linux-kernel - Re: [PATCH 16/16] dma-mapping: use exact allocation in dma_alloc

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Message-ID: <4113cd5f-5c13-e9c7-bc5e-dcf0b60e7054@arm.com>
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.