linux-kernel - Re: [RFC 2/4] dma: add dmaengine driver for Samsung s3c24xx SoCs

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Date:	Tue, 14 May 2013 16:21:36 +0200
From:	Tomasz Figa <t.figa@...sung.com>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Heiko Stübner <heiko@...ech.de>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Dan Williams <djbw@...com>, Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 2/4] dma: add dmaengine driver for Samsung s3c24xx SoCs

Hi Linus, Heiko,

On Tuesday 14 of May 2013 14:47:19 Linus Walleij wrote:
> On Sat, May 11, 2013 at 1:31 PM, Heiko Stübner <heiko@...ech.de> wrote:
> > Conceptually the s3c24xx-dma feels like a distant relative of the pl08x
> > with numerous virtual channels being mapped to a lot less physical ones.
> > The driver therefore borrows a lot from the amba-pl08x driver in this
> > regard. Functionality-wise the driver gains a memcpy ability in addition
> > to the slave_sg one.
> > 
> > The driver currently only supports the "newer" SoCs which can use any
> > physical channel for any dma slave. Support for the older SoCs where
> > each channel only supports a subset of possible dma slaves will have to
> > be added later.
> > 
> > Tested on a s3c2416-based board, memcpy using the dmatest module and
> > slave_sg partially using the spi-s3c64xx driver.
> > 
> > Signed-off-by: Heiko Stuebner <heiko@...ech.de>
> 
> So have I understood correctly if I assume that *some* S3C
> variants, i.e. this: arch/arm/mach-s3c64xx/dma.c
> have a vanilla, unmodified, or just slightly modified
> PL08x block, while this DMAC is something probably based on
> the PL08x where some ASIC engineer has had a good time hacking
> around in the VHDL code to meet some feature requirements.
> Correct? Or plausible guess?
> 
> Exactly *how* far away from the pl08x hardware is it?

AFAIK the DMAC of S3C24xx is completely different from PL08x. I think Heiko 
just meant that it uses similar concepts, like virtual channels.

> I guess you have already come to the conclusion that the
> amba-pl08x.c driver cannot be augmented to handle this hardware
> after some educated reading of that code?
> 
> But are really no parts reusable?
> 
> For example, if the LLIs have the same layout, could we split
> out the LLI handling from amba-pl08x into a separate file and reuse
> that?
> 
> The more you share with amba-pl08x the better for everyone,
> I am positively sure. And please include Russell on the review
> chain, he wrote the virtual channel abstraction.
> 
> If I was given the option amongst S3C work I would definatley
> have augmented the amba-pl08x.c to handle the stuff in
> arch/arm/mach-s3c64xx/dma.c *first* then started to work on
> this driver, but I guess you might not be working on s3c64xx?

Well, I still hope that someone will pick up the work on adapting PL08x driver 
to support PL080s of S3C64xx as well, but most likely it will end up with me 
doing it, as a part of DT support for S3C64xx.

Best regards,
-- 
Tomasz Figa
Samsung Poland R&D Center
SW Solution Development, Kernel and System Framework

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