On Tue, Jan 28, 2014 at 10:16:53AM +0100, Arnd Bergmann wrote: > On Tuesday 28 January 2014 10:05:35 Lars-Peter Clausen wrote: > > > + > > > +Clients must use the format described in the dma.txt file, using a three cell > > > +specifier for each channel. > > > + > > > +The three cells in order are: > > > + 1. A phandle pointing to the DMA controller > > > + 2. The channel number > > > + 3. Direction of the fixed unidirectional channel > > > + 0 - Memory to Device > > > + 1 - Device to Memory > > > + 2 - Device to Device > > > + > > > > Why does the direction needs to be specified in specifier? I see two > > options, either the direction per is fixed in hardware. In that case the DMA > > controller node should describe which channel is which direction. Or the > > direction is not fixed in hardware and can be changed at runtime in which > > case it should be set on a per descriptor basis. > > Normally the direction is implied by dmaengine_slave_config(). > Note that neither the dma slave API nor the generic DT binding > can actually support device-to-device transfers, since this > normally implies using two dma-request lines rather than one. > > There might be a case where the direction is required in order > to allocate a channel, because the engine has specialized channels > per direction, and might connect any of them to any dma request > line. This does not seem to be the case for "bam", because > the DMA specifier already contains a specific channel number, not > a request line or slave ID number. > In the case of BAM, the channels are hardcoded based on the attached peripheral. For instance, if the BAM is attached to the BLSP UART, channel 0 is uart0-RX, channel 1 is uart0-TX, channel 2 is uart1-RX... etc, etc. So not only is the direction hardcoded, but also the function. -- sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
