Hi Rob,

On 25 August 2015 at 10:22, Rob Herring [off-list ref] wrote:

On Sat, Aug 15, 2015 at 8:46 AM, Simon Glass [off-list ref] wrote:

quoted

Hi Rob,

On 14 August 2015 at 14:29, Rob Herring [off-list ref] wrote:

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On Fri, Aug 14, 2015 at 1:34 PM, Simon Glass [off-list ref] wrote:

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-linux-tegra

Hi,

On 12 August 2015 at 07:21, Simon Glass [off-list ref] wrote:

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Hi Lucas,

On 11 August 2015 at 11:05, Lucas Stach [off-list ref] wrote:

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Hi Simon,

why did you send this to the Tegra ML?

Am Dienstag, den 11.08.2015, 08:25 -0600 schrieb Simon Glass:

quoted

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.

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Signed-off-by: Simon Glass <redacted>
---

 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.

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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.

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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?

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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?

Regards,
Simon
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