linux-kernel - Re: [PATCH 0/4] Add global CMA areas for few TI SoCs

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Message-ID: <09979b2a-73b0-4a74-978c-b082764e777e@ti.com>
Date: Mon, 28 Apr 2025 09:42:59 -0500
From: Andrew Davis <afd@...com>
To: Jayesh Choudhary <j-choudhary@...com>, <nm@...com>, <vigneshr@...com>,
        <linux-kernel@...r.kernel.org>
CC: <robh@...nel.org>, <krzk+dt@...nel.org>, <conor+dt@...nel.org>,
        <kristo@...nel.org>, <u-kumar1@...com>, <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <devarsht@...com>
Subject: Re: [PATCH 0/4] Add global CMA areas for few TI SoCs

On 4/24/25 3:44 AM, Jayesh Choudhary wrote:
> Hello All,
> 
> Following AM62* platforms[0], this patches add global CMA reserve area for:
> - J721S2-EVM (1056 MiB)
> - J784S4-EVM (1920 MiB)
> - AM68-SK (1008 MiB)
> - AM69-SK (1904 MiB)
> 
> These SoCs does not have MMU and hence they require contiguous memory
> pool to support various multimedia usecase.
> 
> The calculation was done considering H264 codec requirements, dual-display
> supported in each platforms and multicamera use-cases.
> Additional buffer was kept for other peripheral s and to account for
> fragmentation.

What if I'm not going to use 8 cameras, 2 displays, and decode 16 H264
streams all at the same time? Why should I always lose 2GB of DRAM
unconditionally, just in-case someone someday runs this imagined
worst-case situation?

If I *do* intend to have my device perform some nightmare use-case like
the above, I can pass the needed CMA size in on the kernel command line.
That is a configuration for my specific use-case after all, and DT is
*not* for configuration, especially not insane configurations like this.

Andrew

> The breakdown is mentioned in each commit message.
> 
> [0]: https://lore.kernel.org/all/20240613150902.2173582-1-devarsht@ti.com/
> 
> Jayesh Choudhary (4):
>    arm64: dts: ti: k3-j721s2-som-p0: Reserve 1056MiB of global CMA
>    arm64: dts: ti: k3-j784s4-j742s2-evm-common: Reserve 1920MiB of global
>      CMA
>    arm64: dts: ti: k3-am68-sk-som: Reserve 1008MiB of global CMA
>    arm64: dts: ti: k3-am69-sk: Reserve 1904MiB of global CMA
> 
>   arch/arm64/boot/dts/ti/k3-am68-sk-som.dtsi              | 8 ++++++++
>   arch/arm64/boot/dts/ti/k3-am69-sk.dts                   | 8 ++++++++
>   arch/arm64/boot/dts/ti/k3-j721s2-som-p0.dtsi            | 8 ++++++++
>   arch/arm64/boot/dts/ti/k3-j784s4-j742s2-evm-common.dtsi | 8 ++++++++
>   4 files changed, 32 insertions(+)
>