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

From: Simon Glass
Date: Fri Aug 28 2015 - 14:27:29 EST

Hi Rob,

On 25 August 2015 at 10:22, Rob Herring <robherring2@xxxxxxxxx> wrote:
> On Sat, Aug 15, 2015 at 8:46 AM, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
>> Hi Rob,
>> On 14 August 2015 at 14:29, Rob Herring <robherring2@xxxxxxxxx> wrote:
>>> On Fri, Aug 14, 2015 at 1:34 PM, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
>>>> -linux-tegra
>>>> Hi,
>>>> On 12 August 2015 at 07:21, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
>>>>> Hi Lucas,
>>>>> On 11 August 2015 at 11:05, Lucas Stach <dev@xxxxxxxxxx> 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)
>>> I find it quite strange that you must explicitly enable the parent
>>> node, but not the uart node.
>> U-Boot tries hard to find and bind the UART using the stdout-path
>> link. I would like to add it in the UART node also but we were able to
>> work around it so far.
>>>>>>> Signed-off-by: Simon Glass <sjg@xxxxxxxxxxxx>
>>>>>>> ---
>>>>>>> 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.
>>> How do you debug the DM itself? It seems like you still would need
>>> something earlier for debug like earlycon in the kernel. u-boot DM is
>>> probably simple enough you can get away with using it early, but you
>>> often can't as the complexity increases. Ultimately you need something
>>> simple that just hits all the registers needed to get characters out.
>>> What happens when you add pinmux, clocks, PMIC, power domains, etc. to
>>> the DM and they all become dependencies for the UART?
>> This doesn't seem like a device tree binding question but let me try
>> to answer it.
>> This is a problem - one of the challenges of driver model is that the
>> UART gets further away from the reset vector.
> This is a common problem for any complex embedded system. Nothing
> special about u-boot here. So either we don't need anything because
> everyone else has dealt with the problem in some way or we need a
> common solution because we all have this problem.

I'm struggling to understand the value of this statement - is it just
philosophizing? I do have a real problem and would like to solve it.
Please make a concrete suggestion.

>> In U-Boot there is a single debug UART which can be supported by the
>> various serial drivers (only one can be selected on each platform).
>> There is a special debug_uart_init() function which is supposed to set
>> up the UART for that platform. After that, and until driver model UART
>> is up, console output can go through the debug UART.
> So you don't need this for the common case of an early console, but
> only for platforms needing some other platform specific setup?

The debug UART is generally disabled and has very limited use. It is
hard-coded and does not use the device tree. I mentioned it because
you asked "How do you debug the DM itself". You dropped that from the
context so this thread is now quite confusing.

Let's ignore the debug UART.

>>>>> 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.
>>> Can't the need for that property change over time? Either as more
>>> drivers are converted to DM you need to add this or you add some
>>> feature that depends on a driver (e.g. get a board rev or boot mode
>>> from GPIO). You would have backwards compatibility issues with this.
>>> I'm somewhat less worried about that for u-boot as we should be
>>> bundling the dtb and bootloader rather than kernel and dtb. For the
>>> UART, you can just get which UART to initialize early from
>>> stdout-path. But for other cases, couldn't you just have the platform
>>> provide the list of needed drivers. Then when u-boot needs change, you
>>> just change u-boot.
>> Yes U-Boot and its device tree are normally built from the same tree
>> at the same time. We don't expect to have to support an older or newer
>> device tree with the same U-Boot binary. So I don't see a problem
>> here.
> My point is that if I had to pick how bootloader+dtb+kernel are
> bundled or not, I would rather see the dtb in sync with the bootloader
> rather than the kernel. But I can't decide that for everyone and
> neither can you. You still have a compatibility problem and that has
> to be addressed.

What are you getting at here? If I move to a new kernel and still use
an old device tree I may be missing features, or fail to boot. Don't
do that!

But do we need to talk about this? How does this affect my patch?

>> The UART should have as few dependencies as possible. But if the
>> particular platform needs to check a GPIO to find out the UART number
>> (i.e. stdout-path cannot be used) then we would need to support that
>> case. It hasn't come up yet but it might. I certainly see platforms
>> where we need some other nodes before relocation and would like to
>> mark them as such. But I hope that for UART we can generally use
>> stdout-path.
>> The list of drivers doesn't help that much. What would I do with such
>> a list, and where would I keep such platform-specific information
>> other than in the device tree? It seems much simpler to hold the
>> information about what the platform needs to get through early boot in
>> one place. Then we have just a few lines of generic code to deal with
>> it.
> If you need certain drivers such as gpio, then you obviously have some
> board specific code using those drivers already. If you can tell the
> DM which devices to enumerate via DT, you can also tell the DM with C
> code.
> While doing everything generically without board code is nice, we
> should only do that for the common cases and handle the screwy cases
> with board code.

OK so let's ignore the GPIO example.

After reading your email I am none the wiser about what you are suggesting.

This is not a screwy case. Every board will have a console. In some
cases it is inside a 'soc {' node and in some cases it is not. The
pure solution would be to mark every UART node with
u-boot,dm-pre-reloc and we can do that if you prefer. It isn't
necessary though for the reasons I previously explained.

It seems reasonable that U-Boot should be able to add private
properties to the device tree, intended for U-Boot, just as Linux
does. What is the problem here?

If are you asking for a generic property that any bootloader could
use, to locate what is needed to bring up the serial console then
please propose something and I can take a look. But I know that not
all bootloaders work like U-Boot (small space-constrained SPL,
relocatable main loader, try to output something on a serial port as
early as possible).

How can we resolve this so that the device tree files in U-Boot and
Linux can stay in sync?

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