linux-kernel - Re: [PATCH v2 2/4] dt-bindings: timer: exynos4210-mct: Support using only local timer

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Date:   Tue, 8 Mar 2022 15:44:48 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        tglx@...utronix.de, daniel.lezcano@...aro.org
Cc:     kernel@...s.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, devicetree@...r.kernel.org,
        alim.akhtar@...sung.com, robh+dt@...nel.org
Subject: Re: [PATCH v2 2/4] dt-bindings: timer: exynos4210-mct: Support using
 only local timer

On 08/03/2022 15:24, Vincent Whitchurch wrote:
> The ARTPEC-8 SoC has a quad-core Cortex-A53 and a single-core Cortex-A5
> which share one MCT with one global and eight local timers.  The
> Cortex-A53 and Cortex-A5 do not have cache-coherency between them, and
> therefore run two separate kernels.
> 
> The Cortex-A53 boots first and starts the global FRC and also registers
> a clock events device using the global timer.  (This global timer clock
> events is usually replaced by arch timer clock events for each of the
> cores.)
> 
> When the A5 boots, we should not use the global timer interrupts or
> write to the global timer registers.  This is because even if there are
> four global comparators, the control bits for all four are in the same
> registers, and we would need to synchronize between the cpus.  Instead,
> the global timer FRC (already started by the A53) should be used as the
> clock source, and one of the local timers which are not used by the A53
> can be used for clock events on the A5.
> 
> To support this usecase, add a property to the binding to specify the
> first local timer index to be used. If this parameter is non-zero, the
> global timer interrupts will also not be used.
> 
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> ---
> 
> Notes:
>     v2: New.
> 
>  .../bindings/timer/samsung,exynos4210-mct.yaml           | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/timer/samsung,exynos4210-mct.yaml b/Documentation/devicetree/bindings/timer/samsung,exynos4210-mct.yaml
> index dce42f1f7574..46f466081836 100644
> --- a/Documentation/devicetree/bindings/timer/samsung,exynos4210-mct.yaml
> +++ b/Documentation/devicetree/bindings/timer/samsung,exynos4210-mct.yaml
> @@ -47,6 +47,15 @@ properties:
>    reg:
>      maxItems: 1
>  
> +  local-timer-index:

You need vendor prefix. Also this should describe the actual hardware,
not driver behavior, so rather:
"samsung,local-timers"
with a uint32-array type and list of timers to use.

You also need separate property to skip FRC, so something like:
"samsung,frc-shared"
of type boolean.

In the bindings please describe the hardware, not the result you want to
achieve from driver model point of view.

Also disallow this for all other compatibles:
allOf:
 - if:
     not:
       properties:
       ...
   then:
     properties:
       samsung,local-timers: false
       samsung,frc-shared: false

The property simply should not be used outside of Artpec8. It's not
valid in other configurations.

> +    $ref: /schemas/types.yaml#/definitions/uint32
> +    default: 0
> +    maximum: 15     # Last local timer index
> +    description: |
> +      If present, sets the first local timer index to use.  If this value is
> +      set to a non-default value, the global timer will not be used for
> +      interrupts.

Do not describe the driver, but the hardware. Instead explain which
local timers are allowed to be used.

> +
>    interrupts:
>      description: |
>        Interrupts should be put in specific order. This is, the local timer


Best regards,
Krzysztof