[PATCH 7/8] ARM: dts: imx: add IMX50 SoC device tree bindings

[Greg Ungerer]

Hi Benoit, Matt, Sascha,

On 23/10/13 10:17, Benoît Thébaudeau wrote:
> Hi Matt,
> 
> On Wednesday, October 23, 2013 1:06:44 AM, Matt Sealey wrote:
>> On Tue, Oct 22, 2013 at 5:42 PM, Benoît Thébaudeau
>> <benoit.thebaudeau at advansee.com> wrote:
>>> Dear Sascha Hauer,
>>>
>>> On Tuesday, October 22, 2013 11:57:47 PM, Sascha Hauer wrote:
>>>>>>
>>>>>> 0 is definitely wrong here. We have 0x80000000 for "Don't touch
>>>>>> padctrl", but otherwise this should contain some real padctrl
>>>>>> settings.
>>>>>
>>>>> A more pressing question is in what world did the bootloader not
>>>>> already set these pins up and if they are already set up, why are they
>>>>> loitering in the device tree?
>>>>
>>>> Having NO_PAD_CTRL in the devicetree doesn't make sense, you're right.
>>>> Either a pin has to be configured by the bootloader completely or not at
>>>> all. Having the mux configured by the kernel and the drive strength by
>>>> the bootloader is broken by design. All pins should have a complete
>>>> padctrl setup and NO_PAD_CTRL should be dropped.
>>>
>>> Some pins may be configured completely by the kernel, and not at all by the
>>> bootloader. In that case, the device tree may wish to keep the SoC reset
>>> pad
>>> configuration. NO_PAD_CTRL is useful in that case, although one might argue
>>> that
>>> the reset values should not be trusted for various reasons.
>>
>> No, that's totally illogical. The magic bit to "leave my pad controls
>> alone" makes no sense since you're re-configuring mux mode and input
>> select (even if they're the same as the bootloader, you're doing it
>> twice).
>>
>> There are two reasons this gets done, both of which are very very odd..
>>
>> 1) The definition of the pinmux tuple of 3 register/value pairs is
>> ordered such that the pad control can be set independently of the
>> preprocessor definition for the pad (which supplies the first 5 values
>> in the 6 value pin definition). This is weird since one of the values
>> doesn't belong to the register the value pair is intended for, and
>> saying "do not
>>
>> 2) The reason we have to define the three registers even if we "don't
>> change values" is so users (or userspace) can fiddle with the GPIOs
>> through some never-used non-existent (since it doesn't expose these
>> values) sysfs pinmuxing API.
>>
>> In this case, you don't "set the pad control to 'do not touch'" but
>> supply the exact values from the manual or empirically derived from
>> inspection at SoC power on. There is ZERO use for a 'do not touch'
>> bit. In the case where it turns out the bootloader is wrong, you'll
>> have to supply a value here anyway. So supply the default.
> 
> That makes sense, but it has the disadvantage of being more error prone than
> keeping reset values if they're fine, because errors in pad settings may lead to
> bugs less obvious than bugs in mux settings. On the other hand, forcing the
> developers to check all pad settings has the advantage of not using reset values
> that may be wrong for some applications. All in all, removing NO_PAD_CTRL seems
> better here.
> 
>> In reality what would happen is the binding gets a kick in the pants
>> so it makes more sense, and we define that the bootloader MUST set up
>> all static pin multiplexing configuration at the earliest opportunity
>> and save Linux the hassle. pinctrl is for microcontrollers and
>> robotics boards - Arduino or BeagleBone Black where you have expansion
>> connectors which could have multiple useful configurations, or be
>> changed on a hotpluggable attachment.
>>
>> Even if your bootloader doesn't use the pins, it should at least set
>> them up for later, for electrical reasons. If the default direction
>> for a gpio mux is output, who knows what it is for a custom peripheral
>> mux, and if your peripheral has an output on that pin.. then you're
>> driving outputs from both sides, which is nonsensical. If the default
>> direction is input, and your peripheral is an input, you can set up
>> conditions where essentially the value floats (or at least is
>> unpredictable) at the peripheral side.
>>
>> Your hardware guy will usually slap you for leaving it in the wrong
>> state for the time it takes to load Linux from SD card or SPI NOR, and
>> decompress, and get to several seconds into the boot process before
>> configuring pin muxing, or even to past the init daemon, so it can do
>> a module or firmware load first.
> 
> I agree, except that a properly designed board should not connect pins set as
> outputs upon reset to the outputs of external devices, since this leaves
> potential electrical level conflicts from reset at least until the software
> cleans the mess. However, you're right that such things happen and should be
> handled as you say.
> 
> But if some SoC pins are used as inputs, set up as inputs by the SoC reset but
> with the wrong mux value, and unused by the bootloader, there is no reason to
> set up these pins in the bootloader rather than in the DT. Same for output pins
> connected to a bus disabled by default and unused in the bootloader. Actually,
> the bootloader really has to set up pins because it uses them, or for electrical
> reasons, or to disable some unused feature, but the two latter are most of the
> time an exception among all connected pins.
> 
>>> But there is another case. One SoC pin may be connected to several external
>>> devices, e.g. through an external analog mux. In that case, the bootloader
>>> may
>>> configure the SoC mux and pad for one usage, while the kernel may
>>> afterwards
>>> change only the mux configuration of this pin for the other usage.
>>
>> This is the ONLY use case it makes any sense in a static
>> configuration, but at this point, why not just supply the required
>> value, even if it's the chip default? If the configuration for "kernel
>> time" is fixed, the bootloader should quiesce the device and configure
>> it for the kernel to match the device tree. There are a bunch of
>> configurations on i.MX51 where the DT is redefining the default pinmux
>> for the SoC... pointlessly..
> 
> This is a possibility.
> 
>>> So NO_PAD_CTRL is not strictly required for pinctrl, but it can be handy.
>>
>> Given that 0x0 (or 0x80000000) takes up exactly the same amount of
>> space and time to research and put in the tree as the ACTUAL required
>> pad settings from the manual or empirically supplied, supplying the
>> pad settings makes more sense (and is far, far more descriptive).
>> There is a case here for cross-checking ALL DT hardware configuration
>> details against the actual configuration of the hardware, and
>> supplying a weird pinctrl definition (for instance, setting a RAZ bit
>> as 1, or a RAO bit to 0) could be caught this way (there are *lots* of
>> pin definitions out there in the world for i.MX at least which are
>> doing this, some of them in the Boot ROM DCD table...)
> 
> Agreed. It may be better to drop NO_PAD_CTRL from DT, but I still find debatable
> whether to compel to move all pin configurations to the bootloader for static
> configurations, since the less the kernel has to rely on the bootloader, the
> more reliable and self-sufficient it is (with DT).
> 
> Note that in the patch above, 0 may be intended to be the actual pad setting,
> and not to mean by mistake NO_PAD_CTRL.

And 0 was the intended pad control setting here. I wasn't even aware of
NO_PAD_CTRL.

Regards
Greg