linux-kernel - Re: [PATCH] arm64: dts: ti: k3-am62x-sk-common: Reserve 128MiB of global CMA

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Date: Tue, 14 May 2024 19:59:23 +0530
From: Devarsh Thakkar <devarsht@...com>
To: Andrew Davis <afd@...com>, Nishanth Menon <nm@...com>,
        "Raghavendra,
 Vignesh" <vigneshr@...com>
CC: <kristo@...nel.org>, <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>, <conor+dt@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <a-bhatia1@...com>, <j-luthra@...com>,
        <praneeth@...com>, <j-choudhary@...com>, <gregkh@...uxfoundation.org>,
        Brandon <b-brnich@...com>, "Pothukuchi, Vijay" <vijayp@...com>,
        "Etheridge,
 Darren" <detheridge@...com>
Subject: Re: [PATCH] arm64: dts: ti: k3-am62x-sk-common: Reserve 128MiB of
 global CMA

Hi Andrew, Vignesh, Nishanth,

On 08/08/23 13:44, Devarsh Thakkar wrote:
[...]
>>>>> Reserve 128MiB of global CMA which is also marked as re-usable
>>>>> so that OS can also use the same if peripheral drivers are not using the
>>>>> same.
>>>>>

I wanted to re-initiate discussion on this. Per discussion with few of the
maintainers in OSS 2023 Summit, the suggestion was either to have MMU in the
devices or use the CMA region as done with this patch.
There was not much traction for the idea/suggestion to have some updates in
CMA API to dynamically increase the CMA region per the requirement (for e.g.
move the MIGRATE_MOVABLE pages to CMA area dynamically). Also I see some
efforts in the past made on that direction for introducing CMA_AGGRESSIVE
Kconfig to allocate from ZONE_MOVEABLE but were not accepted [1] and
nevertheless it still requires CMA region to be available to begin with and
only gives a bit of extra margin for the CMA.

My vote is still to use global cma region with re-usable flag (as done with
this patch) and 128 MiB is well estimated per the use-cases AM62x EVM is meant
to cater (the vendor specific kernel has been using it) and is very much
needed to provided out-of-box experience as otherwise basic display and camera
use-cases don't work out-of-box. Besides I see many other boards [presumably
mmu-less] using a similar approach :

     $git grep linux,cma-default arch/ | wc
     36      72    2538

Kindly let us know your opinion on this if we are aligned to proceed with this
approach or not.

[1]:
https://lore.kernel.org/all/20141107070655.GA3486@bbox/
https://linux-mm.kvack.narkive.com/LCGSAqAp/patch-0-4-cma-aggressive-make-cma-memory-be-more-aggressive-about-allocation

Regards
Devarsh