linux-kernel - Re: [PATCH v2] arm64: dts: ti: k3-j721e: Add support for DFS in J721E A72

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Message-ID: <20240104151650.my3cuhgase4yf7gj@luminance>
Date: Thu, 4 Jan 2024 09:16:50 -0600
From: Nishanth Menon <nm@...com>
To: Neha Malcom Francis <n-francis@...com>
CC: <robh+dt@...nel.org>, <krzysztof.kozlowski+dt@...aro.org>,
        <vigneshr@...com>, <linux-arm-kernel@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <a-nandan@...com>, <kristo@...nel.org>, <u-kumar1@...com>
Subject: Re: [PATCH v2] arm64: dts: ti: k3-j721e: Add support for DFS in
 J721E A72

On 16:49-20240104, Neha Malcom Francis wrote:
> Add 2G, 1G, 500M and 250M as the supported frequencies for A72. This
> enables support for Dynamic Frequency Scaling (DFS). Note that Dynamic
> Voltage and Frequency Scaling (DVFS) is not supported on J7 devices.
> 
> J721E SoC has three different speed grade devices (see [1], 7.5
> Operating Performance Points) which as of today are indiscernible in
> software, users of a different speed grade device must manually change
> the DTS to ensure their maximum speed frequency is supported.
> 
> To obtain clock-latency-ns, the maximum time was found to switch from/to
> any frequency for a CPU and this value was rounded off and set.
> 
> [1] https://www.ti.com/lit/gpn/tda4vm
> 
> Signed-off-by: Neha Malcom Francis <n-francis@...com>
> ---
> Test and boot logs:
> https://gist.github.com/nehamalcom/33608837ab5ad3332ff11a7fa7a602e2
> 
> Changes since v1:
> https://lore.kernel.org/all/20231214075637.176586-1-n-francis@ti.com/
> - removed OPPs 1.5G and 750M as they introduced boot regression in
>   J721E-SK

I do not think this is the right approach precisely for the above
reason.

See my comment in V1: https://lore.kernel.org/all/20231214125130.zqtq6ioj4c533wha@elbow/

"
I am also concerned if the table should be separated out as a dtsi and
included at board.dts level to prevent downstream users going crazy..
"

I suspect there is no magic opp configuration that will work with all
downstream and board variations. instead of creating a trimmed down
non-datasheet tuples of OPP configuration, use the data sheet provided
OPP configurations into each dtsi and the boards can apply the dtsi
based on the type of sample they have.

I don't see any other scheme (overlays, maybe?).. but this approach is
broken and your note above proves why this approach is broken.

> - Nishanth
> 	- indicated DVFS not supported in commit message
> 	- moved critical data sheet info from below tear line to commit
> 	  message
> 	- added opp-shared property
> 	- added clock-latency-ns property
> 
>  arch/arm64/boot/dts/ti/k3-j721e.dtsi | 31 ++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/arch/arm64/boot/dts/ti/k3-j721e.dtsi b/arch/arm64/boot/dts/ti/k3-j721e.dtsi
> index a200810df54a..5de6c70bd989 100644
> --- a/arch/arm64/boot/dts/ti/k3-j721e.dtsi
> +++ b/arch/arm64/boot/dts/ti/k3-j721e.dtsi
> @@ -48,6 +48,9 @@ cpu0: cpu@0 {
>  			d-cache-line-size = <64>;
>  			d-cache-sets = <256>;
>  			next-level-cache = <&L2_0>;
> +			clocks = <&k3_clks 202 2>;
> +			clock-names = "cpu";
> +			operating-points-v2 = <&cpu0_opp_table>;
>  		};
>  
>  		cpu1: cpu@1 {
> @@ -62,6 +65,34 @@ cpu1: cpu@1 {
>  			d-cache-line-size = <64>;
>  			d-cache-sets = <256>;
>  			next-level-cache = <&L2_0>;
> +			clocks = <&k3_clks 203 0>;
> +			clock-names = "cpu";
> +			operating-points-v2 = <&cpu0_opp_table>;
> +		};
> +	};
> +
> +	cpu0_opp_table: opp-table {
> +		compatible = "operating-points-v2";
> +		opp-shared;
> +
> +		opp6-2000000000 {
> +			opp-hz = /bits/ 64 <2000000000>;
> +			clock-latency-ns = <300000>;
> +		};
> +
> +		opp4-1000000000 {
> +			opp-hz = /bits/ 64 <1000000000>;
> +			clock-latency-ns = <300000>;
> +		};
> +
> +		opp2-500000000 {
> +			opp-hz = /bits/ 64 <500000000>;
> +			clock-latency-ns = <300000>;
> +		};
> +
> +		opp1-250000000 {
> +			opp-hz = /bits/ 64 <250000000>;
> +			clock-latency-ns = <300000>;
>  		};
>  	};
>  
> -- 
> 2.34.1
> 

-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D