linux-kernel - Re: [PATCH] external references for device tree overlays

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Date:   Tue, 6 Jun 2017 00:20:15 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     Stefani Seibold <stefani.seibold.ext@...wei.com>,
        Stefani Seibold <stefani@...bold.net>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Holm Rauchfuss <holm.rauchfuss@...wei.com>
Subject: Re: [PATCH] external references for device tree overlays

On 06/05/17 05:59, Stefani Seibold wrote:
> From: Stefani Seibold <stefani@...bold.net>
> 
> This patch enables external references for symbols which are not
> exported by the current device tree. For example
> 
> // RASPI example (only for testing)
> /dts-v1/;
> /plugin/;
> 
> / {
>     compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";
> 
>     fragment@0 {
>         target-path = "/soc/i2s@...03000";
>         __overlay__ {
>             #address-cells = <0x00000001>;
>             #size-cells = <0x00000001>;
>             test = "test";
>             timer = <&timer>;
>         };
>     };
> 
>     __external_symbols__ {
>         timer = "/soc/timer@...03000";
>     };
> };

My hope is that the dtc compiler will stop supporting specification of the
__symbols__ node in dts source, and only generate it automatically in the dtb.
That change to dtc would not allow any node name specified in a dts to begin
with an underscore.  Thus node __external_symbols__ would not be allowed.


> 
> The "timer" symbol is not exported by the RASPI device tree, because it is
> missing in the __symbols__ section of the device tree.
> 
> In case of the RASPI device tree this could be simple fixed by modifing
> the device tree source, but when the device tree is provided by a closed
> source BIOS this kind of missing symbol could not be fixed.

Is there a real example of this issue, or is this a theoretical concern?
If this is a real example, we should be discouraging such behavior.

The suggestion by Pantelis should work, but that is just a hack to get
you out of a bad situation, not a good practice.

> 
> An additional benefit is to override a (possible broken) symbol exported
> by the currect live device tree.
> 
> The patch is based and tested on linux 4.12-rc3.
> 
> Signed-off-by: Stefani Seibold <stefani.seibold.ext@...wei.com>
> Signed-off-by: Stefani Seibold <stefani@...bold.net>
> ---
>  drivers/of/overlay.c  | 19 +++++++++++++++++++
>  drivers/of/resolver.c | 27 ++++++++++++++++++++++-----
>  2 files changed, 41 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> index 7827786718d8..de6516ea0fcd 100644
> --- a/drivers/of/overlay.c
> +++ b/drivers/of/overlay.c

< snip >

-Frank