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

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Date:   Fri, 19 Jul 2019 15:08:52 +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 13:18 +0200, Nicolas Saenz Julienne wrote:
> 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.

Never mind this suggestion, I don't think it makes any sense. If anything arm64
seems to fit the ZONE_DMA usage pattern of arm and powerpc: where ZONE_DMA's
size is decided based on ram size and/or board configuration. It was actually
set-up like this until Christoph's ad67f5a6545f7 ("arm64: replace ZONE_DMA with
ZONE_DMA32").

So the easy solution would be to simply revert that commit. On one hand I feel
it would be a step backwards as most 64 bit architectures have been moving to
use ZONE_DMA32. On the other, current ZONE_DMA32 usage seems to be heavily
rooted on having a 32 bit DMA mask*, which will no longer be the case on arm64
if we want to support the RPi 4.

So the way I see it and lacking a better solution, the argument is stronger on
moving back arm64 to using ZONE_DMA. Any comments/opinions?

Note that I've been looking at all the DMA/CMA/swiotlb code to see if this
would break anything or change behaviors and couldn't find anything obvious. I
also tested the revert on my RPi4 and nothing seems to fail.

* A good example is dma-direct's implementation.

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