linux-kernel - Re: [PATCH v2 0/2] dma-buf: heaps: Use constant name for CMA heap

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Message-ID: <CAO_48GELW3ax5Q3h9=qpWBJJa0Uy3eJwFkEcbaz4ZT56Gq513A@mail.gmail.com>
Date: Wed, 23 Apr 2025 20:53:20 +0530
From: Sumit Semwal <sumit.semwal@...aro.org>
To: Jared Kangas <jkangas@...hat.com>
Cc: benjamin.gaignard@...labora.com, Brian.Starkey@....com, jstultz@...gle.com, 
	tjmercier@...gle.com, christian.koenig@....com, mripard@...nel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] dma-buf: heaps: Use constant name for CMA heap

Hello Jared,

On Wed, 23 Apr 2025 at 00:49, Jared Kangas <jkangas@...hat.com> wrote:
>
> Hi all,
>
> This patch series is based on a previous discussion around CMA heap
> naming. [1] The heap's name depends on the device name, which is
> generally "reserved", "linux,cma", or "default-pool", but could be any
> arbitrary name given to the default CMA area in the devicetree. For a
> consistent userspace interface, the series introduces a constant name
> for the CMA heap, and for backwards compatibility, an additional Kconfig
> that controls the creation of a legacy-named heap with the same CMA
> backing.
>
> The ideas to handle backwards compatibility in [1] are to either use a
> symlink or add a heap node with a duplicate minor. However, I assume
> that we don't want to create symlinks in /dev from module initcalls, and
> attempting to duplicate minors would cause device_create() to fail.
> Because of these drawbacks, after brainstorming with Maxime Ripard, I
> went with creating a new node in devtmpfs with its own minor. This
> admittedly makes it a little unclear that the old and new nodes are
> backed by the same heap when both are present. The only approach that I
> think would provide total clarity on this in userspace is symlinking,
> which seemed like a fairly involved solution for devtmpfs, but if I'm
> wrong on this, please let me know.

Thanks indeed for this patch; just one minor nit: the link referred to
as [1] here seems to be missing. Could you please add it? This would
make it easier to follow the chain of discussion in posterity.
>
> Changelog:
>     v2: Use tabs instead of spaces for large vertical alignment.
>
> Jared Kangas (2):
>   dma-buf: heaps: Parameterize heap name in __add_cma_heap()
>   dma-buf: heaps: Give default CMA heap a fixed name
>
>  Documentation/userspace-api/dma-buf-heaps.rst | 11 ++++---
>  drivers/dma-buf/heaps/Kconfig                 | 10 +++++++
>  drivers/dma-buf/heaps/cma_heap.c              | 30 ++++++++++++++-----
>  3 files changed, 40 insertions(+), 11 deletions(-)
>
> --
> 2.49.0
>

Best,
Sumit