linux-kernel - Re: remove the ->mapping_error method from dma_map

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Message-ID: <20181128194708.GP30658@n2100.armlinux.org.uk>
Date:   Wed, 28 Nov 2018 19:47:09 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     David Miller <davem@...emloft.net>
Cc:     torvalds@...ux-foundation.org, hch@....de,
        linux-arch@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org, robin.murphy@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-alpha@...r.kernel.org, xen-devel@...ts.xenproject.org,
        dwmw2@...radead.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: remove the ->mapping_error method from dma_map_ops V2

On Wed, Nov 28, 2018 at 11:27:17AM -0800, David Miller wrote:
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> Date: Wed, 28 Nov 2018 10:00:06 -0800
> 
> > Not all memory is accessible even to the kernel. If you have memory
> > that shows up in the last page of phys_addr_t, you just mark it
> > reserved at boot-time.
> 
> It's not the physical memory at the end that needs to be reserved.
> 
> It's the IOMMU mapping arena.

True, if and only if you have an IOMMU.

Where there isn't an IOMMU, then we'd have to reserve every page that
that translates to a bus address in the top 4K of dma_addr_t on any
bus in the system - that means knowing early in the kernel
initialisation about all buses in the system so we can detect and
reserve these pages.

I don't think that's trivial to do.

-- 
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