Message-ID: <CALszF6BU2uReeTdx6+FZG7Qx1TxCBNk_gzp00WWADi-oe-wyHA@mail.gmail.com>
Date:	Mon, 18 May 2015 14:21:42 +0200
From:	Maxime Coquelin <mcoquelin.stm32@...il.com>
To:	Arnd Bergmann <arnd@...db.de>,
	Daniel Thompson <daniel.thompson@...aro.org>
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Subject: Re: [PATCH v8 14/16] ARM: dts: Introduce STM32F429 MCU

Hi Arnd, Daniel,

2015-05-13 18:54 GMT+02:00 Maxime Coquelin <mcoquelin.stm32@...il.com>:
> 2015-05-13 18:37 GMT+02:00 Arnd Bergmann <arnd@...db.de>:
>> On Wednesday 13 May 2015 18:29:05 Maxime Coquelin wrote:
>>> 2015-05-13 17:28 GMT+02:00 Arnd Bergmann <arnd@...db.de>:
>>> > On Wednesday 13 May 2015 16:20:34 Daniel Thompson wrote:
>>> >>
>>> >> That would suit me very well (although is the 0x20/0x40 not the 8 that
>>> >> we would need in the middle column).
>>> >
>>> > We don't normally use register offsets in DT. The number 8 here instead
>>> > would indicate block 8, where each block is four bytes wide. Using the
>>> > same index here for reset and clock would also help readability.
>>>
>>> My view is that it makes the bindings usage very complex.
>>> Also, it implies we have a specific compatible for stm32f429, whereas
>>> we didn't need with my earlier proposals.
>>> Indeed, the reset driver will need to know the offset of every reset
>>> registers, because:
>>>   1. The AHB registers start at RCC offset 0x10 (up to 0x18)
>>>   2. The APB registers start at RCC offset 0x20 (up to 0x24).
>>> We have a gap between AHB and APB registers, so how do we map the
>>> index for the block you propose?
>>> Should the gap be considered as a block, or we should skip it?
>>>
>>> I'm afraid it will not be straightforward for a reset user to
>>> understand how to use this bindings.
>>>
>>> Either my v7 or v8 versions would have made possible to use a single
>>> compatible for STM32 series.
>>> If we stick with one of these, we could even think to have a "generic"
>>> reset driver, as it could be compatible with sunxi driver bindings.
>>
>> We should definitely try to use the same compatible string for all of
>> them, and make a binding that is easy to use.
>>
>> I haven't fully understood the requirements for the various parts that
>> are involved here. My understanding so far was that the driver could
>> use the index from the first cell and compute
>>
>>         void __iomem *reset_reg = rcc_base + 0x10 + 4 * index;
>>         void __iomem *clock_reg = rcc_base + 0x30 + 4 * index;
>
> This calculation is true, but we have to take into account there is a
> hole in the middle, between AHB3, and APB1 register:
>
> AHB1RSTR : offset = 0x10, index = 0
> AHB2RSTR : offset = 0x14, index = 1
> AHB3RSTR : offset = 0x18, index = 2
> <HOLE >     : offset = 0x1c, index = 3
> APB1RSTR : offset = 0x20, index = 4
> APB2RSTR : offset = 0x24, index = 5
>
> So we have to carefully document this hole in the bindings, maybe by
> listing indexes in the documentation?
>
>>
>> Are there parts that need something else? If the 0x10 offset is
>> different, we probably want a different compatible string, and I'd
>> consider it a different part at that point. If there are chips
>> that do not spread the clock from the reset by exactly 256 bits,
>> we could add a DT property in the rcc node for that.
>
> I will check other chips, to see if this is valid generally.

I checked on other chips, and the assumption looks valid generally.

Kind regards,
Maxime

>
> Thanks for your feedback,
> Maxime
>
>>
>>         Arnd
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