[U-Boot] [PATCH v2] serial: ns16550: Add register shift variable

[Alexander Graf]

On 07/17/2018 03:34 PM, Felix Brack wrote:
>
> On 17.07.2018 15:21, Tom Rini wrote:
>> On Tue, Jul 17, 2018 at 12:45:51PM +0000, Alexey Brodkin wrote:
>>> Hi Felix,
>>>
>>>> -----Original Message-----
>>>> From: Felix Brack [mailto:fb at ltec.ch]
>>>> Sent: Tuesday, July 17, 2018 3:13 PM
>>>> To: Alexander Graf <agraf at suse.de>; Lokesh Vutla <lokeshvutla at ti.com>; u-boot at lists.denx.de
>>>> Cc: Wolfgang Denk <wd at denx.de>; Tom Rini <trini at konsulko.com>; Marek Vasut <marek.vasut at gmail.com>; Patrice Chotard
>>>> <patrice.chotard at st.com>; Michal Simek <michal.simek at xilinx.com>; Simon Glass <sjg at chromium.org>; Alexey Brodkin
>>>> <Alexey.Brodkin at synopsys.com>; Bin Meng <bmeng.cn at gmail.com>; Ley Foon Tan <ley.foon.tan at intel.com>; Patrick Delaunay
>>>> <patrick.delaunay at st.com>; Mario Six <mario.six at gdsys.cc>; Stefan Roese <sr at denx.de>; Bernhard Messerklinger
>>>> <bernhard.messerklinger at br-automation.com>
>>>> Subject: Re: [PATCH v2] serial: ns16550: Add register shift variable
>>> [snip]
>>>   
>>>> Adding a separate PORT in ns16550_serial_ids for a particular
>>>> architecture, platform or SoC would be an option. However the patch I
>>>> posted is much more generic as it offers to set the reg-shift property
>>>> for no matter what architecture, platform or SoC. It can also easily be
>>>> extended by adding more conditional defaults to the Kconfig file.
>>> I'd say we're dealing with just one corner-case here.
>>> If I understand a concept of Device Tree it is supposed to describe your
>>> hardware. Thus if reg shift exists in your HW it should be explicitly mentioned in
>>> your .dts. If for some [historical] reason you have to deal with "incorrect" .dts then
>>> I'd prefer to have mentioned quirk with a separate PORT in ns16550_serial_ids
>>> instead of adding yet another Kconfig option.
>> So, this is part of the problem I suppose.  I don't know _why_ we can't
>> just add the correct and valid reg-shift property to the dtsi file in
>> Linux and be done with it.  Then the U-Boot driver would work because we
>> parse that property.
>>
> The only reason I can see why the <reg-shift> property "can't be added"
> to the Linux .dtsi file is that there is nothing broken in Linux. Hence
> we would actually ask Linux to add a property required by U-Boot.

In the DT you can describe hardware specifics by either a different 
compatible string or by additional properties on a generic compatible. So

   compatible = "ti,omap3-uart";

is correct, as is

   compatible = "ns16550";
   reg-shift = 2;

There might be more that gets implied by the omap3-uart compatible that 
I'm not aware of, but in a nutshell it's all about whether you use a 
generic compatible string or a device specific one. Linux went for the 
specific one, so it didn't need reg-shift. U-Boot (incorrectly) treats 
the device specific compatible string as generic which is why you see 
the failure.


Alex