linux-kernel - Re: [PATCH] arm: rpi: Device tree modifications for U-Boot

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Message-ID: <CAPnjgZ0hzyjy=8o8ocaGCoOC7Js4CirhystidbvtabzbhRi4RQ@mail.gmail.com>
Date:	Fri, 14 Aug 2015 12:34:41 -0600
From:	Simon Glass <sjg@...omium.org>
To:	Lucas Stach <dev@...xeye.de>
Cc:	lak <linux-arm-kernel@...ts.infradead.org>,
	Stephen Warren <swarren@...dotorg.org>,
	Stephen Warren <swarren@...dia.com>,
	Russell King <linux@....linux.org.uk>,
	Lee Jones <lee@...nel.org>,
	Devicetree Discuss <devicetree@...r.kernel.org>,
	Kumar Gala <galak@...eaurora.org>,
	lk <linux-kernel@...r.kernel.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Rob Herring <robh+dt@...nel.org>,
	linux-rpi-kernel@...ts.infradead.org,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] arm: rpi: Device tree modifications for U-Boot

-linux-tegra

Hi,

On 12 August 2015 at 07:21, Simon Glass <sjg@...omium.org> wrote:
> Hi Lucas,
>
> On 11 August 2015 at 11:05, Lucas Stach <dev@...xeye.de> wrote:
>> Hi Simon,
>>
>> why did you send this to the Tegra ML?
>>
>> Am Dienstag, den 11.08.2015, 08:25 -0600 schrieb Simon Glass:
>>> This updates the device tree from the kernel version to something suitable
>>> for U-Boot:
>>>
>>> - Add stdout-path alias for console
>>> - Mark the /soc node to be available pre-relocation so that the early
>>> serial console works (we need the 'ranges' property to be available)
>>>
>>> Signed-off-by: Simon Glass <sjg@...omium.org>
>>> ---
>>>
>>>  arch/arm/boot/dts/bcm2835.dtsi | 4 +++-
>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm/boot/dts/bcm2835.dtsi b/arch/arm/boot/dts/bcm2835.dtsi
>>> index 301c73f..bd6bff6 100644
>>> --- a/arch/arm/boot/dts/bcm2835.dtsi
>>> +++ b/arch/arm/boot/dts/bcm2835.dtsi
>>> @@ -8,6 +8,7 @@
>>>
>>>       chosen {
>>>               bootargs = "earlyprintk console=ttyAMA0";
>>> +             stdout-path = &uart;
>>>       };
>>>
>>>       soc {
>>> @@ -16,6 +17,7 @@
>>>               #size-cells = <1>;
>>>               ranges = <0x7e000000 0x20000000 0x02000000>;
>>>               dma-ranges = <0x40000000 0x00000000 0x20000000>;
>>> +             u-boot,dm-pre-reloc;
>>
>> Why do you need this and why should upstream carry your favourite
>> bootloaders configuration? This is in no way hardware description.
>
> I'm not sure how much you know about U-Boot, so let me know if you
> need more info.
>
> U-Boot normally starts up by setting up its serial UART and displaying
> a banner message. At this stage typically only a few devices are
> initialised (e.g. maybe just the UART). It then relocates itself to
> the top of memory and starts up all the devices. It throws away any
> previous devices that it set up before relocation and starts again.
>
> U-Boot uses a thing called driver model (dm) which handles driver
> binding and probing. Driver model has the device tree and would
> normally scan through it and create devices for everything it finds.
>
> Before relocation we don't need every device. Also the CPU is often
> running slowly, perhaps without the cache enabled. SDRAM may not be
> available yet so space is short. We want to avoid starting up things
> that will not be used.
>
> So this property indicates that the device is needed before relocation
> and should be set up by driver model. We need it to avoid a very slow
> and memory-hungry startup.
>
> As to why upstream should accept it, my understanding of upstream is
> that people can send patches to it and in fact are encouraged to do
> so, to avoid misunderstandings and duplication. The device tree files
> are stored in Linux so any binding or source file changes should end
> up there. Otherwise the files tend to diverge and we end up with
> multiple bindings and multiple versions of the same source file.

Is the above explanation sufficient? Will this patch be applied?

Regards,
Simon
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