linux-kernel - Re: [PATCH] fsldma: Support 40 bit DMA addresses where capable

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Message-ID: <2025042316-nippy-lemur-debd6b@boujee-and-buff>
Date: Wed, 23 Apr 2025 16:41:58 -0400
From: Ben Collins <bcollins@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: dmaengine@...r.kernel.org, Vinod Koul <vkoul@...nel.org>, 
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org, 
	Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH] fsldma: Support 40 bit DMA addresses where capable

On Wed, Apr 23, 2025 at 03:49:16PM -0500, Arnd Bergmann wrote:
> On Tue, Apr 22, 2025, at 23:10, Ben Collins wrote:
> > On Tue, Apr 22, 2025 at 11:25:40AM -0500, Arnd Bergmann wrote:
> >> On Tue, Apr 22, 2025, at 10:56, Ben Collins wrote:
> >>
> >> >> > I'll check on this, but I think it's a seperate issue. The main thing is
> >> >> > just to configure the dma hw correctly.
> >> >> 
> >> >> I think it's still important to check this before changing the
> >> >> driver: if the larger mask doesn't actually have any effect now
> >> >> because the DT caps the DMA at 4GB, then it might break later
> >> >> when someone adds the correct dma-ranges properties.
> >> >
> >> > I'm adding dma-ranges to my dt for testing.
> >> 
> >> Ok. The other thing you can try is to printk() the dev->bus_dma_limit
> >> to see if it even tries to use >32bit addressing.
> >
> > Did that. Every combination of IOMMU on/off and dma-ranges in my dt always
> > showed bus_dma_limit as 0x0.

> There was originally a hack for powerpc that allowed DMA to be
> done in the absence of a dma-ranges property in the bus node, but
> limit it to 32-bit addressing for backwards compatibility, while
> all other architectures should require either an empty dma-ranges
> to allow full addressing or a specific translation if there is
> a bus specific limit and/or offset.
> 
> Looking at the current code I don't see that any more, so it's
> possible that now any DMA is allowed even if there is no
> dma-ranges property at all.

It's still there. It hardcodes zone_dma_limit to 31-bits:

arch/powerpc/mm/mem.c: paging_init()

I'm digging into this more. I'll check back when I have a better
understanding.

> > As an aside, if you could give this a quick check, I can send the revised
> > patch. Appreciate the feedback.
> >
> > https://github.com/benmcollins/linux/commit/2f2946b33294ebff2fdaae6d1eadc976147470d6
> 
> This looks correct to me, but I would change two things:
> 
>  - remove the debug message, which you probably left by accident
>  - instead of the explicit of_device_is_compatible(), change it
>    to use the .data field of the of_device_id table instead.

Will do.

-- 
 Ben Collins
 https://libjwt.io
 https://github.com/benmcollins
 --
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