linux-kernel - Re: [PATCH v3 2/2] arm64: dts: mt8173: Add I2C device node

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Date:	Mon, 15 Jun 2015 15:46:46 +0800
From:	Eddie Huang <eddie.huang@...iatek.com>
To:	Sascha Hauer <s.hauer@...gutronix.de>
CC:	Daniel Kurtz <djkurtz@...omium.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	"open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<linux-mediatek@...ts.infradead.org>,
	"Linus Walleij" <linus.walleij@...aro.org>
Subject: Re: [PATCH v3 2/2] arm64: dts: mt8173: Add I2C device node

Hi Sascha,

On Mon, 2015-06-15 at 08:12 +0200, Sascha Hauer wrote:
> On Fri, Jun 12, 2015 at 08:28:51PM +0800, Daniel Kurtz wrote:
> > On Fri, Jun 12, 2015 at 5:27 PM, Eddie Huang <eddie.huang@...iatek.com> wrote:
> > >
> > > Add MT8173 I2C device nodes, include I2C controllers and pins.
> > > MT8173 has six I2C controllers, from i2c0 to i2c6, exclude i2c5.
> > > The 6th I2C controller register base doesn't next to 5th I2C,
> > > and there is a hardware between 5th and 6th I2C controller. So
> > > SoC designer name 6th controller as "i2c6", not "i2c5".
> > 
> > 
> > This is slightly misleading.  There are in fact 7 I2C controllers, but
> > "i2c5" is dedicated for use by HDMI for ddc & hdcp.
> > Is there a reason why the HDMI I2C port cannot be controlled by the
> > generic i2c driver?
> > 
> > Of course the hdmiddc / i2c5 node can always be added in a later
> > patch, so this is no reason to hold up this patch.
> > 
> > > Signed-off-by: Eddie Huang <eddie.huang@...iatek.com>
> > > ---
> > >  arch/arm64/boot/dts/mediatek/mt8173-evb.dts | 50 ++++++++++++++++++++
> > >  arch/arm64/boot/dts/mediatek/mt8173.dtsi    | 72 +++++++++++++++++++++++++++++
> > >  2 files changed, 122 insertions(+)
> > >
> > > diff --git a/arch/arm64/boot/dts/mediatek/mt8173-evb.dts b/arch/arm64/boot/dts/mediatek/mt8173-evb.dts
> > > index 43d5401..2e01988 100644
> > > --- a/arch/arm64/boot/dts/mediatek/mt8173-evb.dts
> > > +++ b/arch/arm64/boot/dts/mediatek/mt8173-evb.dts
> > > @@ -33,6 +33,56 @@
> > >         chosen { };
> > >  };
> > >
> > > +&pio {
> > 
> > I don't think we needed to move these i2c pinmux descriptions from
> > mt8173.dtsi to the board .dts file.
> > 
> > AFAICT, the "i2cN_pins_a" nodes defined here are the pinctl
> > configuration that the corresponding enabled i2c nodes would choose.
> > Thus, they are generic to the MT8173 SoC, not specific to any board.
> > By themselves, these nodes do not actually select a pin configuration.
> 
> On i.MX we started with i2cN_pins_[abcde] groups in the SoC dtsi file
> aswell. At some point we realized that this does not scale anymore.
> The problems were:
> 
> - Whenever a new board came along which needed some pin setting which
>   didn't already exist a new group was created in the SoC dtsi file using
>   the next free letter from the alphabet. The ordering of the group names
>   was rather arbitrary and often enough there were merge conflicts to
>   resolve when during one merge window two new boards showed up which both
>   added i2cN_pin_x with different content.
> - When a new board needs the same pins but with different drive strength
>   settings a new group was required
> - With SD/MMC we had groups for 4bit data, groups adding the remaining
>   pins for 8bit data
> - With UARTs we had groups for RX/TX and additionally groups adding
>   RTS/CTS
> - For graphics we had groups for 16bit data and 24bit data
> - There is no way to remove the unused nodes from the binary dtb, so
>   every board contained all existing pingroups for every other board,
>   so the dtbs became quite big
> 
> So with only a few existing boards with very similar pinmux groups
> it may work fine to put the groups into the SoC dtsi, but this way
> may also explode quite fast with more and different boards.
> I don't know how much variation there will be with Mediatek boards, so
> I'm fine with either way. You may want to look at:
> 
> 5a2a7d5 ARM: dts: imx51: make pinctrl nodes board specific
> 7ac0f70 ARM: dts: imx53: make pinctrl nodes board specific
> 817c27a ARM: dts: imx6qdl: make pinctrl nodes board specific
> 

We tend to put basic and fixed pins in SoC dtsi, platform-variant pins
to board dts.Since these pins already move once, I hope we reach a
consensus that I put i2c pins back to SoC dtsi.

Eddie
Thanks


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