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

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Date:   Fri, 14 Jun 2019 15:16:43 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Robin Murphy' <robin.murphy@....com>,
        'Christoph Hellwig' <hch@....de>
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

From: Robin Murphy
> Sent: 14 June 2019 16:06
...
> 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."

I knew it was somewhere :-)
Interestingly that also implies that the address returned for a size
of (say) 128 will also be page aligned.
In that case 128 byte alignment should probably be ok - but it is still
an API change that could have horrid consequences.

	David

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