently already the default for the other UARTS? And then make
> > > > > > hw-flow-control the optional state for serial0?
> > > > > > 
> > > > > > That also has the advantage that 'default' has the same meaning with older DT's.
> > > > > 
> > > > > The reason it was done is this was because none of the other UARTs
> > > > > require 2 separate Pinctrl configurations, only this one.  Moreover,
> > > > > if they support RTS/CTS then I believe that the lines should be
> > > > > defined in Pinctrl.
> > > > 
> > > > Yes I agree with that.
> > > > 
> > > > > Thus, it was my plan to update all UART's default
> > > > > Pinctrl configs to include the RTS/CTS lines.
> > > > > 
> > > > 
> > > > I still don't see the point in changing the meaning of 'default' group and breaking
> > > > ABI if you don't need to?
> > > > 
> > > > As far as I can tell if you swap the meaning of 'default' and 'maunal-rts'
> > > > groups you get all the benefits of this series whilst also maintaining backwards
> > > > compatbility with older DT's.
> > > 
> > > What makes you think this will break ABI?
> > 
> > I've not tried it, but an older DT defines one group, 'default' which contains
> > the same pin config as your new optional 'manual-rts' group.
> > 
> > The driver now reads like the manual-rts pin config is optional and should be stored in
> > ascport->states[MANUAL_RTS]. An older DT will pass that same pin config as the default
> > group and it will be stored in ascport->states[DEFAULT].
> > 
> > That seems wrong to me, and if it executes OK it wouldn't be what you
> > expect by reading the code.
> 
> This makes no sense at a functional level.
> 
> Old kernel, old DTB:
> 
> ASC driver doesn't understand Pinctrl, but since only the "default"
> state is defined, that's what will be used as a matter of course.
> RTS/CTS aren't configured, but that doesn't matter because the DTS
> does not advertise that HW flow-control is available.  In this
> use-case neither HW flow-control, nor manual toggling of the RTS line
> is possible.
> 
> New kernel, old DTB:
> 
> ASC driver demands "default" and requests "manual-rts" Pinctrl states,
> but "manual-rts" isn't available so "default" will be the only
> utilised state.  Unlike the first example above, "default" now
> contains the RTS and CTS lines,

No it doesn't, default just contains 'tx' & 'rx' pins, as it has always
done until now.

Which is IMO where the condusion arises, as it is the same pin configuration
as what you are now calling 'manual-rts' which the driver just tried and failed
to obtain (although in reality it has actually obtained those pins but stored
them in DEFAULT instead.

I presume this is why it didn't make sense to you above.

>but since the DTS does not advertise
> HW flow-control as available they will be harmlessly unused.  This
> configuration culminates in the same result as the first example
> i.e. no HW flow-control and no manual toggling.  However, there are no
> detremental effects to the driver's functions. 
>

<snip>

>New kernel, new DTB:
> 
> ASC driver demands "default" and requests "manual-rts" Pinctrl
> states.  If DTS advertises that HW flow-control is possible and the
> client requests it, ASC will use the "default" state and HW
> flow-control will commence.  If HW flow-control is not requested by
> the client and "manual-rts" is available, then ASC will request the
> RTS line is handled by GPIO until such times as the client requests
> HW flow-control, at which point ASC will disable GPIO and request the
> "default" state again.

Unless it is uart 1 or 2, in which case 'default' still only contains
tx & rx pins, and you have the same situation as above. 

> 
> It is not possible to read C-code and make assumptions that the DTB
> will be in a particular state as you suggest.
> No disparity ever
> exists and the code is always clear IMHO.
> 

Really?

ascport->states[DEFAULT]: may contain "tx, rx" or "tx, rx, cts & rts"
ascport->states[MANUAL_RTS]: may contain "tx, rx", or it could be stored in DEFAULT

And as the series currently is you have a mixture of the two in the same kernel
depending on what instance of the UART you are.

regards,

Peter.

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