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

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Date:	Thu, 9 Jun 2011 12:24:33 +0530
From:	viresh kumar <viresh.kumar@...com>
To:	"Koul, Vinod" <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<anemo@....ocn.ne.jp>, Shiraz HASHIM <shiraz.hashim@...com>,
	Armando VISCONTI <armando.visconti@...com>,
	Bhupesh SHARMA <bhupesh.sharma@...com>
Subject: Why move all map_sg/unmap_sg for slave channel to its client?


Hi,

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:

commit 657a77fa7284d8ae28dfa48f1dc5d919bf5b2843
Author: Atsushi Nemoto <anemo@....ocn.ne.jp>
Date:   Tue Sep 8 17:53:05 2009 -0700

    dmaengine: Move all map_sg/unmap_sg for slave channel to its client
    
    Dan Williams wrote:
    ... DMA-slave clients request specific channels and know the hardware
    details at a low level, so it should not be too high an expectation to
    push dma mapping responsibility to the client.
    
    Also this patch includes DMA_COMPL_{SRC,DEST}_UNMAP_SINGLE support for
    dw_dmac driver.
    
    Acked-by: Maciej Sosnowski <maciej.sosnowski@...el.com>
    Acked-by: Nicolas Ferre <nicolas.ferre@...el.com>
    Signed-off-by: Atsushi Nemoto <anemo@....ocn.ne.jp>
    Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
 drivers/dma/at_hdmac.c       |   43 +++++++++++++++++++++--------------------
 drivers/dma/dw_dmac.c        |   31 ++++++++++++++++++-----------
 drivers/mmc/host/atmel-mci.c |    9 +++++++-
 3 files changed, 49 insertions(+), 34 deletions(-)


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.

Why to replicate similar code in client drivers if they need to unmap? What do you say?

-- 
viresh
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