Hi Arnd On Fri, 6 Jul 2012, Arnd Bergmann wrote: > On Friday 06 July 2012, Guennadi Liakhovetski wrote: > > On Mon, 25 Jun 2012, Arnd Bergmann wrote: > > > > [snip] > > > > > The channel data in the device tree is still in a format > > > that is specific to that dmaengine driver and interpreted > > > by it. Using the regular dma_filter_fn prototype is not > > > necessary, but it would be convenient because the dmaengine > > > code already knows how to deal with it. If we don't use this > > > method, how about adding another callback to struct dma_device > > > like > > > > > > bool (*device_match)(struct dma_chan *chan, struct property *req); > > > > I like this idea, but why don't we extend it to also cover the non-DT > > case? I.e., why don't we add the above callback (call it "match" or > > "filter" or anything else) to dmaengine operations and inside (the > > extended) dma_request_channel(), instead of calling the filter function, > > passed as a parameter, we loop over all registered DMAC devices and call > > their filter callbacks, until one of them returns true? In fact, it goes > > back to my earlier proposal from > > http://thread.gmane.org/gmane.linux.kernel/1246957 > > which I, possibly, failed to explain properly. So, the transformation > > chain from today's API would be (all code is approximate): > > > > ... > > <dmaengine_core> > > for_each_channel() { > > ret = chan->device->device_alloc_chan_resources(chan, filter_arg); > > if (!ret) > > return chan; > > else if (ret != -ENODEV) > > return ret; > > /* -ENODEV - try the next channel */ > > } > > > > Which is quite similar to my above mentioned proposal. Wouldn't this both > > improve the present API and prepare it for DT? > > How would the individual driver know the size of the filter_arg? In exactly the same way as most dmaengine drivers do it today: they don't touch filter_arg until they're sure this is one of their channels. And this they typically do by comparing the driver pointer, e.g.: bool sa11x0_dma_filter_fn(struct dma_chan *chan, void *param) { if (chan->device->dev->driver == &sa11x0_dma_driver.driver) { Thanks Guennadi > In the > device tree code, we would know if from #dma-cells of the engine node, > and that gets checked when reading the property, but when you have > a free-form data structure, it's much less clear. > > Also, you could easily have an argument that looks valid for more than one > dmaengine, but really isn't. > > I think for your proposal to work, we would need something like the > phandle for the dmaengine device that is at the start of the property > in the DT case. > > Arnd > --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/ -- 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
