[PATCH] Documentation: dma-buf: heaps: Add naming guidelines

[John Stultz]

On Tue, May 20, 2025 at 3:00 AM Maxime Ripard <mripard at kernel.org> 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.
>
> Signed-off-by: Maxime Ripard <mripard at kernel.org>
> ---
>  Documentation/userspace-api/dma-buf-heaps.rst | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/Documentation/userspace-api/dma-buf-heaps.rst b/Documentation/userspace-api/dma-buf-heaps.rst
> index 535f49047ce6450796bf4380c989e109355efc05..b24618e360a9a9ba0bd85135d8c1760776f1a37f 100644
> --- a/Documentation/userspace-api/dma-buf-heaps.rst
> +++ b/Documentation/userspace-api/dma-buf-heaps.rst
> @@ -21,5 +21,24 @@ 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
> +=================
> +
> +A good heap name is a name that:
> +
> +- Is stable, and won't change from one version to the other;
> +
> +- Describes the memory region the heap will allocate from, and will
> +  uniquely identify it in a given platform;
> +
> +- Doesn't use implementation details, such as the allocator;
> +
> +- Can describe intended usage.
> +

Thanks again for sending this out. Sorry I've been spotty in some of
the discussions (I'm really trying to step back from this area).

I might only suggest you provide a bit more "why" to the list of
qualities you highlight here, just to communicate more of the
spirit/philosophy of how these might be judged or reviewed in the
future.

> +For example, assuming a platform with a reserved memory region located
> +at the RAM address 0x42000000, intended to allocate video framebuffers,
> +and backed by the CMA kernel allocator. Good names would be
> +`memory at 42000000` or `video at 42000000`, but `cma-video` wouldn't.

The point about avoiding the allocator details, just so I better
understand the criteria: Would distinguishing from a contiguous pool
vs non-contiguous in the name be considered ok? As it's a property of
the buffer returned, and one that is critically important for some
devices. Or do you intend that to be opaque, and the usage->heap
mapping logic to provide that itself? (Effectively avoiding folks from
being able to make potentially problematic assumptions from the name?)

Similarly, how would you suggest naming a heap that provides 32bit
allocations?   Similarly for "protected' allocations, which are being
proposed?

thanks
-john