RPI CM4 - I2C BUS and Device Tree

[Simon Glass]

Hi Romain,

On Mon, 19 Aug 2024 at 01:15, Romain Crausaz <crausaz.romain at gmail.com> wrote:
>
> Hi Simon, Hi Peter,
>
> First of all, thanks a lot for your answers.
>
> 1. When I enable the i2c bus in the config.txt (which is parsed by the rpi first stage bootloader), the overlay put this as compatible:"brcm,bcm2835". I checked in the source of U-Boot and the driver is not available.
> 2. One solution could be to use the i2c-gpio driver instead of the default i2c controller. U-Boot has this driver.
> 3. Since the device tree passed by the rpi bootloader is stored in the variable fw_dtb_pointer (see the lowlevel_init.S) would I be able to do the following:
>     3.1 remove the automatic selection for the device tree passed by the rpi bootload
>     3.2 system starts and use the default device tree where the i2c bus is defined with i2c-gpio, access the device I need and store the information needed
>     3.3 get the device tree address from fw_dtb_pointer, set fdt address fdt addr with the new device tree and then start the kernel
>
> I have another question, what triggers the DM to check for the device (with dm tree command there is a X under the column probed). Because here we could have a really simple device tree with the minimum configuration for just the i2c and load it from U-Boot, perform the action needed and then boot the kernel by passing the device tree provided by the first stage bootloder.

Just a little niggle, please avoid top-posting as it messes up the
history for later readers.

U-Boot devices are probed when used. So you could use the 'i2c'
command to make it be probed.

Regards,
Simon



>
> Regards
> Romain
>
>
>
> Le sam. 17 août 2024 à 17:57, Simon Glass <sjg at chromium.org> a écrit :
>>
>> +Peter Robinson
>> +Matthias Brugger
>>
>> Hi Romain,
>>
>> On Sat, 17 Aug 2024 at 07:01, Romain Crausaz <crausaz.romain at gmail.com> wrote:
>> >
>> > Hi all,
>> >
>> >
>> >
>> > I am currently facing an issue with U-Boot and a raspberry pi CM4. Here is
>> > the setup:
>> >
>> >
>> >
>> > 1. Yocto Scarthgap
>> >
>> > 2. U-Boot 2024.01
>> >
>> > 3. rpi_arm64_defconfig
>> >
>> >
>> >
>> > I am trying to access the i2c bus from U-Boot. I have been trying to modify
>> > the device tree which is referenced as
>> > “CONFIG_DEFAULT_DEVICE_TREE="bcm2711-rpi-4-b”” in the defconfig without
>> > success. I can remove everything, set all nodes to disabled and I see no
>> > change in U-Boot (I am using dm commands to check the status of the
>> > peripherals).
>> >
>> >
>> >
>> > It seems that with the rpi, U-Boot will only configure itself against the
>> > device tree passed by the bootloader of RPI. Could someone confirm this ?
>>
>> Yes. It isn't quite obvious, but this is a clue:
>>
>> Core:  211 devices, 16 uclasses, devicetree: board
>>
>> 'board' means that the board can do whatever it wants.
>>
>> >
>> >
>> >
>> > Is there a way to configure / modify U-Boot to use its own devicetree and
>> > simply pass the device tree generated by the rpi bootloader to the kernel ?
>> >
>> >
>> >
>> > Because the second issue is that the i2c driver defined in the device tree
>> > passed to u-boot is not available in u-boot.
>> >
>> >
>> >
>> > Thanks for your help
>>
>> Since OF_HAS_PRIOR_STAGE is a hidden option, you cannot disable it in
>> the defconfig. The only way I know of is to comment out the 'imply
>> OF_HAS_PRIOR_STAGE' in ARCH_BCM283X in arch/arm/Kconfig
>>
>> For me, that stops the rpi_4 booting, so perhaps the devicetree is
>> out-of-date or ft_board_setup() is doing something strange, or
>> something else. The in-tree devicetree may have got out-of-date, so
>> perhaps use the Linux one.
>>
>> Regards,
>> Simon