[U-Boot] [RFC] arm: dts: am33xx: Sync DTS with Linux 4.16.11

[Lokesh Vutla]

Hi Felix,

On Wednesday 11 July 2018 06:59 PM, Felix Brack wrote:
> Hi Lokesh, many thanks for the feedback!
> 
> On 10.07.2018 08:42, Lokesh Vutla wrote:
>> Hi Felix,
>>
>> On Thursday 24 May 2018 02:32 PM, Felix Brack wrote:
>>> Hello,
>>>
>>> I am working on a patch to synchronize the DTS files of the am33xx SoC
>>> with those from Linux 4.16.11 (current stable).
>>>
>>> After some tiny modifications to the boards am335x-pdu001, am335x-evm,
>>> am335x-rut, am437x-gp-evm and am43x-epos-evm buildman passes without any
>>> warnings on the 46 am33xx based boards. This is required but not
>>> sufficient at all.
>>>
>>> As I'm the maintainer of the am335x-pdu001 board I will use it to
>>> illustrate a sample problem.
>>> The am335x-pdu001 board uses the ns16550 driver which reads the property
>>> <reg-shift> from the DT. This property is required and must be set to a
>>> value of 2, otherwise it would default to 0.
>>> The am33xx.dtsi file currently used by U-Boot sets this property
>>> correctly for all 6 uarts. The am33xx.dtsi file from Linux 4.16.11
>>> however does not define this property anymore.
>>> For the am335x-pdu001 board the fix is trivial: just add the property
>>> <reg-shift> to the board and U-Boot specific am335xx-pdu001-u-boot.dtsi
>>> file.
>>> But this has a major drawback: only the am335x-pdu001 board gets fixed.
>>> What about other boards requiring the <reg-shift> property?
>>
>> I agree this is a problem. Also not all kernel drivers need this
>> property as there are separate driver files for some of the compatibles.
>> And few drivers has this property hard-coded.
>>
>>> The first idea that would probably come into mind is to put the property
>>> <reg-shift> into the SoC specific am33xx-u-boot.dtsi file. But this file
>>> is ignored if a board specific file already exists. Hence this can not
>>> be done.
>>
>> One solution I can think of is add a Kconfig entry
>> CONFIG_SYS_NS16550_REG_SHIFT. A default value of 0 can be assigned for
>> this and can be updated in as necessary.
>>
>> So when doing dev_read_u32_default for reg-shift,
>> CONFIG_SYS_NS16550_REG_SHIFT can be passed as default instead of 0. This
>> way no DTS files needs to be updated.
>>
> This sounds like a very good solution for the <reg-shift> property. I'm
> thinking about sending in a patch ...

That would be great.

Thanks and regards,
Lokesh