Hello,

[expanded Cc: to include William Zhang and Florian Fainelli]

On Mon, Apr 11, 2022 at 12:59:39AM +0200, Aurelien Jarno wrote:
> Add the '-@' DTC option for the Raspberry Pi devices. This option
> populates the '__symbols__' node that contains all the necessary symbols
> for supporting device-tree overlays (for instance from the firmware or
> the bootloader) on these devices.
> 
> The Rasbperry Pi devices are well known for their GPIO header, that
> allow various "HATs" or other modules do be connected and this enables
> users to create out-of-tree device-tree overlays for these modules.
> 
> Please note that this change does increase the size of the resulting DTB
> by ~40%. For example, with v5.17 increase in size is as follows:
> 
>   bcm2711-rpi-400.dtb       26481 -> 36830 bytes
>   bcm2711-rpi-4-b.dtb       26537 -> 36886 bytes
>   bcm2711-rpi-cm4-io.dtb    26426 -> 36945 bytes
>   bcm2837-rpi-3-a-plus.dtb  14133 -> 19740 bytes
>   bcm2837-rpi-3-b.dtb       14310 -> 20006 bytes
>   bcm2837-rpi-3-b-plus.dtb  14670 -> 20474 bytes
>   bcm2837-rpi-cm3-io3.dtb   13680 -> 19266 bytes
> 
> Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
> ---
>  arch/arm64/boot/dts/broadcom/Makefile | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/boot/dts/broadcom/Makefile b/arch/arm64/boot/dts/broadcom/Makefile
> index 5082fcd1fea5..0aa8ee2afdd5 100644
> --- a/arch/arm64/boot/dts/broadcom/Makefile
> +++ b/arch/arm64/boot/dts/broadcom/Makefile
> @@ -1,4 +1,15 @@
>  # SPDX-License-Identifier: GPL-2.0
> +
> +# Enables support for device-tree overlays
> +DTC_FLAGS_bcm2711-rpi-400 := -@
> +DTC_FLAGS_bcm2711-rpi-4-b := -@
> +DTC_FLAGS_bcm2711-rpi-cm4-io := -@
> +DTC_FLAGS_bcm2837-rpi-3-a-plus := -@
> +DTC_FLAGS_bcm2837-rpi-3-b := -@
> +DTC_FLAGS_bcm2837-rpi-3-b-plus := -@
> +DTC_FLAGS_bcm2837-rpi-cm3-io3 := -@
> +DTC_FLAGS_bcm2837-rpi-zero-2-w := -@

Instead of setting these all explicitly a single

DTC_FLAGS := -@

would work, too, without changing how the dts in subdirs are compiled.
IMHO this would be suiteable for arch/arm64/boot/dts/broadcom.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |