Date: Thu, 16 May 2024 14:21:45 +0200
From: Maxime Ripard <mripard@...nel.org>
To: John Stultz <jstultz@...gle.com>
Cc: Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>, 
	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>, Mattijs Korpershoek <mkorpershoek@...libre.com>, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH 0/8] dma-buf: heaps: Support carved-out heaps and ECC
 related-flags

Hi John,

Thanks for your feedback

On Wed, May 15, 2024 at 11:42:58AM -0700, John Stultz wrote:
> On Wed, May 15, 2024 at 6:57 AM Maxime Ripard <mripard@...nel.org> wrote:
> > This series is the follow-up of the discussion that John and I had a few
> > months ago here:
> >
> > https://lore.kernel.org/all/CANDhNCquJn6bH3KxKf65BWiTYLVqSd9892-xtFDHHqqyrroCMQ@mail.gmail.com/
> >
> > The initial problem we were discussing was that I'm currently working on
> > a platform which has a memory layout with ECC enabled. However, enabling
> > the ECC has a number of drawbacks on that platform: lower performance,
> > increased memory usage, etc. So for things like framebuffers, the
> > trade-off isn't great and thus there's a memory region with ECC disabled
> > to allocate from for such use cases.
> >
> > After a suggestion from John, I chose to start using heap allocations
> > flags to allow for userspace to ask for a particular ECC setup. This is
> > then backed by a new heap type that runs from reserved memory chunks
> > flagged as such, and the existing DT properties to specify the ECC
> > properties.
> >
> > We could also easily extend this mechanism to support more flags, or
> > through a new ioctl to discover which flags a given heap supports.
> 
> Hey! Thanks for sending this along! I'm eager to see more heap related
> work being done upstream.
> 
> The only thing that makes me a bit hesitant, is the introduction of
> allocation flags (as opposed to a uniquely specified/named "ecc"
> heap).
> 
> We did talk about this earlier, and my earlier press that only if the
> ECC flag was general enough to apply to the majority of heaps then it
> makes sense as a flag, and your patch here does apply it to all the
> heaps. So I don't have an objection.
> 
> But it makes me a little nervous to add a new generic allocation flag
> for a feature most hardware doesn't support (yet, at least). So it's
> hard to weigh how common the actual usage will be across all the
> heaps.
> 
> I apologize as my worry is mostly born out of seeing vendors really
> push opaque feature flags in their old ion heaps, so in providing a
> flags argument, it was mostly intended as an escape hatch for
> obviously common attributes. So having the first be something that
> seems reasonable, but isn't actually that common makes me fret some.
> 
> So again, not an objection, just something for folks to stew on to
> make sure this is really the right approach.

I understand your hesitation and concern :) Is there anything we could
provide that would help moving the discussion forward?

> Another thing to discuss, that I didn't see in your mail: Do we have
> an open-source user of this new flag?

Not at the moment. I guess it would be one of the things that would
reduce your concerns, but is it a requirement?

Thanks!
Maxime

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