On 15/11/2016 12:02, Mason wrote: > I would like to be able to setup my board's DMA engine at the time when > dma_request_chan() is called by the driver using that specific channel. > (Or some time later, but before an actual transfer is requested.) > > Does dma_request_chan() call back into the DMA driver at some point? > If yes, through which function pointer? > > I see that of_dma_request_slave_channel() calls ofdma->of_dma_xlate() > which is the function registered through of_dma_controller_register() > in the DMA driver, right? > > Currently, the driver sets up the DMA engine at the time when the > device_issue_pending() method is called, but this creates a catch-22 > situation where A must be before B, and B must be before A. > > Looking at struct dma_device ... > http://lxr.free-electrons.com/source/include/linux/dmaengine.h#L637 > Could I use the device_config() call-back for this? > > https://www.kernel.org/doc/Documentation/dmaengine/client.txt Let me try describing the problem on a higher level. I'm currently using this DMA driver on my board: https://github.com/mansr/linux-tangox/blob/master/drivers/dma/tangox-dma.c The driver configures the DMA HW in device_issue_pending() in two steps A) switch box (sbox) setup B) memory bus (mbus) channel setup And the DMA engine client (a NAND Flash controller driver) starts a transfer by programming the NFC registers. The problem is that the steps need to be done in this order: 1) SBOX 2) NAND 3) MBUS But in the current design, there is no way to have the NAND step *between* SBOX and MBUS steps. For now, I have hard-coded the SBOX setup in the driver's probe function, but that's just a hack. IMO, it makes sense to configure the SBOX only once, when the client driver "registers" with the DMA driver (dma_request_chan if I understand correctly) and release the HW channel when the client driver "unbinds". I'm eager to hear anyone's comments on this issue. Vinod, I would like to upstream this DMA driver. What do you think? I'll run a few more tests tomorrow. Regards. -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
