linux-kernel - Re: [PATCHv6 2/2] ARM: dma-mapping: remove custom consistent dma region

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Date:	Tue, 21 Aug 2012 14:59:45 +0200
From:	Hiroshi Doyu <hdoyu@...dia.com>
To:	"linux@....linux.org.uk" <linux@....linux.org.uk>
CC:	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kyungmin.park@...sung.com" <kyungmin.park@...sung.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"chunsang.jeong@...aro.org" <chunsang.jeong@...aro.org>,
	Krishna Reddy <vdumpa@...dia.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"subashrp@...il.com" <subashrp@...il.com>,
	"minchan@...nel.org" <minchan@...nel.org>
Subject: Re: [PATCHv6 2/2] ARM: dma-mapping: remove custom consistent dma
 region

Russell King - ARM Linux <linux@....linux.org.uk> wrote @ Tue, 21 Aug 2012 14:34:51 +0200:

> On Tue, Aug 21, 2012 at 02:22:35PM +0300, Hiroshi Doyu wrote:
> > The following "__get_vm_area_node()" can take gfp_mask, it means that
> > this function is expected to be called from atomic context, but why
> > it's _NOT_ allowed _ONLY_ from interrupt context?
> 
> One reason is it takes read/write locks without using the IRQ safe
> versions for starters (vmap_area_lock and vmlist_lock).  I don't see
> any other reasons in that bit of code though.

IIRC, if *_{irqsave,irqrestore} versions were introduced to protect
from IRQ context, could we remove this BUG_ON(in_interrupt()) in
__get_vm_area_node() at least? Or is it not encouraged from
performance POV?

It seems that the solution to allow IOMMU'able device driver to
allocate from ISR are:
(1) To provide the pre-allocate area like arm_dma_alloc() does,
or
(2) __get_vm_area_node() can be called from ISR.
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