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

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Message-Id: <06765168-a36a-4229-b03b-6ea91157237a@app.fastmail.com>
Date: Wed, 23 Apr 2025 15:49:16 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ben Collins" <bcollins@...nel.org>
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 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.

Strange, either something changed since I last looked at this code,
or there is something on Freescale SoCs that avoids the
default logic.

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.

> 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.

       Arnd