linux-kernel - Re: Why move all map_sg/unmap_sg for slave channel to its client?

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Message-ID: <4DF19251.1090100@st.com>
Date:	Fri, 10 Jun 2011 09:11:05 +0530
From:	viresh kumar <viresh.kumar@...com>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	"Koul, Vinod" <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"anemo@....ocn.ne.jp" <anemo@....ocn.ne.jp>,
	Shiraz HASHIM <shiraz.hashim@...com>,
	Armando VISCONTI <armando.visconti@...com>,
	Bhupesh SHARMA <bhupesh.sharma@...com>,
	linux-raid <linux-raid@...r.kernel.org>
Subject: Re: Why move all map_sg/unmap_sg for slave channel to its client?

On 06/09/2011 11:58 PM, Dan Williams wrote:
> On Thu, Jun 9, 2011 at 2:38 AM, Linus Walleij <linus.walleij@...aro.org> wrote:
>> On Thu, Jun 9, 2011 at 8:54 AM, viresh kumar <viresh.kumar@...com> wrote:
>>
>>> I thought map_sg/unmap_sg for slave channels will be handled according
>>> to the flags passed in prep_slave_sg(). But then i found following patch:
>>> (...)
>>> I don't have much knowledge about that discussion, but i think this should be left
>>> configurable.
>>> If the client wants to control map/unmap then it can simply pass
>>> DMA_COMPL_SKIP_DEST_UNMAP | DMA_COMPL_SKIP_SRC_UNMAP in flags. I didn't wanted to
>>> skip this in my driver and so i don't pass them.
>>
>> What if the same driver is used on many different platforms like say
>> drivers/tty/serial/amba-pl011.c, and some of the platforms using it
>> has DMA engines that does not implement mapping/unmapping of
>> the passed sglist?
>>
>> In that case I think you have to modify all drivers in drivers/dma/*
>> to do this mapping, and then you could just make it a required behaviour
>> and skip the flags altogether.
>>
>> But apparently that approach was blocked at one point so let's see
>> what the others say.
> 
> My problem with automatic unmapping support is that the dma-driver
> really does not have a chance to get it right except for the trivially
> straightforward cases.  One need only look at the current bustage of
> raid5 acceleration with respect to overlapping mappings and arm v6.
> The dma-driver just knows how to perform "this" operation on "this"
> dma address.  It does not know the lifetime of the mapping, or even if
> it has the actual dma handle for unmapping versus an offset
> 
> For the raid case I've currently convinced myself that the raid client
> needs to get directly involved in dma mapping management, rather than
> teach all dma drivers a language of how to unmap and when.  Not only
> will this fix the overlapping, but it also eliminates the need to map
> and remap because the raid client knows the lifetime of  a stripe_head
> while the driver only knows the lifetime of a given stripe operation.
> 
> For slave-dma maybe there is a lot of common un-mapping logic that can
> be reused, but I think that comes from a separate smart library that
> understands the dma mapping lifetimes of a given class of clients.
> Leave the dma-drivers to just be dumb operators on anonymous dma
> addresses.
> 

Linus, Dan,

Got it. Thanks for your replies.

-- 
viresh
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