linux-kernel - Re: [PATCH v2 0/2] dma-buf: heaps: Use constant name for CMA heap

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Message-ID: <aAkN7BXIT7RR85PR@jkangas-thinkpadp1gen3.rmtuswa.csb>
Date: Wed, 23 Apr 2025 08:57:32 -0700
From: Jared Kangas <jkangas@...hat.com>
To: Sumit Semwal <sumit.semwal@...aro.org>
Cc: benjamin.gaignard@...labora.com, Brian.Starkey@....com,
	jstultz@...gle.com, tjmercier@...gle.com, christian.koenig@....com,
	mripard@...nel.org, linux-media@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] dma-buf: heaps: Use constant name for CMA heap

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@...hat.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
>