linux-kernel - Re: [PATCH 1/2] dmaengine: TXx9 Soc DMA Controller driver

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Message-Id: <20090321.212920.25912728.anemo@mba.ocn.ne.jp>
Date:	Sat, 21 Mar 2009 21:29:20 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	dan.j.williams@...el.com
Cc:	linux-mips@...ux-mips.org, ralf@...ux-mips.org,
	linux-kernel@...r.kernel.org, haavard.skinnemoen@...el.com
Subject: Re: [PATCH 1/2] dmaengine: TXx9 Soc DMA Controller driver

On Wed, 18 Mar 2009 10:26:13 -0700, Dan Williams <dan.j.williams@...el.com> wrote:
> > The atmel-mci does not select "channel".  It just pick the first
> > usable channel of the dma_device specified by platform_data.  I
> > suppose dw_dmac is symmetric (it can use any channel for any slave).
> 
> You are right, it does not hardwire the channel, but it does hardwire
> the device, see at32_add_device_mci [1].
> 
> > But TXx9 SoC DMAC channels are hardwired to each peripheral devices.
> 
> I think creating a dma_device instance per channel and specifying that
> device like atmel-mci is the more future-proof way to go.

Well, I have considered it but it looks overkill for me at that time.
Maybe time to think again...

> > And I want to call Channel-3 of DMAC-0 "dma0chan3" even if Channel-2
> > was assigned to for public memcpy channel.
> 
> The problem is you could pass in the chan_id to guarantee 'chan3', but
> there is no guarantee that you will get 'dma0', as the driver has no
> knowledge of what other dma devices may be in the system.

Yes, I do not expect 'dma0'.  My filter function uses
dev_name(chan->device->dev), which is "txx9dmac.0" in this case.

Anyway, "one dma-device per channel" manner will make things much simpler.

---
Atsushi Nemoto
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