linux-kernel - Re: [PATCH 2/3] spi: mediatek: Add spi bus for Mediatek MT8173

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Message-ID: <20150617124758.GE3214@sirena.org.uk>
Date:	Wed, 17 Jun 2015 13:47:58 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Eddie Huang <eddie.huang@...iatek.com>
Cc:	Leilk Liu (刘磊) <Leilk.Liu@...iatek.com>,
	Mark Rutland <mark.rutland@....com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	HongZhou Yang <hongzhou.yang@...iatek.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
	"linux-mediatek@...ts.infradead.org" 
	<linux-mediatek@...ts.infradead.org>,
	srv_heupstream <srv_heupstream@...iatek.com>
Subject: Re: [PATCH 2/3] spi: mediatek: Add spi bus for Mediatek MT8173

On Wed, Jun 17, 2015 at 05:08:03PM +0800, Eddie Huang wrote:

> Here comes the problem, although total length of tx, rx is the same,
> each entry in rx and tx scatterlist may not be the same (in the case
> data buffer allocate from vmalloc). Other vendor have dmaengine driver
> to send entry-by-entry automatically, so it is ok due to total length is
> the same.But mediatek hw can only send tx and rx scatterlist entry one
> by on, which means in full duplex, mediatek SPI hardware need send each
> entry separately, will cause full duplex fail because tx/rx entry size
> or entry number may not be the same.

I don't see why this is a problem - your driver is going to have to do
more work to overcome the limitations of the hardware but surely it's
just a question of how you parse the scatterlists (or rewriting them if
that's appropriate)?

> The problem is not dma map discuss earlier, it is mediatek spi hardware
> limitation that can't support scatterlist dma transfer in full-duplex
> mode. We can only support both tx and rx has the same size continuous
> memory data in full-duplex mode. I don't know whether should modify
> spi.c to support mediatek spi, or we just return can_dma() false and do
> transfer one continuous data in transfer function.

> By the way, I think maybe it is better to change can_dma() to
> can_dma_sg().

Don't you just need to handle the scatterlists in page sized chunks
here?  There's nothing about passing information about the memory that
was mapped around in a scatterlist that means you have to pass the whole
list to the hardware at once - at heart the scatterlist is just a
convenient structure for passing around where the data to be transferred
is.

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