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.

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

Hello Jared,

On Wed, 23 Apr 2025 at 00:49, Jared Kangas <jkangas@redhat.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

Hi Sumit,

On Wed, Apr 23, 2025 at 08:53:20PM +0530, Sumit Semwal wrote:
> Hello Jared,
> 
> On Wed, 23 Apr 2025 at 00:49, Jared Kangas <jkangas@redhat.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.

My bad, I must have dropped the link while revising the cover letter.
Here's the dropped reference:

[1]: https://lore.kernel.org/all/f6412229-4606-41ad-8c05-7bbba2eb6e08@ti.com/

Thanks to you and John for looking this over,
Jared

> >
> > 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
>