On Mon, Dec 15, 2014 at 11:42:46AM +0200, Laurent Pinchart wrote: > Hi Vinod, > > On Monday 15 December 2014 14:43:14 Vinod Koul wrote: > > On Mon, Dec 15, 2014 at 09:31:01AM +0100, Geert Uytterhoeven wrote: > > > Please let me summarize... > > > > Thanks :) > > > > > During probe of a DMA client driver, the DMA engine driver may not be > > > available, causing dma_request_slave_channel*() to return -EPROBE-DEFER. > > > There are actually two different reasons that the DMA engine driver may > > > not be available: > > > > > > 1. The DMA engine driver hasn't been initialized yet, due to probe order. > > > This is more likely to happen with i2c client drivers, as they are > > > initialized from subsys_initcall() instead of module_init() (E.g. I > > > never saw it with the spi-rspi driver). > > > => The DMA client driver wants to return -EPROBE_DEFER too, and > > > retry later. > > > > > > 2. The DMA engine driver is not included in the kernel build. > > > => The DMA client driver wants to fall back to PIO. > > > > > > Now, how to distinguish between the two cases above? > > > > Quite right, this is a good question. Today we cannot distinguish between > > the two. Should we improve the deferred probe to tell us when the init is > > complete and all the modules have been initialized? > > I don't think that's possible, as you can never know when a module will be > loaded. In a production system it is quite reasonable to assume that usermode will find all the modules available for the devices detected and insert them post boot within a reasonable amount of time. > > If we ever have such a mechanism to check then we know no modules are to be > > inserted then we can fall back to PIO mode. Without that we should use some > > timeout counter to fall back on, say try requesting 5 times and give up and > > move to PIO after that > > That could be a performance improvement, but I wonder whether it's worth it. > If DT specifies DMA channels for the I2C controller, and the DMA core is > compiled in, and the DMA engine driver is compiled as a module and never > loaded, then yes, there will be a small overhead for each I2C transaction, but > I'd argue that such a combination of conditions is asking for trouble anyway > :-) Yes that is quite reasonable. For a proper production system it should be a fair assumption that drivers for hardware available should be available and if not possibly a buggy configuration which must be fixed anyway. -- ~Vinod -- 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
