From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Date: Wed, 28 Nov 2018 19:47:09 +0000 Subject: Re: remove the ->mapping_error method from dma_map_ops V2 Message-Id: <20181128194708.GP30658@n2100.armlinux.org.uk> List-Id: References: <20181128174545.GJ30658@n2100.armlinux.org.uk> <20181128.112717.149234960284639699.davem@davemloft.net> In-Reply-To: <20181128.112717.149234960284639699.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: David Miller Cc: torvalds@linux-foundation.org, hch@lst.de, linux-arch@vger.kernel.org, linux-ia64@vger.kernel.org, linux-parisc@vger.kernel.org, robin.murphy@arm.com, x86@kernel.org, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, linux-alpha@vger.kernel.org, xen-devel@lists.xenproject.org, dwmw2@infradead.org, linux-arm-kernel@lists.infradead.org On Wed, Nov 28, 2018 at 11:27:17AM -0800, David Miller wrote: > From: Linus Torvalds > 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. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up