On 28/04/2020 18:57, Simon Glass wrote:
Hi,
sorry for the delay, found this, slightly mouldy already, in my draft folder.
First, thanks for the review! I saw the Tom merged this already, but wanted to come back to the DT hacks:
Hi Andre,
On Mon, 27 Apr 2020 at 12:18, Andre Przywara andre.przywara@arm.com wrote:
Even though the PL011 UART driver claims to be DM compliant, it does not really a good job with parsing DT nodes. U-Boot seems to adhere to a non-standard binding, either requiring to have a "skip-init" property in the node, or to have an extra "clock" property holding the base *frequency* value for the baud rate generator. DTs in the U-Boot tree seem to have been hacked to match this requirement.
One problem is that we want a 'debug UART' to work before the clock driver is running, so we want to do the *minimum possible* amount of init to get the UART running. So we don't want to start up driver model, clock drivers, etc.
I understand this very well - having an UART up and running as early as possible is crucial for debugging.
I think we should have useful helpers like the 'clock' property to avoid lots of parsing very early in U-Boot. Of course such things are hard for kernel people to understand / agree to but that doesn't make them wrong.
I agree, but I don't think we should mess around with the DT for this purpose. This is basically a U-Boot requirement or debug feature, not a machine property. And deviating from the official DT binding is not a good idea.
I think for those U-Boot specific debug features we can just have CONFIG options - for instance we already have CONFIG_PL011_CLOCK. Also I strongly believe that skip-init does not belong into the DT. It's a *U-Boot* decision to not *re*-init the UART, not a machine property. There are PL011 compatible UARTs which should *not* be initialised (SBSA-UART), but both TX1 and RPi don't have those, but instead real PL011s. So if we desperately wanted this in the DT, we could actually use compatible = "arm,sbsa-uart", then we don't need any clock at all.
But I was more thinking about turning skip-init into a config symbol and defining this for TX1 and RPi. We do already something similar for the RPi4 in Trusted Firmware [1]. This would allow us to remove the skip-init property from the u-boot.dtsi, and would help with booting with the DT from the SD card instead (for which the GPU firmware puts the pointer to into the beginning of memory [2]).
I have a patch for that already, will send it soonish.
Cheers, Andre
[1] https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=0eda7... [2] https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=c4597...
The official binding does not mention any of these properties, instead recommends a standard "clocks" property to point to the baud base clock.
Some boards use simple "fixed-clock" providers, which U-Boot readily supports, so let's add some simple DM clock code to the PL011 driver to learn the rate of the first clock, as described by the official binding.
These clock nodes seem to be not ready very early in the boot process, so provide a fallback value, by re-using the already existing CONFIG_PL011_CLOCK variable.
Signed-off-by: Andre Przywara andre.przywara@arm.com
drivers/serial/serial_pl01x.c | 47 +++++++++++++++++++++++------------ 1 file changed, 31 insertions(+), 16 deletions(-)
Reviewed-by: Simon Glass sjg@chromium.org
diff --git a/drivers/serial/serial_pl01x.c b/drivers/serial/serial_pl01x.c index 2a5f256184..14040f32ef 100644 --- a/drivers/serial/serial_pl01x.c +++ b/drivers/serial/serial_pl01x.c @@ -12,6 +12,7 @@
Regards, Simon