linux-kernel - Re: [RFC PATCH v1] dma-buf: heaps: move the verification of heap

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Message-ID: <CANDhNCq50zPB+TS+_Oo0HY0aUuBAdik2KrC8eJRTygbis293sw@mail.gmail.com>
Date: Mon, 3 Jun 2024 09:01:52 -0700
From: John Stultz <jstultz@...gle.com>
To: hailong.liu@...o.com
Cc: Sumit Semwal <sumit.semwal@...aro.org>, 
	Benjamin Gaignard <benjamin.gaignard@...labora.com>, Brian Starkey <Brian.Starkey@....com>, 
	"T.J. Mercier" <tjmercier@...gle.com>, Christian König <christian.koenig@....com>, 
	21cnbao@...il.com, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1] dma-buf: heaps: move the verification of
 heap_flags to the corresponding heap

On Mon, Jun 3, 2024 at 4:40 AM <hailong.liu@...o.com> wrote:
>
> From: "Hailong.Liu" <hailong.liu@...o.com>
>
> This help module use heap_flags to determine the type of dma-buf,
> so that some mechanisms can be used to speed up allocation, such as
> memory_pool, to optimize the allocation time of dma-buf.

This feels like it's trying to introduce heap specific flags, but
doesn't introduce any details about what those flags might be?

This seems like it would re-allow the old opaque vendor specific heap
flags that we saw in the ION days, which was problematic as different
userspaces would use the same interface with potentially colliding
heap flags with different meanings. Resulting in no way to properly
move to an upstream solution.

With the dma-heaps interface, we're trying to make sure it is well
defined. One can register a number of heaps with different behaviors,
and the heap name is used to differentiate the behavior. Any flags
introduced will need to be well defined and behaviorally consistent
between heaps. That way when an upstream solution lands, if necessary
we can provide backwards compatibility via symlinks.

So I don't think this is a good direction to go for dma-heaps.

It would be better if you were able to clarify what flag requirements
you need, so we can better understand how they might apply to other
heaps, and see if it was something we would want to define as a flag
(see the discussion here for similar thoughts:
https://lore.kernel.org/lkml/CANDhNCoOKwtpstFE2VDcUvzdXUWkZ-Zx+fz6xrdPWTyciVXMXQ@mail.gmail.com/
)

But if your vendor heap really needs some sort of flags argument that
you can't generalize, you can always implement your own dmabuf
exporter driver with whatever ioctl interface you'd prefer.

thanks
-john