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

[Felix Brack]

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 ...

> Thanks and regards,
> Lokesh
> 
>>
>> For the patch I'm working on there is the general question about how to
>> proceed. Should I just fix the board I'm maintaining and then post the
>> patch? This would most certainly break some of the remaining 45 boards
>> which I think is a no-go.
>>
>> A more specific question dealing with the "missing SoC property"
>> <reg-shift> above is about <>-u-boot.dtsi files: would it be possible,
>> useful and hazard-free to change the build process to use, for example,
>> SoC specific _and_ board specific files? Of course some sort of
>> hierarchy would be necessary to make sure that properties defined in the
>> board specific dtsi file would overwrite the same properties defined in
>> the SoC specific file ect.
>>
>> I believe that keeping DT files (especially include files) in sync
>> between U-Boot and Linux is of big importance. Maybe there is already a
>> standard solution to problems similar to the one depicted above. Anyway,
>> I would appreciate any feedback.
>>
>> regards, Felix
>>

regards, Felix