Re: [PATCH 2/2] dt-bindings: timer: imx: gpt: Add pin group bindings for input capture

[Steve Longerbeam]

On 10/29/19 12:58 PM, Rob Herring wrote:
> On Mon, Oct 28, 2019 at 6:59 PM Steve Longerbeam <slongerbeam@xxxxxxxxx> wrote:
> > Hi Rob,
> >
> > Thanks for reviewing.
> >
> > On 10/27/19 2:21 PM, Rob Herring wrote:
> > > On Tue, Oct 15, 2019 at 06:05:44PM -0700, Steve Longerbeam wrote:
> > > > Add pin group bindings to support input capture function of the i.MX
> > > > GPT.
> > > >
> > > > Signed-off-by: Steve Longerbeam <slongerbeam@xxxxxxxxx>
> > > > ---
> > > >    .../devicetree/bindings/timer/fsl,imxgpt.txt  | 28 +++++++++++++++++++
> > > >    1 file changed, 28 insertions(+)
> > > >
> > > > diff --git a/Documentation/devicetree/bindings/timer/fsl,imxgpt.txt b/Documentation/devicetree/bindings/timer/fsl,imxgpt.txt
> > > > index 5d8fd5b52598..32797b7b0d02 100644
> > > > --- a/Documentation/devicetree/bindings/timer/fsl,imxgpt.txt
> > > > +++ b/Documentation/devicetree/bindings/timer/fsl,imxgpt.txt
> > > > @@ -33,6 +33,13 @@ Required properties:
> > > >               an entry for each entry in clock-names.
> > > >    - clock-names : must include "ipg" entry first, then "per" entry.
> > > >
> > > > +Optional properties:
> > > > +
> > > > +- pinctrl-0: For the i.MX GPT to support the Input Capture function,
> > > > +         the input capture channel pin groups must be listed here.
> > > > +- pinctrl-names: must be "default".
> > > > +
> > > > +
> > > >    Example:
> > > >
> > > >    gpt1: timer@10003000 {
> > > > @@ -43,3 +50,24 @@ gpt1: timer@10003000 {
> > > >                <&clks IMX27_CLK_PER1_GATE>;
> > > >       clock-names = "ipg", "per";
> > > >    };
> > > > +
> > > > +
> > > > +Example with input capture channel 0 support:
> > > > +
> > > > +pinctrl_gpt_input_capture0: gptinputcapture0grp {
> > > > +    fsl,pins = <
> > > > +            MX6QDL_PAD_SD1_DAT0__GPT_CAPTURE1 0x1b0b0
> > > > +    >;
> > > > +};
> > > > +
> > > > +gpt: gpt@2098000 {
> > > timer@...
> > Ok.
> >
> > > I don't really think this merits another example though.
> > Ok.
> >
> > But for version 2 of this patch-set I'd like to run some ideas by you.
> >
> > Because in this version I did not make any attempt to create a generic
> > timer capture framework. I just exported a couple imx-specific functions
> > to request and free a timer input capture channel in the imx-gpt driver.
> >
> > So for version 2 I am thinking about a simple framework that other SoC
> > timers with timer input capture support can make use of.
> >
> > To begin with I don't see that timer input capture warrants the
> > definition of a new device. At least for imx, timer input capture is
> > just one function of the imx GPT, where the other is Output Compare
> > which is used for the system timer. I think that is likely the case for
> > most all SoC timers, that is, input capture and output compare are
> > tightly interwoven functions of general purpose timers.
> >
> > So I'm thinking there needs to be an additional #input-capture-cells
> > property that defines how many input capture channels the timer
> > contains, where a channel refers to a single input signal edge that can
> > capture the timer counter. The imx GPT has two input capture channels (2
> > separate input signals).
> #foo-cells is not how many of something, but how many u32 parameters a
> 'foos' consumer property has. But seems like that's what you meant
> based on the example.

Sorry yes that's what I meant, my wording was imprecise. If a timer has 
only one input capture channel, no arguments are needed to specify the 
channel in the timer-input-capture property and #input-capture-cells 
would be <0>.


> > For example, on imx:
> >
> > gpt: timer@2098000 {
> >          compatible = "fsl,imx6q-gpt", "fsl,imx31-gpt";
> >          /* ... */
> >          #input-capture-cells = <1>;
> >          pinctrl-names = "default", "icap1";
> >          pinctrl-0 = <&pinctrl_gpt_input_capture0>;
> >          pinctrl-1 = <&pinctrl_gpt_input_capture1>;
> > };
> >
> >
> > A device that is a listener/consumer of an timer capture event would then refer to a timer capture channel:
> >
> > some-device {
> >          /* ... */
> >          timer-input-capture = <&gpt 0>;
> > };
> We'd want to be more consistent in the naming, but seems reasonable.

Yeah, maybe rename the properties to #timer-capture-cells and timer-capture.


> One of the challenges with timers is selecting which timer is used for
> what function. This helps as you can know if a timer is used for input
> capture or not. One issue will be is having '#input-capture-cells'
> enough to decide that,


Yes, it does bother me somewhat that

timer-capture = <&gpt 0>;

is referring to the timer itself and not its input-capture functionality.

Maybe it would be better, since the timer has multiple functions, to 
make the timer compatible with simple-mfd, so that a timer-capture 
sub-device can be defined, for example on i.MX6:

gpt: timer@2098000 {
        compatible = "fsl,imx6q-gpt", "fsl,imx31-gpt", "simple-mfd";
        /* ... */

	tcap: timer-capture {
		compatible = "fsl,imx6q-gpt-capture";
		#timer-capture-cells = <1>;
		pinctrl-names = "default", "icap1";
        	pinctrl-0 = <&pinctrl_gpt_input_capture0>;
        	pinctrl-1 = <&pinctrl_gpt_input_capture1>;
	};
};

some-device {
        /* ... */
        timer-capture = <&tcap 0>;
};


>   or does one have to walk the DT and find all
> the 'timer-input-capture' properties (shouldn't be a lot)?
>   You could
> also want to use input capture, but not describe the connection in DT.

That's a thought, but I'm not sure how the kernel API would look in that 
case, i.e. it would not be as straightforward to locate the timer 
clocksource driver that contains the timer capture support. The 
advantage of using a 'timer-capture' property that contains a timer 
phandle, is that it is simple to locate the clocksource driver that has 
the timer capture function.

> Another thought is should it be just 'timers' to cover both input
> capture and output compare with those being selected with flags (like
> GPIO).
>
> My other question is just what are some real examples of devices
> needing to describe this connection. Timers have had input capture
> forever, but I've rarely seen it used. Output compare even less so.

In this specific use-case, the i.MX6 CSI often cannot recover from 
corrupted frame synchronization info in the incoming video frames, 
especially for BT.656 sources (too many or too few lines between two 
SAV/EAV codes, or missing codes altogether). The result is loss of 
vertical sync in the captured frames. The only indication of this error 
condition on i.MX6 is a drop in the captured frame intervals. So a 
workaround is to implement a frame interval monitor that measures the 
FI's and reports a V4L2 event to userspace when a FI falls outside some 
tolerance value. Userspace can then take corrective action such as 
restarting video streaming. Finally getting to the use-case here, the 
most accurate way to measure FI's is to capture a timer counter between 
two falling edges of a VSYNC signal from the video source.

Steve