Re: [PATCH v2 1/3] dt-bindings: reserved-memory: rmtfs: Allow guard pages

[Stephan Gerhold <stephan@gerhold.net>]

On Tue, May 30, 2023 at 04:36:41PM -0700, Bjorn Andersson wrote:
> On some Qualcomm platforms it's required that the rmtfs memory is not
> placed adjacent to allocations performed by other clients. Some
> DeviceTree authors have solved this by reserving the space around
> the region, but this prevents such author to use rely on the OS to place
> the region, through the use of "size" (instead of a fixed location).
> 
> So introduce a flag to indicate that guard pages should be carved at the
> beginning and end of the memory region. The user shall account for the
> two 4k blocks in the defined size.
> 
> Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
> ---
> 
> Changes since v1:
> - Drop qcom,alloc-size in favour of using reserved-memory/size
> - Introduce explicit property to signal that guard pages should be
>   carved out from this region (rather than always do it in the dynamic
>   case).
> 
>  .../bindings/reserved-memory/qcom,rmtfs-mem.yaml           | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/qcom,rmtfs-mem.yaml b/Documentation/devicetree/bindings/reserved-memory/qcom,rmtfs-mem.yaml
> index bab982f00485..26e24e7b08cf 100644
> --- a/Documentation/devicetree/bindings/reserved-memory/qcom,rmtfs-mem.yaml
> +++ b/Documentation/devicetree/bindings/reserved-memory/qcom,rmtfs-mem.yaml
> @@ -26,6 +26,13 @@ properties:
>      description: >
>        identifier of the client to use this region for buffers
>  
> +  qcom,use-guard-pages:
> +    type: boolean
> +    description: >
> +      Indicates that the OS should ignore the first and last 4k block of the
> +      memory region, for the purpose of ensuring that the allocation is not
> +      adjacent to other protected regions.
> +

The name "page(s)" is kind of ambiguous nowadays given that systems are
configured with a variety of page sizes other than 4k.

I wonder if it would be more clear to add the actual size of the guard
pages to the device tree, e.g.

	rmtfs {
		compatible = "qcom,rmtfs-mem";
		size = <(2*1024*1024 + 2*4096)>;
		qcom,guard-size = <4096>;
	};

This could also handle a different padding - in the unlikely case that
this is needed on some platform...

It's probably also fine the way you added it though, this is just
a random thought I had. :)

Thanks,
Stephan