linux-kernel - Re: [PATCH 1/2] pinmux: Add TB10x pinmux driver

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Message-ID: <51B2320B.6060101@wwwdotorg.org>
Date:	Fri, 07 Jun 2013 13:18:35 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Christian Ruppert <christian.ruppert@...lis.com>
CC:	Haojian Zhuang <haojian.zhuang@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Shiraz HASHIM <shiraz.hashim@...com>,
	Patrice CHOTARD <patrice.chotard@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>,
	Sascha Leuenberger <sascha.leuenberger@...lis.com>,
	Pierrick Hascoet <pierrick.hascoet@...lis.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH 1/2] pinmux: Add TB10x pinmux driver

On 06/06/2013 09:30 AM, Christian Ruppert wrote:
> On Thu, Jun 06, 2013 at 10:32:21PM +0800, Haojian Zhuang wrote:
>> On 6 June 2013 22:11, Christian Ruppert <christian.ruppert@...lis.com> wrote:
>>> On Wed, Jun 05, 2013 at 09:44:27AM +0800, Haojian Zhuang wrote:
>>>> On 3 June 2013 20:30, Christian Ruppert <christian.ruppert@...lis.com> wrote:
>>>>> OK, here's a simplified example of what we would like to do (this seems
>>>>> pretty common so I suppose there is a way I haven't understood). Our
>>>>> situation is slightly more complex but for the purpose of discussion
>>>>> let's assume a chip with 8 pins which can be configured for the
>>>>> following functions:
>>>>>
>>>>> Pin   GPIO-A    I2C    SPI0     SPI1
>>>>> ------------------------------------
>>>>>  1    GPIOA0    SDA             MISO1
>>>>>  2    GPIOA1    SCL             MOSI1
>>>>>  3    GPIOA2                    SS1_B
>>>>>  4    GPIOA3                    SCLK1
>>>>>  5    GPIOA4           MISO0
>>>>>  6    GPIOA5           MOSI0
>>>>>  7    GPIOA6           SS0_B
>>>>>  8    GPIOA7           SCLK0
>>>>>
>>>>> We can now define the following pinctrl-single:
>>>>>
>>>>> pinmux: pinmux@...FEE0000 {
>>>>>         compatible = "pinctrl-single";
>>>>>         reg = <0xFFEE0000 0x8>;
>>>>>         #address-cells = <1>;
>>>>>         #size-cells = <0>;
>>>>>         #gpio-range-cells = <3>;
>>>>>         pinctrl-single,register-width = <32>;
>>>>>         pinctrl-single,function-mask = <0xffffffff>;
>>>>>         pinctrl-single,gpio-range = <&range 1 8 0>;
>>>>>         gpioa_pins: pinmux_gpioa_pins {
>>>>>                 pinctrl-single,pins = <0x0 0 0x4 0>
>>>>>         };
>>>>>         i2c_pins: pinmux_i2c_pins {
>>>>>                 pinctrl-single,pins = <0x0 1>
>>>>>         };
>>>>>         spi0_pins: pinmux_spi0_pins {
>>>>>                 pinctrl-single,pins = <0x1 1>
>>>> <0x1 1>?
>>>>
>>>> If each pinmux register is only for one pin in your SoC.
>>>> I think that your definitions are wrong above. We use
>>>> register offset as the first argument, not pin number.
>>>> And the second argument should be pin function number.
>>>
>>> In our case each pinmux register (bit field) actually controls an entire
>>> group of pins.
>>>
>>>> If multiple pins are sharing one register with different bits,
>>>> you need to enable "pinctrl-single,bit-per-mux".
>>>
>>> Multiple pins are sharing the same bits in the same register. Do you
>>> think this prevents us from using pinctrl-single?
>>>
>> Could you give me your register definition? Then I can understand you
>> better.
> 
> In our example, the register map would look a bit like the following.
> Note that every register configures four pins at a time.
> 
> Register 0x0:
>  Mode  GPIO-A    I2C    SPI1
>  Value 0x0       0x1    0x2
>  ---------------------------
>  Pin1  GPIOA0    SDA    MISO1
>  Pin2  GPIOA1    SCL    MOSI1
>  Pin3  GPIOA2           SS1_B
>  Pin4  GPIOA3           SCLK1
> 
> Register 0x4:
>  Mode  GPIO-A    SPI0
>  Value 0x0       0x1
>  ---------------------
>  Pin5  GPIOA4    MISO0
>  Pin6  GPIOA5    MOSI0
>  Pin7  GPIOA6    SS0_B
>  Pin8  GPIOA7    SCLK0

My suggestion here is that pinctrl-single isn't appropriate. The only
way it could work is if you pretend that each group-of-pins is actually
a single pin.

However, then the correlation between these pretend pins (i.e. really
the groups) and GPIOs won't work, because each "pin" is really 4 pins,
and hence 4 GPIOs, and hence you won't be able to gpio_get() more than 1
GPIO per pin group, I think.

It's not hard (although possibly data intensive depending on your SoC)
to represent your HW just fine with a native pinctrl driver; pinctrl
itself has the ability to separate the concepts of pins, groups-of-pins,
and the mux-functions-that-are-assigned-to-groups. If any of your HW
registers actually do control only a single pin, you can simply create
both a pin and a group that contains only that one pin. This is all very
similar to how Tegra works, although it sounds like your registers may
be a bit more regular than Tegras - Tegra has a very variable number of
pins in each grop, and even some overlap between groups (mux function
groups and pin configuration groups aren't aligned).
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