On Wed, Feb 01, 2012 at 11:50:30AM +0100, Cousson, Benoit wrote: > Hi Russell, > > On 2/1/2012 12:09 AM, Russell King - ARM Linux wrote: >> On Sat, Jan 28, 2012 at 11:06:02AM -0700, Grant Likely wrote: >>> This makes the assumption that dma specifiers will only ever be 1 >>> cell. I think to be generally useful, the full dma specifier needs to >>> be either handed to the dma controller to get a cookie or passed back >>> to the caller in its entirety. >> >> More to the point, who says that the DMA specifier is even an integer? >> When people are using DMA engines, it (probably) isn't an integer at >> all. Several platforms I know of use strings for this. >> >> Some platforms can even select between two or more DMA engines for >> handling the same peripheral - I believe Samsung do this depending >> on their individual workloads. >> >> However, the opaque DMA engine API for requesting a channel doesn't >> lend itself well to DT, as the match data and match function are >> entirely left to the individual DMA engine driver and/or platform >> itself. >> >> As far as creating another linear number space for DMA stuff, I'd >> really suggest against that - you're going to need some additional >> code in place to manage that numberspace. If you at least use a two- >> paid cookie, eg 'dma controller phandle + request signal' then that >> makes all the stuff we're starting to see with the IRQ subsystem, >> IRQ domains etc become completely unnecessary. >> >> I guess what I'm saying is ignore the flat number space, and go >> straight to some kind of 'dma domains' solution from the start. > > Fully agree, and this is exactly the idea of this DMA binding: First > argument is always a DMA controller phandle and then you can add 0, 1 or > more cells to define extra specifiers dependent of the DMA controller > driver expectation. The one cell Grant was referring was just the extra > specifier that is needed for a simple DMA engine like the SDMA we have > inside OMAP. But the whole idea is to have a flexible enough mechanism > to allow any kind of specifier. > > No more global linear number space like for IRQ! How does this work when you're stuffing a number into a struct resource as a plain DMA number? That looks very much like a linear number space, as you don't have a way to associate that number with anything else. -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
