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Date: Fri, 14 Jun 2019 16:05:33 +0100 From: Robin Murphy <robin.murphy@....com> To: 'Christoph Hellwig' <hch@....de>, David Laight <David.Laight@...LAB.COM> Cc: Maxime Ripard <maxime.ripard@...tlin.com>, Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>, "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>, "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>, "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>, David Airlie <airlied@...ux.ie>, "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>, Intel Linux Wireless <linuxwifi@...el.com>, "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Jani Nikula <jani.nikula@...ux.intel.com>, Ian Abbott <abbotti@....co.uk>, Rodrigo Vivi <rodrigo.vivi@...el.com>, Sean Paul <sean@...rly.run>, "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, H Hartley Sweeten <hsweeten@...ionengravers.com>, "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>, Daniel Vetter <daniel@...ll.ch> Subject: Re: [PATCH 16/16] dma-mapping: use exact allocation in dma_alloc_contiguous On 14/06/2019 15:50, 'Christoph Hellwig' wrote: > On Fri, Jun 14, 2019 at 02:15:44PM +0000, David Laight wrote: >> Does this still guarantee that requests for 16k will not cross a 16k boundary? >> It looks like you are losing the alignment parameter. > > The DMA API never gave you alignment guarantees to start with, > and you can get not naturally aligned memory from many of our > current implementations. Well, apart from the bit in DMA-API-HOWTO which has said this since forever (well, before Git history, at least): "The CPU virtual address and the DMA address are both guaranteed to be aligned to the smallest PAGE_SIZE order which is greater than or equal to the requested size. This invariant exists (for example) to guarantee that if you allocate a chunk which is smaller than or equal to 64 kilobytes, the extent of the buffer you receive will not cross a 64K boundary." That said, I don't believe this particular patch should make any appreciable difference - alloc_pages_exact() is still going to give back the same base address as the rounded up over-allocation would, and PAGE_ALIGN()ing the size passed to get_order() already seemed to be pointless. Robin.
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