linux-kernel - Re: CMA alignment question

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Message-ID: <CAL1ERfMk+rhd=-MaLC2VVj61-T17_SVgKL4=Z_okhEYktFJ+tQ@mail.gmail.com>
Date:	Thu, 6 Nov 2014 09:00:33 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	Gregory Fong <gregory.0xf0@...il.com>, linux-mm@...ck.org,
	Laura Abbott <lauraa@...eaurora.org>, iamjoonsoo.kim@....com,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Brian Norris <computersforpeace@...il.com>
Subject: Re: CMA alignment question

On Thu, Nov 6, 2014 at 6:01 AM, Michal Nazarewicz <mina86@...a86.com> wrote:
>> On Tue, Nov 04 2014, Gregory Fong wrote:
>>> The alignment in cma_alloc() is done w.r.t. the bitmap.  This is a
>>> problem when, for example:
>>>
>>> - a device requires 16M (order 12) alignment
>>> - the CMA region is not 16 M aligned
>
> On Wed, Nov 05 2014, Weijie Yang wrote:
>> I think the device driver should ensure that situation could not occur,
>> by assign suitable alignment parameter in cma_declare_contiguous().
>
> What about default CMA area? Besides, I think principle of least
> surprise applies here and alignment should be physical.

I agree the current code doesn't handle this issue properly.
However, I prefer to add specific usage to CMA interface rather than
modify the cma code, Because the latter hide the issue and could waste
memory.

> --
> Best regards,                                         _     _
> .o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
> ..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
> ooo +--<mpn@...gle.com>--<xmpp:mina86@...ber.org>--ooO--(_)--Ooo--
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