Re: [PATCH/RFC v2 1/2] arm64: dts: r8a7795: Add support for R-Car H3 ES2.0

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Laurent,

On Thu, Apr 20, 2017 at 12:42 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> On Thursday 20 Apr 2017 11:36:59 Geert Uytterhoeven wrote:
>> On Mon, Mar 27, 2017 at 10:48 AM, Laurent Pinchart wrote:
>> > On Friday 24 Mar 2017 14:37:44 Geert Uytterhoeven wrote:
>> >> Update r8a7795.dtsi so it corresponds to R-Car H3 ES2.0 or later:
>> >>   - The following devices no longer exist on ES2.0, and are thus removed:
>> >>     fcpf2, fcpvd3, fcpvi2, fdp1-2, usb3-if1, vspd3, vspi2.
>> >>
>> >>   - The DU <-> VSPD topology is different on ES2.0, hence remove the
>> >>     "vsps" property from the DU node until the driver can handle this.
>> >
>> > I think I'll need a different compatible string between ES1.x and ES2 for
>> > the DU. It could make sense to move the whole DU node to *-es1.dtsi. We
>> > can decide about that later when I'll have a DU driver prototype ready.
>>
>> Why would you need a different compatible string?
>> Can't you use soc_device_match() to handle ES1.x SoCs?
>>
>> The different DU <-> VSPD topology is handled through the vsps property in
>> DTS. Are the ports different, too? That can be handled in DTS.
>
> My point (not expressed clearly) was that, as I'll need a different vsps
> property, I can as well go for a different compatible string.

Do you need a different vsps property?
AFAIK, the current array links each DU channel to a VSPD.
When a VSPD is shared between multiple channels, you can still link these
channels to the same VSPD.

Or is my understanding incorrect?

>> The main reason why I kept the DU node in r8a7795.dtsi is that the board DTS
>> refers to it.  Sharing board DTS means there needs to be at least a
>> placeholder node for the DU in r8a7795.dtsi, unless you want to keep on
>> shuffling board overrides around.
>
> As the ports are identical it makes sense to share the same board DTS, I agree
> with you. We could override the compatible string in r8a7795-es1.dtsi and
> leave it blank in r8a7795.dtsi for now, as there's no driver support for
> ES2.0. That's a bit of a workaround as it shouldn't matter to DT whether
> driver support is available or not. On the other hand, leaving the vsps
> property out is a workaround too.
>
> The current driver will fail probing if the number of VSPs is different than
> the number of CRTCs, so I believe you can keep the vsps property in
> r8a7795.dtsi with 3 VSPS without causing any problem. However, there's no DT
> bindings for the H3 ES2.0 DU yet, so we would end up merging DT without
> bindings, which is not good. I think that regardless of how we proceed,
> keeping the DU node in the ES2.0 .dtsi will be a violation of that policy.

What does the current driver do if the number of VSPs is the same as the
number of CRTCs, but some VSPs are listed more than once?
I guess it will break in a subtle way, instead of refusing to probe?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds