linux-kernel - Re: [RFC 3/6] dmaengine: core: Introduce new, universal API to request a channel

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Date:	Fri, 27 Nov 2015 12:00:10 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Peter Ujfalusi <peter.ujfalusi@...com>, vinod.koul@...el.com,
	andy.shevchenko@...il.com, linux-mmc@...r.kernel.org,
	nsekhar@...com, linux-kernel@...r.kernel.org,
	dmaengine@...r.kernel.org, linux-omap@...r.kernel.org
Subject: Re: [RFC 3/6] dmaengine: core: Introduce new, universal API to request a channel

On Friday 27 November 2015 10:29:39 Peter Ujfalusi wrote:
> struct dma_chan *dma_request_chan(struct device *dev, const char *name,
> 				  const dma_cap_mask_t *mask);
> To request a slave channel. The mask parameter is optional and it is used
> to check if the received channel's capabilities can satisfy the requested
> mask. The dma_request_chan() will try to find the channel via DT, ACPI or
> in case if the kernel booted in non DT/ACPI mode it will use a filter
> lookup table and retrieves the RESOURCE_DMA from the requester's device.
> This legacy mode needs changes in platform code, in dmaengine drivers and
> finally the dmaengine user drivers can be converted:

I think we should not introduce the mask parameter at all. In the rare
case that we actually need to pass a mask other than DMA_SLAVE today,
we should be able to encode that in the data we pass to the filter
function.

> RESOURCE_DMA needs to be added to the platform devices with names
> 
> For each dmaengine driver a string array listing the devices handled by the
> given DMA driver:
> 
> static char *da8xx_edma0_devices[] = {
> 	"davinci-mcasp.0",
> 	"da830-mmc.0",
> };
> 
> This information is going to be needed by the dmaengine driver, so
> modification to the platform_data is needed, and the driver map should be
> added to the pdata of the DMA driver:

However, a lot of drivers definitely need to pass a data pointer for
each device. I see that you currently rely on the IORESOURCE_DMA
method that a couple of platforms use today, but I think it would
be backwards to do it that way for the platforms that require passing
more than an integer number today.

Having a pointer as the filter function argument is intentional and I'd
prefer to change the platforms that currently use IORESOURCE_DMA (davinci,
pxa, omap1, blackfin, alchemy) to stop doing it instead, and return
IORESOURCE_DMA to its original meaning that was limited to ISAPNP style
devices not using the dmaengine API.

	Arnd
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