On 12/3/24 1:44 AM, Maxime Ripard wrote:

Hi John,

On Mon, Dec 02, 2024 at 11:12:23AM -0800, John Stultz wrote:

...

On Mon, Dec 2, 2024 at 3:58 AM Maxime Ripard mripard@kernel.org wrote:

...

Following a recent discussion at last Plumbers, John Stultz, Sumit Sewal, TJ Mercier and I came to an agreement that we should document what the dma-buf heaps names are expected to be, and what the buffers attributes you'll get should be documented.

Let's create that doc to make sure those attributes and names are guaranteed going forward.

Signed-off-by: Maxime Ripard mripard@kernel.org

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Changes from v1:

• Add the mention that the cma / reserved heap is optional.

To: Jonathan Corbet corbet@lwn.net To: Sumit Semwal sumit.semwal@linaro.org Cc: Benjamin Gaignard benjamin.gaignard@collabora.com Cc: Brian Starkey Brian.Starkey@arm.com Cc: John Stultz jstultz@google.com Cc: "T.J. Mercier" tjmercier@google.com Cc: "Christian König" christian.koenig@amd.com Cc: dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org Cc: linux-media@vger.kernel.org Cc: linux-doc@vger.kernel.org

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Documentation/userspace-api/dma-buf-heaps.rst | 76 +++++++++++++++++++ Documentation/userspace-api/index.rst | 1 + 2 files changed, 77 insertions(+) create mode 100644 Documentation/userspace-api/dma-buf-heaps.rst

diff --git a/Documentation/userspace-api/dma-buf-heaps.rst b/Documentation/userspace-api/dma-buf-heaps.rst new file mode 100644 index 000000000000..68be7ddea150 --- /dev/null +++ b/Documentation/userspace-api/dma-buf-heaps.rst @@ -0,0 +1,76 @@ +.. SPDX-License-Identifier: GPL-2.0

+============================== +Allocating dma-buf using heaps +==============================

+Dma-buf Heaps are a way for userspace to allocate dma-buf objects. They are +typically used to allocate buffers from a specific allocation pool, or to share +buffers across frameworks.

+Heaps +=====

+A heap represent a specific allocator. The Linux kernel currently supports the +following heaps:

• • The ``system`` heap allocates virtually contiguous, cacheable, buffers
• • The ``reserved`` heap allocates physically contiguous, cacheable,

So, I think calling the heap name the "cma" heap might be clearer here? On some platforms the chardev is "reserved" because the default cma region is setup via dma_contiguous_reserve_area() (which calls it "reserved"), but the dma heap is exporting cma.

Ok, I'll change it

...

Additionally, on some downstream devices there can be multiple purpose separated CMA heaps. This hasn't made it upstream over objections to how downstream devices were enumerating the separate cma regions in their devicetrees. It was proposed to export all cma regions as separate cma heaps, but there was reasonable concern over kernel drivers not expecting userland to be able to allocate from special reservations, so that seemed like a bad idea.

Ideally upstream drivers would want to explicitly export their special CMA region themselves and we would export something like add_cma_heap() to do so.

All to say: this document really only covers the naming of the "default" CMA region that is exported via the cma heap. Eventually there may be other CMA heaps as well upstream.

Sure. My point was to follow-up on the discussion we had at Plumbers that concluded by something along the lines of: heap names provide the semantics and guarantee over how buffers are allocated, and thus we need to document what each name is providing.

If we were to add new ones, we would obviously extend that list to accomodate those changes.

...

...

