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

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Date:   Fri, 14 Jun 2019 15:01:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>
CC:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        "Ian Abbott" <abbotti@....co.uk>,
        H Hartley Sweeten <hsweeten@...ionengravers.com>,
        Intel Linux Wireless <linuxwifi@...el.com>,
        "moderated list:ARM PORT" <linux-arm-kernel@...ts.infradead.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 16/16] dma-mapping: use exact allocation in
 dma_alloc_contiguous

From: 'Christoph Hellwig'
> Sent: 14 June 2019 15:50
> To: David Laight
> 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.

Hmmm...
I thought that was even documented.

I'm pretty sure there is a lot of code out there that makes that assumption.
Without it many drivers will have to allocate almost double the
amount of memory they actually need in order to get the required alignment.
So instead of saving memory you'll actually make more be used.

	David

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