RE: [RFC] Configure i.MX6 RGMII pad group control registers from device tree

["A.s. Dong" <aisheng.dong@xxxxxxx>]

Copy Andy & Frank,

> -----Original Message-----
> From: Michal Vokáč [mailto:michal.vokac@xxxxxxxxx]
> Sent: Tuesday, June 12, 2018 11:10 PM
> To: linux-gpio@xxxxxxxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; Shawn Guo
> <shawnguo@xxxxxxxxxx>; Sascha Hauer <kernel@xxxxxxxxxxxxxx>; Fabio
> Estevam <fabio.estevam@xxxxxxx>; Rob Herring <robh+dt@xxxxxxxxxx>;
> devicetree@xxxxxxxxxxxxxxx; A.s. Dong <aisheng.dong@xxxxxxx>; Fabio
> Estevam <festevam@xxxxxxxxx>; Shawn Guo <shawnguo@xxxxxxxxxx>;
> Stefan Agner <stefan@xxxxxxxx>; Pengutronix Kernel Team
> <kernel@xxxxxxxxxxxxxx>; Linus Walleij <linus.walleij@xxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFC] Configure i.MX6 RGMII pad group control registers from
> device tree
> 
> On 11.6.2018 14:36, Michal Vokáč wrote:
> > Ahoj,
> >
> > To configure individual pad's characteristics on i.MX6 SoC a fsl,pins
> > = <PIN_FUNC_ID CONFIG> property can be used. Is there any convenient
> > way to configure the pad group control registers?
> >
> > The issue is that some bits (DDR_SEL and ODT) in the individual RGMII
> > pad control registers are read-only. To tweak those parameters (signal
> > voltage and termination resistors) one need to write to the pad group
> > control registers for the whole RGMII pad group. Namely
> > IOMUXC_SW_PAD_CTL_GRP_DDR_TYPE_RGMII and
> > IOMUXC_SW_PAD_CTL_GRP_RGMII_TERM. The group registers in general
> are not accessible from the list in arch/arm/boot/dts/imx6dl-pinfunc.h.
> >
> > I could not find any other way to change the group registers than
> > hacking-in some lines into the imx6q_init_machine(void) function in
> > arch/arm/mach-imx/mach-imx6q.c source. As I work towards upstreaming
> > my board this should be done from my device tree or solved in some
> universal way.
> >
> > Any hints will be much appreciated.
> > Michal
> 
> I figured out this is more "pinctrl-imx.c" than "device-tree" related so I am
> kindly adding maintainers of that file in hope somebody will shed some light
> to it.
> 
> I am diving deeper into the code and it seems there really is no generic
> option to set the i.MX6 pad group control registers from device tree.
> Or am I looking at the problem from a wrong angle?
> 

Yes, there's a few special pad group ctrl registers (e.g. DRAM and RGMII for mx6q)
which are not added In the pinctrl driver support.

> How should we deal with boards that need to configure some pad
> characteristics available only through the pad group control registers?
> 

Andy,
How do we handle it internally?

> I also raised this question at the NXP community forum [1] and get quite
> unsatisfying answer so far. I would love to find/implement a proper and
> universal solution.
> 

There're probably two ways to do it:
1) handle it in fec driver by parsing a specific property
2) Add a new pad group into pinctrl driver support
e.g.
MX6Q_PAD_CTL_GRP_RGMII_TERM
MX6Q_PAD_CTL_GRP_DDR_TYPE_RGMII

I may prefer to 2).

Regards
Dong Aisheng

> Thanks in advance for your time,
> Michal
> 
> [1]
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