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

[Simon Glass]

Hi,

On Friday, 28 August 2015, Simon Glass <sjg@xxxxxxxxxxxx> wrote:
>
> 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?


It doesn't look like this thread has made any progress. I'll redo the
patch with a binding file and try again.

Regards,
Simon
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