linux-kernel - Re: Delete flush cache all in arm64 platform.

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Date:	Wed, 23 Mar 2016 11:11:55 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Laura Abbott <labbott@...hat.com>
Cc:	Chen Feng <puck.chen@...ilicon.com>, catalin.marinas@....com,
	akpm@...ux-foundation.org, mhocko@...e.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, xuyiping@...ilicon.com,
	suzhuangluan@...ilicon.com, saberlily.xia@...ilicon.com,
	dan.zhao@...ilicon.com, linux-arm-kernel@...ts.infradead.org
Subject: Re: Delete flush cache all in arm64 platform.

On Mon, Mar 21, 2016 at 08:58:02AM -0700, Laura Abbott wrote:
> On 03/21/2016 03:08 AM, Mark Rutland wrote:
> >On Mon, Mar 21, 2016 at 04:07:47PM +0800, Chen Feng wrote:
> >>But if we use VA to flush cache to do cache-coherency with other
> >>master(eg:gpu)
> >>
> >>We must iterate over the sg-list to flush by va to pa.
> >>
> >>In this way, the iterate of sg-list may cost too much time(sg-table to
> >>sg-list) if the sglist is too long. Take a look at the
> >>ion_pages_sync_for_device in ion.
> >>
> >>The driver(eg: ION) need to use this interface(flush cache all) to
> >>*improve the efficiency*.

> >I'm not sure what to suggest regarding improving efficiency.
> >
> >Is walking the sglist the expensive portion, or is the problem the cost
> >of multiple page-size operations (each with their own barriers)?
> 
> Last time I looked at this, it was mostly the multiple page-size operations.

We may be able to amortize some of that cost if we had non-synchronised
cache maintenance operations for each page, then followed that up with a
single final DSB SY.

There are several places in arch/arm64/mm/dma-mapping.c (practically
every use of for_each_sg) that could potentially benefit. I'm not sure
how much that's likely to gain as it will depend heavily on the
microarchitecture.

Regardless, it looks like that would require ion_pages_sync_for_device
and friends to be reworked, as it seems to only hand single pages down
to the architecture backend rather than a more complete sglist.

Thanks,
Mark.