Date:	Fri, 07 Nov 2014 17:20:42 +0100
From:	Noralf Tronnes <notro@...nnes.org>
To:	Stephen Warren <swarren@...dotorg.org>,
	Matthias Klein <matthias.klein@...ux.com>,
	linux-rpi-kernel@...ts.infradead.org, lee@...nel.org
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ARM: bcm2835: add device tree for Raspberry Pi model
 B+

Den 07.11.2014 05:22, skrev Stephen Warren:
> On 11/06/2014 12:36 PM, Noralf Tronnes wrote:
>> Den 06.11.2014 00:45, skrev Matthias Klein:
>>> The model B and B+ differ in the GPIO lines for ACT and PWR leds, and the
>>> I2S interface.
>>>
>>> Signed-off-by: Matthias Klein <matthias.klein@...ux.com>
>>> ---
>>> Changes in v2:
>>> - move the common parts between the B and B+ model into the new
>>> bcm2835-rpi.dtsi file
>>>
>>> - add the I2S signals to the B+ file which fix the problem that USB is
>>> not working
>>>     with the current bcm2835-rpi-b.dts file on the B+.
>>> ---
>> <snip>
>>> +&gpio {
>>> +    pinctrl-names = "default";
>>> +
>>> +    gpioout: gpioout {
>>> +        brcm,pins = <6>;
>>> +        brcm,function = <1>; /* GPIO out */
>>> +    };
>>> +
>>> +    alt0: alt0 {
>>> +        brcm,pins = <0 1 2 3 4 5 7 8 9 10 11 14 15 40 45>;
>>> +        brcm,function = <4>; /* alt0 */
>>> +    };
>>> +
>>> +    alt3: alt3 {
>>> +        brcm,pins = <48 49 50 51 52 53>;
>>> +        brcm,function = <7>; /* alt3 */
>>> +    };
>>> +};
>> AFAIK these pins will always be configured regardless of whether they
>> are used by a driver or not.
> Yes.
>
>> Could we do something like this for SPI and I2C, configuring only when
>> needed?
> ...
>> &spi {
>>      pinctrl-names = "default";
>>      pinctrl-0 = <&spi_pins>;
>> };
> It's certainly possible, but I don't really see any advantage. I'd much
> rather see the pinmux set up correctly all in one go as early as possible.
>
>>> +&i2c0 {
>>> +    status = "okay";
>>> +    clock-frequency = <100000>;
>>> +};
>>> +
>>> +&i2c1 {
>>> +    status = "okay";
>>> +    clock-frequency = <100000>;
>>> +};
>>> +
>> Should the I2C busses be enabled by default?
> Yes, I think so. Whichever I2C controllers are actually connected to
> something (even bare pins on an IO connector) should be enabled by the
> DT, so that they're available for use.
What makes i2c so special that it should be enabled by default?
SPI is not enabled and not 1wire, i2s, lirc either, they must be explicitly
enabled. The problem I see, is that this is a newbie platform, and having
pins driven increases the chance of shorting something.
And it's quite easy to enable these devices with fdtput.

>> On Raspian, i2c is disabled
>> by blacklisting the module (/etc/modprobe.d/raspi-blacklist.conf).
>> At least i2c0 should be left disabled due to the HAT EEPROM and camera.
>> The bus number has also changed with revisions:
>> http://www.raspberrypi.org/forums/viewtopic.php?p=603950#p603950
> It sounds like for I2C-0 on the B+ should use the i2c-mux-pinctrl.c to
> switch the bus between the two destinations as required, although we'd
> have to confirm with Broadcom or the RPi Foundation that that would work
> with this SoC.
>
> There's certainly no reason to believe that the kernel wouldn't want to
> read the HAT EEPROM. After all, it has to identify what's connected there.
This will happen solely in the firmware. The firmware reads the eeprom,
and if it contains a DT fragment, it is merged with the dtb before handing
it over to the kernel. As far as I know, this hasn't been implemented yet.

I asked if we couldn't use u-boot as the boot loader, but I was turned down.
Personally I'm not happy about putting a lot stuff in a closed firmware,
especially on a platform dedicated to getting kids into programming.

 From the HAT specification:
GPIO pins ID_SC and ID_SD (GPIO0 and GPIO1) are reserved for use solely
for board detection / identification. The only allowed connections to
the ID_ pins are an ID EEPROM plus 3.9K pull up resistors. Do not connect
anything else to these pins!

Ref: 
https://github.com/raspberrypi/hats/blob/master/designguide.md#gpio-requirements

I asked in the forum if I could use an app for writing to the eeprom to
change things like display rotation in the DT fragment, but was told 
that this
was a no-no. i2c0 is also used together with the camera port, which is then
controlled by the firmware.

When talking about firmware, there's also a /boot/dt-blob.bin used for 
configuring pins and clocks.
https://github.com/raspberrypi/documentation/blob/master/configuration/pin-configuration.md
This is the source of that file: 
https://github.com/raspberrypi/documentation/blob/master/configuration/images/dt-blob.dts
I haven't studied this, so I don't know how/if it affects our work here.

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