Re: device tree on Pi4B

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On 8/4/22 02:46 AM, Peter Robinson wrote:
On Wed, Aug 3, 2022 at 8:04 PM Steven A. Falco <stevenfalco@xxxxxxxxx> wrote:

On 8/3/22 02:24 PM, Peter Robinson wrote:
On Wed, Aug 3, 2022 at 4:17 PM Steven A. Falco <stevenfalco@xxxxxxxxx> wrote:

I have a Pi4B that is working pretty well with the aarch64 version of Fedora Rawhide, but I have one issue that I haven't been able to find a solution to.

I have an Adafruit RTC connected via I2C (  If I use the kernel device tree, this device is not found.

I then tried switching to the firmware device tree via the instructions in  That works pretty well - I can now add an overlay for the RTC in /boot/efi/config.txt and the RTC is found during boot.  dtoverlay=i2c-rtc,pcf8523,addr=0x68

However, once I've switched to the firmware device tree, I no longer have /dev/vchiq, which means that I can no longer use vcgencmd.  Instead, vcgencmd gives me "VCHI initialization failed".

It should be the same as required in Raspbian. Have you checked the
details in /boot/efi/overlays/README? Out of interest what do you use
/dev/vchiq / vcgencmd for?

I've looked at the README, but it doesn't mention VCHI, so I'm not sure how to enable VCHI when using the firmware DT.  I'll look at the kernel DT source - perhaps that gives clues as to how to create an overlay that I could then use with the firmware DT.

I'll take a look when I get a few cycles.

Turns out the vchiq_probe code in vchiq_arm.c is looking for either "brcm,bcm2835-vchiq" or "brcm,bcm2836-vchiq", while the FW DT is supplying "brcm,bcm2711-vchiq".  I attached a patch that lets me insert the vchiq driver; the driver in turn creates the /dev/vchiq node, and I am then able to use vcgencmd.

I have to manually modprobe the driver when using the FW DT.  The driver is automatically inserted during boot when using the kernel DT.  I haven't tracked that part down yet...

One interesting thing is that the kernel DT shows the RTC on bus 3, but the firmware DT shows the RTC on bus 1.

Three's different "modes" (for want of a better description) where the
firmware steals buses for it's own use.

BTW, I'm using vcgencmd to measure the temperature and to check the "get_throttled" flag to detect voltage dips.

You can also use tmon in the kernel-tools package to monitor temp,
thermal trip points and the like.

Is there any way to get both the RTC and vcgencmd to work?  Which device tree would I choose?  Do I have to build a custom kernel or is there some other way to enable the RTC with the kernel device tree?

You don't need a custom kernel but you'd need to build your own DT as
obviously anything that is not default on the RPi devices themselves
is optional hence the requirement of overlays.

Understood.  I don't know which is better - the firmware DT or the kernel DT.  I guess the FW DT has the advantage that it is easy to turn stuff on/off, but the kernel DT probably better matches the drivers.

For the later point, not really, they are slowly converging closer
together. Fedora as a whole is moving towards SystemReady IR style of
DT where it's provided by the firmware and I hope to stop shipping the
kernel ones all together.

And of course the last thing I want is to have to regenerate a custom DT each time the kernel is updated.

You shouldn't need to do that for each kernel.
--- test_kernel/builddir/kernel_build/BUILD/kernel-5.19/linux-5.19.0-65.local.fc37.aarch64/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c	2022-07-31 17:03:01.000000000 -0400
+++ /home/sfalco/vchiq_arm.c	2022-08-12 11:10:23.609220349 -0400
@@ -81,6 +81,10 @@
 	.cache_line_size = 64,
+static struct vchiq_drvdata bcm2711_drvdata = {
+	.cache_line_size = 64,
 struct vchiq_arm_state {
 	/* Keepalive-related data */
 	struct task_struct *ka_thread;
@@ -1761,6 +1765,7 @@
 static const struct of_device_id vchiq_of_match[] = {
 	{ .compatible = "brcm,bcm2835-vchiq", .data = &bcm2835_drvdata },
 	{ .compatible = "brcm,bcm2836-vchiq", .data = &bcm2836_drvdata },
+	{ .compatible = "brcm,bcm2711-vchiq", .data = &bcm2711_drvdata },
 MODULE_DEVICE_TABLE(of, vchiq_of_match);
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