linux-kernel - Re: [PATCH 3/5] arm64: dts: allwinner: a64: add simplefb for A64 SoC

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Date:   Tue, 13 Mar 2018 17:51:29 +0100
From:   Harald Geyer <harald@...ib.org>
To:     Maxime Ripard <maxime.ripard@...tlin.com>
cc:     Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andre Przywara <andre.przywara@....com>,
        Icenowy Zheng <icenowy@...c.io>, info@...mex.com
Subject: Re: [PATCH 3/5] arm64: dts: allwinner: a64: add simplefb for A64 SoC

Maxime Ripard writes:
> Hi,
> 
> On Tue, Mar 13, 2018 at 10:18:22AM +0100, Harald Geyer wrote:
>> Maxime Ripard writes:
>>> On Mon, Mar 12, 2018 at 04:10:48PM +0000, Harald Geyer wrote:
>>>> The A64 SoC features two display pipelines, one has a LCD output, the
>>>> other has a HDMI output.
>>>>
>>>> Add support for simplefb for the LCD output. Tested on Teres I.
>>>>
>>>> This patch was inspired by work of Icenowy Zheng.
>>>>
>>>> Signed-off-by: Harald Geyer <harald@...ib.org>
>>>> ---
>>>> arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi | 20 ++++++++++++++++++++
>>>> 1 file changed, 20 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi
>>> b/arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi
>>>> index ca1b365bc722..05d5e8def68a 100644
>>>> --- a/arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi
>>>> +++ b/arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi
>>>> @@ -52,6 +52,26 @@
>>>> 	#address-cells = <1>;
>>>> 	#size-cells = <1>;
>>>>
>>>> +	chosen {
>>>> +		#address-cells = <1>;
>>>> +		#size-cells = <1>;
>>>> +		ranges;
>>>> +
>>>> +/*
>>>> + * The pipeline mixer0-lcd0 depends on clock CLK_MIXER0 from DE2 CCU.
>>>> + * However there is no support for this clock on A64 yet, so we depend
>>>> + * on the upstream clocks here to keep them (and thus CLK_MIXER0) up.
>>>> + */
>>> 
>>> There's definitely support for the CLK_MIXER0 clock in the DE2 CCU
>>> driver, so I guess this would need to be amended / fixed
>> 
>> AFAIK on the A64 a special sram quirk is necessary, that never got merged:
>> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1574303.html
>> 
>> I asked Icenowy if I should resubmit her patch as part of this series,
>> but was told further discussion is necessary. I'm sure she can better
>> explain the details than me.
>> 
>> Actually if it wasn't for the sram quirk, there is a simplefb patch by
>> her, that could have been merged long ago:
>> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1574304.html
> 
> The issue with your patch is that, as soon as that clock is
> functional, the DT with the node you were introducing here will no
> longer be.
> 
> And since people use their firmware DT or put them in NOR these days,
> you can't really expect them to update them every release either.

How is this a problem? - The clock can't be functional without adding
a DT node for it's driver. So the breakage can only happen on DT update,
which means we can avoid it, by moving the clocks to the proper places
when we add the node actually using them for real.

> I guess a proper solution would be to respin that patch.

Sure, but as mentioned above I offered to do that and was told not to.

As a compromise I could also move the simple framebuffer node from the
A64 dtsi to the teres-i dts, where we know that the DT can be updated
easily?

Thanks for your consideration,
Harald