linux-kernel - Re: [RFC 3/4] dma-direct: add dma_direct_min

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Message-ID: <13dd1a4f33fcf814545f0d93f18429e853de9eaf.camel@suse.de>
Date:   Thu, 18 Jul 2019 13:18:42 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, catalin.marinas@....com,
        will@...nel.org, phil@...pberrypi.org, stefan.wahren@...e.com,
        f.fainelli@...il.com, mbrugger@...e.com,
        Jisheng.Zhang@...aptics.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 3/4] dma-direct: add dma_direct_min_mask

On Thu, 2019-07-18 at 11:15 +0200, Christoph Hellwig wrote:
> On Wed, Jul 17, 2019 at 05:31:34PM +0200, Nicolas Saenz Julienne wrote:
> > Historically devices with ZONE_DMA32 have been assumed to be able to
> > address at least the lower 4GB of ram for DMA. This is still the defualt
> > behavior yet the Raspberry Pi 4 is limited to the first GB of memory.
> > This has been observed to trigger failures in dma_direct_supported() as
> > the 'min_mask' isn't properly set.
> > 
> > We create 'dma_direct_min_mask' in order for the arch init code to be
> > able to fine-tune dma direct's 'min_dma' mask.
> 
> Normally we use ZONE_DMA for that case.

Fair enough, I didn't think of that possibility.

So would the arm64 maintainers be happy with something like this:

- ZONE_DMA: Follows standard definition, 16MB in size. ARCH_ZONE_DMA_BITS is
	    left as is.
- ZONE_DMA32: Will honor the most constraining 'dma-ranges'. Which so far for
	      most devices is 4G, except for RPi4.
- ZONE_NORMAL: The rest of the memory.


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