Re: [PATCH] dt-bindings: arm,gic-v3: require that reserved cells are always 0

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

On Wed, Feb 03, 2016 at 06:00:58PM +0000, Will Deacon wrote:
> The arm,gic-v3 binding was written with good intentions and doesn't
> enforce interrupt-cells to be 3, therefore making it easy to extend
> the irq description in future if necessary:
> 
>   > Cells 4 and beyond are reserved for future use.
> 
> Unfortunately, this sentence is immediately followed up with:
> 
>   > When the 1st cell has a value of 0 or 1, cells 4 and beyond act as
>   > padding, and may be ignored. It is recommended that padding cells
>   > have a value of 0.

The initial intention of this was to allow for new, longer entries which
were identified by a new value in cell 1.

Regardless, I agree that it should be a requirement that padding cells
must be zero.

> Consequently, any extensions to the PPI or SPI interrupt specifiers must
> be able to work with random crap from legacy DTs, effectively
> necessitating a new interrupt type in the first cell. Sigh.

I'm not sure that's true if we allocate a new value for the 1st cell for
"extended" PPI or SPI interrupt-sepcficiers (which presumably add
restrictions). That would also allow old kernels to safely skip such
interrupts rather than mis-parsing them.

> This patch fixes the text so that additional, reserved cells are
> required to be zero. This looks like a reasonable thing to require and
> is already satisifed by the .dts files in-tree.
> 
> Cc: Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>
> Cc: Marc Zyngier <marc.zyngier-5wv7dgnIgG8@public.gmane.org>
> Signed-off-by: Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>

This patch itself makes sense, so FWIW:

Acked-by: Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>

Any proposed extention to the binding requires its own discussion.

Thanks,
Mark.

> ---
>  .../devicetree/bindings/interrupt-controller/arm,gic-v3.txt          | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.txt b/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.txt
> index 7803e77d85cb..007a5b46256a 100644
> --- a/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.txt
> +++ b/Documentation/devicetree/bindings/interrupt-controller/arm,gic-v3.txt
> @@ -24,9 +24,8 @@ Main node required properties:
>  		1 = edge triggered
>  		4 = level triggered
>  
> -  Cells 4 and beyond are reserved for future use. When the 1st cell
> -  has a value of 0 or 1, cells 4 and beyond act as padding, and may be
> -  ignored. It is recommended that padding cells have a value of 0.
> +  Cells 4 and beyond are reserved for future use and must have a value
> +  of 0 if present.
>  
>  - reg : Specifies base physical address(s) and size of the GIC
>    registers, in the following order:
> -- 
> 2.1.4
> 
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