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

[Lokesh Vutla]

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.

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
>