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

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Message-Id: <7d914aa8-dc6f-426c-b7fc-dbb03c6b676c@app.fastmail.com>
Date: Wed, 23 Apr 2025 22:56:01 +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 Wed, Apr 23, 2025, at 22:41, Ben Collins wrote:
> On Wed, Apr 23, 2025 at 03:49:16PM -0500, Arnd Bergmann wrote:
>> 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.

zone_dma_limit is the other side of this: you need a ZONE_DMA
and/or ZONE_DMA32 memory zone that lets a driver allocate buffers
from low addresses if the DMA mask is smaller than the available
memory.

      Arnd