linux-kernel - Re: [PATCH RFC 06/12] dma: direct: Provide accessor to dmem region

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Message-ID: <20250310-expert-piculet-of-fascination-3813cd@houat>
Date: Mon, 10 Mar 2025 17:28:26 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Robin Murphy <robin.murphy@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Marek Szyprowski <m.szyprowski@...sung.com>, Sumit Semwal <sumit.semwal@...aro.org>, 
	Christian König <christian.koenig@....com>, Benjamin Gaignard <benjamin.gaignard@...labora.com>, 
	Brian Starkey <Brian.Starkey@....com>, John Stultz <jstultz@...gle.com>, 
	"T.J. Mercier" <tjmercier@...gle.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Tomasz Figa <tfiga@...omium.org>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Hans Verkuil <hverkuil@...all.nl>, 
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	iommu@...ts.linux.dev, linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH RFC 06/12] dma: direct: Provide accessor to dmem region

On Mon, Mar 10, 2025 at 02:56:37PM +0000, Robin Murphy wrote:
> On 2025-03-10 12:06 pm, Maxime Ripard wrote:
> > Consumers of the direct DMA API will have to know which region their
> > device allocate from in order for them to charge the memory allocation
> > in the right one.
> 
> This doesn't seem to make much sense - dma-direct is not an allocator
> itself, it just provides the high-level dma_alloc_attrs/dma_alloc_pages/etc.
> interfaces wherein the underlying allocations _could_ come from CMA, but
> also a per-device coherent/restricted pool, or a global coherent/atomic
> pool, or the regular page allocator, or in one weird corner case the SWIOTLB
> buffer, or...

I guess it wasn't super clear, but what I meant is that it's an
allocator to the consumer: it gets called, and returns a buffer. How it
does so is transparent to the device, and on the other side of the
abstraction.

I do agree that the logic is complicated to follow, and that's what I
was getting at in the cover letter.

Maxime

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