linux-kernel - Re: [PATCH 00/11] ARM: PrimeCell DMA Interface v5

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Date:	Sat, 1 May 2010 16:28:59 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Linus Walleij <linus.ml.walleij@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linus WALLEIJ <linus.walleij@...ricsson.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	STEricsson_nomadik_linux <STEricsson_nomadik_linux@...t.st.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/11] ARM: PrimeCell DMA Interface v5

On Sat, May 1, 2010 at 4:04 PM, Linus Walleij
<linus.ml.walleij@...il.com> wrote:
> 2010/5/2 Russell King - ARM Linux <linux@....linux.org.uk>:
>
>> Versatile has some MUXing on three of the DMA signals, so (eg) we
>> really don't want UARTs claiming DMAs just because they're in existence
>> and not in use - that would prevent DMAs from being used for (eg) AACI
>> or MMC.
>
> As long as Versatile doesn't specify any filter function or
> data for the channel allocation function (it currently doesn't and defaults
> to NULL) it won't even try to call the DMA engine to allocate a channel
> for say the UART.
>
> There is nothing blocking some other peripheral from grabbing a
> muxed channel in that case.
>
> But the implementation of the DMA engine would be better of
> handling the muxing dynamically I believe, so when the PL011
> driver (say) requests a DMA channel, it doesn't mean it requests the
> *physical* channel and holds it (unless the driver is very naďvely
> implemented) it nominally means it reserves a placeholder in the
> DMA engine.
>
> When the driver issues a request to perform a DMA transfer, it will pull
> out a physical channel and use that, then return it. If there is too
> much combat about the physical channels, you configure out DMA
> for the least wanted PrimeCells.
>

Could you simulate this by publishing more struct dma_chans than are
physically present, and then handle the muxing internal to the driver?
 Or am I misunderstanding the usage model?
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