linux-kernel - Re: [PATCH] arm64: mm: fix zone_dma

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Message-ID: <Z0oYMXMGYgXoyon7@arm.com>
Date: Fri, 29 Nov 2024 19:38:25 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Robin Murphy <robin.murphy@....com>
Cc: Yang Shi <yang@...amperecomputing.com>,
	Baruch Siach <baruch@...s.co.il>, will@...nel.org,
	ptesarik@...e.com, hch@....de, jiangyutang@...amperecomputing.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: mm: fix zone_dma_limit calculation

On Fri, Nov 29, 2024 at 06:06:50PM +0000, Robin Murphy wrote:
> On 2024-11-27 5:49 pm, Catalin Marinas wrote:
> > If IORT or DT indicate a large mask covering the whole RAM (i.e. no
> > restrictions), in an ideal world, we should normally extend ZONE_DMA to
> > the same.
> 
> That's not right, ZONE_DMA should still be relatively limited in size
> (unless we really do only have a tiny amount of RAM) - just because a DT
> dma-ranges property says the system interconnect can carry >32 address bits
> in general doesn't mean that individual device DMA masks can't still be
> 32-bit or smaller. IIRC we're still implicitly assuming that if DT does
> describe an offset range into "high" RAM, it must represent a suitable
> lowest common denominator for all relevant devices already, and therefore we
> can get away with sizing ZONE_DMA off it blindly.

Fine by me to keep ZONE_DMA in the low range always. I was thinking of
only doing this if ZONE_DMA32 is enabled.

> After staring at it for long enough, I think $SUBJECT patch is actually
> correct as it is.

Thanks Robin for having a look. Can I add your reviewed-by?

> In fact I'm now wondering why the fix was put inside
> max_zone_phys() in the first place, since it's clearly pointless to clamp
> DMA_BIT_MASK(32) to U32_MAX in the dma32_phys_limit case...

I came to the same conclusion. I think it might have been some left-over
from when we had a ZONE_DMA32 in the above 4GB (AMD Seattle?). Than we
changed it a few times but only focused on this function for setting the
limits.

> However the commit message is perhaps not as clear as it could be -
> technically we are correctly *calculating* the appropriate effective
> zone_dma_limt value within the scope of zone_sizes_init(), we're just
> failing to properly update the actual zone_dma_limit variable for the
> benefit of other users.

I'll have a look next week at rewriting the commit message, unless Yang
does it first. I'm planning to queue this patch for -rc2.

-- 
Catalin