netdev - Re: [RFC PATCH 2/3] dt-bindings: can: ctucanfd: add properties for HW timestamping

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Date:   Mon, 16 May 2022 18:34:45 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Rob Herring <robh@...nel.org>
Cc:     Matej Vasilevski <matej.vasilevski@...nam.cz>,
        linux-can@...r.kernel.org, pisa@....felk.cvut.cz,
        devicetree@...r.kernel.org, netdev@...r.kernel.org,
        ondrej.ille@...il.com, martin.jerabek01@...il.com
Subject: Re: [RFC PATCH 2/3] dt-bindings: can: ctucanfd: add properties for
 HW timestamping

On 16.05.2022 11:02:50, Rob Herring wrote:
> On Fri, May 13, 2022 at 01:27:06AM +0200, Matej Vasilevski wrote:
> > Extend dt-bindings for CTU CAN-FD IP core with necessary properties
> > to enable HW timestamping for platform devices. Since the timestamping
> > counter is provided by the system integrator usign those IP cores in
> > their FPGA design, we need to have the properties specified in device tree.
> > 
> > Signed-off-by: Matej Vasilevski <matej.vasilevski@...nam.cz>
> > ---
> >  .../bindings/net/can/ctu,ctucanfd.yaml        | 34 +++++++++++++++++--
> >  1 file changed, 31 insertions(+), 3 deletions(-)
> 
> What's the base for this patch? Doesn't apply for me.
> 
> > 
> > diff --git a/Documentation/devicetree/bindings/net/can/ctu,ctucanfd.yaml b/Documentation/devicetree/bindings/net/can/ctu,ctucanfd.yaml
> > index fb34d971dcb3..c3693dadbcd8 100644
> > --- a/Documentation/devicetree/bindings/net/can/ctu,ctucanfd.yaml
> > +++ b/Documentation/devicetree/bindings/net/can/ctu,ctucanfd.yaml
> > @@ -41,9 +41,35 @@ properties:
> >  
> >    clocks:
> >      description: |
> > -      phandle of reference clock (100 MHz is appropriate
> > -      for FPGA implementation on Zynq-7000 system).
> > +      Phandle of reference clock (100 MHz is appropriate for FPGA
> > +      implementation on Zynq-7000 system). If you wish to use timestamps
> > +      from the core, add a second phandle with the clock used for timestamping
> > +      (can be the same as the first clock).
> > +    maxItems: 2
> 
> With more than 1, you have to define what each entry is. IOW, use 
> 'items'.
> 
> > +
> > +  clock-names:
> > +    description: |
> > +      Specify clock names for the "clocks" property. The first clock name
> > +      doesn't matter, the second has to be "ts_clk". Timestamping frequency
> > +      is then obtained from the "ts_clk" clock. This takes precedence over
> > +      the ts-frequency property.
> > +      You can omit this property if you don't need timestamps.
> > +    maxItems: 2
> 
> You must define what the names are as a schema.
> 
> > +
> > +  ts-used-bits:
> > +    description: width of the timestamping counter
> > +    maxItems: 1
> > +    items:
> 
> Not an array, so you don't need maxItems nor items.
> 
> > +      minimum: 8
> > +      maximum: 64
> > +
> > +  ts-frequency:
> 
> Use a standard unit suffix.
> 
> > +    description: |
> > +      Frequency of the timestamping counter. Set this if you want to get
> > +      timestamps, but you didn't set the timestamping clock in clocks property.
> >      maxItems: 1
> > +    items:
> 
> Not an array.
> 
> 
> Is timestamping a common feature for CAN or is this specific to this 
> controller? In the latter case, you need vendor prefixes on these 
> properties. In the former case, you need to define them in a common 
> schema.

This property describes the usable with of the free running timer and
the timestamps generated by it. This is similar to the free running
timer and time stamps as found on PTP capable Ethernet NICs. But the
ctucanfd comes in a hardware description language that can be
parametrized and synthesized into your own FPGA.

To answer your question, timestamping is common in newer CAN cores, but
the width of the timestamping register is usually fixed and thus hard
coded in the driver.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
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