• buffers. Only present if a CMA region is present. Such a region is
• 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 differently:
• • Acer Iconia Tab A500: ``linux,cma``
• • Allwinner sun4i, sun5i and sun7i families: ``default-pool``
• • Amlogic A1: ``linux,cma``
• • Amlogic G12A/G12B/SM1: ``linux,cma``
• • Amlogic GXBB/GXL: ``linux,cma``
• • ASUS EeePad Transformer TF101: ``linux,cma``
• • ASUS Google Nexus 7 (Project Bach / ME370TG) E1565: ``linux,cma``
• • ASUS Google Nexus 7 (Project Nakasi / ME370T) E1565: ``linux,cma``
• • ASUS Google Nexus 7 (Project Nakasi / ME370T) PM269: ``linux,cma``
• • Asus Transformer Infinity TF700T: ``linux,cma``
• • Asus Transformer Pad 3G TF300TG: ``linux,cma``
• • Asus Transformer Pad TF300T: ``linux,cma``
• • Asus Transformer Pad TF701T: ``linux,cma``
• • Asus Transformer Prime TF201: ``linux,cma``
• • ASUS Vivobook S 15: ``linux,cma``
• • Cadence KC705: ``linux,cma``
• • Digi International ConnectCore 6UL: ``linux,cma``
• • Freescale i.MX8DXL EVK: ``linux,cma``
• • Freescale TQMa8Xx: ``linux,cma``
• • Hisilicon Hikey: ``linux,cma``
• • Lenovo ThinkPad T14s Gen 6: ``linux,cma``
• • Lenovo ThinkPad X13s: ``linux,cma``
• • Lenovo Yoga Slim 7x: ``linux,cma``
• • LG Optimus 4X HD P880: ``linux,cma``
• • LG Optimus Vu P895: ``linux,cma``
• • Loongson 2k0500, 2k1000 and 2k2000: ``linux,cma``
• • Microsoft Romulus: ``linux,cma``
• • NXP i.MX8ULP EVK: ``linux,cma``
• • NXP i.MX93 9x9 QSB: ``linux,cma``
• • NXP i.MX93 11X11 EVK: ``linux,cma``
• • NXP i.MX93 14X14 EVK: ``linux,cma``
• • NXP i.MX95 19X19 EVK: ``linux,cma``
• • Ouya Game Console: ``linux,cma``
• • Pegatron Chagall: ``linux,cma``
• • PHYTEC phyCORE-AM62A SOM: ``linux,cma``
• • PHYTEC phyCORE-i.MX93 SOM: ``linux,cma``
• • Qualcomm SC8280XP CRD: ``linux,cma``
• • Qualcomm X1E80100 CRD: ``linux,cma``
• • Qualcomm X1E80100 QCP: ``linux,cma``
• • RaspberryPi: ``linux,cma``
• • Texas Instruments AM62x SK board family: ``linux,cma``
• • Texas Instruments AM62A7 SK: ``linux,cma``
• • Toradex Apalis iMX8: ``linux,cma``
• • TQ-Systems i.MX8MM TQMa8MxML: ``linux,cma``
• • TQ-Systems i.MX8MN TQMa8MxNL: ``linux,cma``
• • TQ-Systems i.MX8MPlus TQMa8MPxL: ``linux,cma``
• • TQ-Systems i.MX8MQ TQMa8MQ: ``linux,cma``
• • TQ-Systems i.MX93 TQMa93xxLA/TQMa93xxCA SOM: ``linux,cma``
• • TQ-Systems MBA6ULx Baseboard: ``linux,cma``

Hrm. I'm not sure I see the value in enumerating things in this way, it seems like it will be a nuisance to keep current? Maybe something like:

On most systems the default cma region is named "linux, cma" or "reserved", with a few exceptions: - Allwinner sun4i, sun5i and sun7i families: ``default-pool``

I'm a bit worried about doing so. What if, on a "linux,cma" system, we have another "reserved" heap created with different semantics?

Having the "default CMA" heap get its dev name based on the method that created it was arguably a mistake made when first upstreaming this heap. We should fix this, then maybe add the old name as a link just for backwards compat as needed.

exp_info.name = "default_cma";

All other CMA and carveout heaps will have names based on their method of creation as there may be multiple of them, but there will only every be one "default CMA" area, and its heap should be named to match.

Andrew