linux-kernel - Re: [PATCH v2] Documentation: dma-buf: heaps: Add naming guidelines

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Message-ID: <2bc21d27-c7e9-4b74-bae5-d10ad0963e70@infradead.org>
Date: Mon, 16 Jun 2025 09:17:03 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Maxime Ripard <mripard@...nel.org>, Sumit Semwal
 <sumit.semwal@...aro.org>,
 Benjamin Gaignard <benjamin.gaignard@...labora.com>,
 Brian Starkey <Brian.Starkey@....com>, John Stultz <jstultz@...gle.com>,
 "T.J. Mercier" <tjmercier@...gle.com>, Jonathan Corbet <corbet@....net>
Cc: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Jared Kangas <jkangas@...hat.com>,
 Mattijs Korpershoek <mkorpershoek@...nel.org>,
 Bagas Sanjaya <bagasdotme@...il.com>
Subject: Re: [PATCH v2] Documentation: dma-buf: heaps: Add naming guidelines



On 6/16/25 8:21 AM, Maxime Ripard wrote:
> We've discussed a number of times of how some heap names are bad, but
> not really what makes a good heap name.
> 
> Let's document what we expect the heap names to look like.
> 
> Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>
> Signed-off-by: Maxime Ripard <mripard@...nel.org>
> ---
> Changes in v2:
> - Added justifications for each requirement / suggestions
> - Added a mention and example of buffer attributes
> - Link to v1: https://lore.kernel.org/r/20250520-dma-buf-heap-names-doc-v1-1-ab31f74809ee@kernel.org
> ---
>  Documentation/userspace-api/dma-buf-heaps.rst | 38 +++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/Documentation/userspace-api/dma-buf-heaps.rst b/Documentation/userspace-api/dma-buf-heaps.rst
> index 535f49047ce6450796bf4380c989e109355efc05..835ad1c3a65bc07b6f41d387d85c57162909e859 100644
> --- a/Documentation/userspace-api/dma-buf-heaps.rst
> +++ b/Documentation/userspace-api/dma-buf-heaps.rst
> @@ -21,5 +21,43 @@ following heaps:
>     usually created either through the kernel commandline through the
>     `cma` parameter, a memory region Device-Tree node with the
>     `linux,cma-default` property set, or through the `CMA_SIZE_MBYTES` or
>     `CMA_SIZE_PERCENTAGE` Kconfig options. Depending on the platform, it
>     might be called ``reserved``, ``linux,cma``, or ``default-pool``.
> +
> +Naming Convention
> +=================
> +
> +``dma-buf`` heaps name should meet a number of constraints:
> +

For these points below, I would s/That name/The name/ (3 places).

> +- That name must be stable, and must not change from one version to the
> +  other. Userspace identifies heaps by their name, so if the names ever
> +  changes, we would be likely to introduce regressions.

     change,

> +
> +- That name must describe the memory region the heap will allocate from,
> +  and must uniquely identify it in a given platform. Since userspace
> +  applications use the heap name as the discriminant, it must be able to
> +  tell which heap it wants to use reliably if there's multiple heaps.

                                              if there are

> +
> +- That name must not mention implementation details, such as the
> +  allocator. The heap driver will change over time, and implementation
> +  details when it was introduced might not be relevant in the future.
> +
> +- The name should describe properties of the buffers that would be
> +  allocated. Doing so will make heap identification easier for
> +  userspace. Such properties are:
> +
> +  - ``cacheable`` / ``uncacheable`` for buffers with CPU caches enabled
> +    or disabled;
> +
> +  - ``contiguous`` for physically contiguous buffers;
> +
> +  - ``protected`` for encrypted buffers not accessible the OS;
> +
> +- The name may describe intended usage. Doing so will make heap
> +  identification easier for userspace applications and users.
> +
> +For example, assuming a platform with a reserved memory region located
> +at the RAM address 0x42000000, intended to allocate video framebuffers,
> +physically contiguous, and backed by the CMA kernel allocator. Good

   ^^^ Not a complete sentence. Change '.' to ',':     allocator, good

> +names would be ``memory@...00000-cacheable-contiguous`` or
> +``video@...00000``, but ``cma-video`` wouldn't.
> 
> ---
> base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494
> change-id: 20250520-dma-buf-heap-names-doc-31261aa0cfe6
> 
> Best regards,

-- 
~Randy