[PATCH v2] Documentation: dma-buf: heaps: Add heap name definitions

Tue Dec 3 07:44:25 UTC 2024

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 at 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 at kernel.org>
> >
> > ---
> > Changes from v1:
> >   - Add the mention that the cma / reserved heap is optional.
> >
> > To: Jonathan Corbet <corbet at lwn.net>
> > To: Sumit Semwal <sumit.semwal at linaro.org>
> > Cc: Benjamin Gaignard <benjamin.gaignard at collabora.com>
> > Cc: Brian Starkey <Brian.Starkey at arm.com>
> > Cc: John Stultz <jstultz at google.com>
> > Cc: "T.J. Mercier" <tjmercier at google.com>
> > Cc: "Christian König" <christian.koenig at amd.com>
> > Cc: dri-devel at lists.freedesktop.org
> > Cc: linaro-mm-sig at lists.linaro.org
> > Cc: linux-media at vger.kernel.org
> > Cc: linux-doc at vger.kernel.org
> > ---
> >  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?

Maxime
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