linux-kernel - Re: [PATCH 4/8] dmaengine: ste_dma40: Do not configure channels during an channel allocation

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Date:	Sun, 14 Apr 2013 18:34:51 +0200
From:	Rabin Vincent <rabin@....in>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	arnd@...db.de, linus.walleij@...ricsson.com,
	Vinod Koul <vinod.koul@...el.com>, Dan Williams <djbw@...com>,
	Per Forlin <per.forlin@...ricsson.com>
Subject: Re: [PATCH 4/8] dmaengine: ste_dma40: Do not configure channels
 during an channel allocation

2013/4/12 Lee Jones <lee.jones@...aro.org>:
> So I need to devise another way, as this function cannot be called
> here either. Using the dmaengine API, allocating a channel and
> configuring it are to be completed using different calls. Using the
> API correctly, there is no way the driver can setup the channel
> with all of the relevant information during allocation time.
>
> The steps are as follows:
>
> dma_request_channel() - here we only allot a channel number and
>                         allocate the appropriate resources for the
>                         channel.
>
> dma_slave_config()    - this is where we're meant to configure the
>                         channel, so d40_config_write() needs to be
>                         called here, as only dma_slave_config() will
>                         carry the information required so as
>                         d40_*_cfg() can make the correct decisions.

The choice between whether a physical or a logical channel is used is
not something that is configurable via dma_slave_config().  And
d40_config_write() only needs that information, and that information is
available in dma_request_channel().  Therefore no more information
relevant to d40_config_write() will be obtained in dma_slave_config().
Hence d40_config_write() can be called in dma_request_channel().
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