linux-kernel - Re: [PATCH 2/2] arch: m68k: Add DMA mapping operations for Coldfire M5441x

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Message-ID: <Z5h70TL6lSWzGA4f@infradead.org>
Date: Mon, 27 Jan 2025 22:40:17 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Jean-Michel Hautbois <jeanmichel.hautbois@...eli.org>
Cc: Christoph Hellwig <hch@...radead.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Greg Ungerer <gerg@...ux-m68k.org>, linux-m68k@...ts.linux-m68k.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] arch: m68k: Add DMA mapping operations for Coldfire
 M5441x

On Tue, Jan 28, 2025 at 07:33:37AM +0100, Jean-Michel Hautbois wrote:
> > Arch dma code should not implement DMA OPS, but use the generic
> > dma-direct code with the right helpes for handling coherency.  This
> > variant looks like it should be using the DMA_COHERENT_POOL allocator
> > for dma coherent memory.
> 
> Thanks for your answer.
> I am not sure to understand it though :-) because I can see a few
> dma_map_ops implementations in arch code.

The last one left for the direct mapping are arm32 and parisc, and
they should go away eventually.

> I tried to let dma_direct do the
> work, I can't remember exactly what happened but it was not great :-).
> I can give it a second try.

It really should not be hard.  The dynamic mappings already work fine
as m68k is using the generic code.  So the only thing you want is
to dip into the glonal pool for coherent allocations. For that you
need to select the DMA_GLOBAL_POOL config option and fill the pool
with dma_init_global_coherent().