linux-kernel - RE: [PATCH 1/2] Documentation: can: flexcan: add PE clock source property to device tree

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Date:   Fri, 21 Dec 2018 03:34:16 +0000
From:   Aisheng Dong <aisheng.dong@....com>
To:     Rob Herring <robh@...nel.org>,
        Joakim Zhang <qiangqing.zhang@....com>,
        Marc Kleine-Budde <mkl@...gutronix.de>
CC:     "mkl@...gutronix.de" <mkl@...gutronix.de>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH 1/2] Documentation: can: flexcan: add PE clock source
 property to device tree

> -----Original Message-----
> From: Rob Herring [mailto:robh@...nel.org]
> Sent: Friday, December 21, 2018 4:23 AM
> 
> On Thu, Dec 13, 2018 at 07:07:57AM +0000, Joakim Zhang wrote:
> > From: Dong Aisheng <aisheng.dong@....com>
> >
> > The FlexCAN controller can parse clock source property from DTS file
> > to select PE clock source.
> >
> > Signed-off-by: Dong Aisheng <aisheng.dong@....com>
> > Signed-off-by: Joakim Zhang <qiangqing.zhang@....com>
> > ---
> >  Documentation/devicetree/bindings/net/can/fsl-flexcan.txt | 8
> > ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/net/can/fsl-flexcan.txt
> > b/Documentation/devicetree/bindings/net/can/fsl-flexcan.txt
> > index bc77477c6878..a04168605998 100644
> > --- a/Documentation/devicetree/bindings/net/can/fsl-flexcan.txt
> > +++ b/Documentation/devicetree/bindings/net/can/fsl-flexcan.txt
> > @@ -32,6 +32,13 @@ Optional properties:
> >  		 ack_gpr is the gpr register offset of CAN stop acknowledge.
> >  		 ack_bit is the bit offset of CAN stop acknowledge.
> >
> > +- fsl,clk-source: Select the clock source to the CAN Protocol Engine (PE).
> > +		  It's SoC Implementation dependent. Refer to RM for detailed
> 
> If SoC dependent, then it should be implied by the SoC specific compatible.
> Also, seems like you should add clock binding support here if you need more
> clock control.

The clock source selection is done by a register bit inside the IP block:
BIT13 CLKSRC CAN Engine Clock Source
0b - The CAN engine clock source is the oscillator clock. 
1b - The CAN engine clock source is the peripheral clock.

Currently it's written 1 by default during driver initialization.
drivers/net/can/flexcan.c
/* select "bus clock", chip must be disabled */
reg = priv->read(&regs->ctrl);
reg |= FLEXCAN_CTRL_CLK_SRC;
priv->write(reg, &regs->ctrl);

I'm not sure if it's a typical case to abstract CLKSRC bit into a common clock mux.
(Is there any similar case in kernel?)
But I think we can also use SoC specific compatible to write 0 for those special
Ones (currently only imx8qxp). Then this patch may not be needed.

Marc,
Please let us know if you have a different idea.

Regards
Dong Aisheng

> 
> > +		  definition. If this property is not set in device tree node
> > +		  then driver selects clock source 1 by default.
> > +		  0: clock source 0 (oscillator clock)
> > +		  1: clock source 1 (peripheral clock)
> > +
> >  Example:
> >
> >  	can@...00 {
> > @@ -40,4 +47,5 @@ Example:
> >  		interrupts = <48 0x2>;
> >  		interrupt-parent = <&mpic>;
> >  		clock-frequency = <200000000>; // filled in by bootloader
> > +		fsl,clk-source = <0>; // select clock source 0 for PE
> >  	};
> > --
> > 2.17.1
> >