Thanks for your suggestion On Wednesday, January 23, 2019 1:15:21 PM CET Vinod Koul wrote: > On 21-01-19, 17:41, Federico Vaga wrote: > > Hello, > > > > I am a bit puzzle about the proper usage these two functions: > > > > void dmaengine_get(void) > > struct dma_chan *dma_find_channel(enum dma_transaction_type tx_type) > > > > Looking into the kernel sources dma_find_channel is not used, despite the > > fact that it looks as one of the suggested mechanism to access a DMA > > engine. So, I cannot find a valid example of its usage. > > > > The same for dmaengine_get(). On the other hand, at least this function is > > used in: drivers/video/fbdev/mx3fb.c. But I believe that its usage is > > wrong > > because it uses: dmaengine_get() + dma_request_channel(), instead of > > dmaengine_get() + dma_find_channel(). > > > > Reading the code my understanding is that dmaengine_get() + > > dma_find_channel() should be used for "public" (not private) DMA engine > > channels. > > > > It's here where things are getting obscure to me. My understanding is that > > DMA channels which are not private can be used by several drivers at the > > same time. In principle they submit all their transfers and the DMA > > engine will process them and then the DMA engine will notify the drivers > > about their completion. > > The concept of public channel is a channel capable of doing any dma > transfer for example a system dmaengine capable of flipping bits from > system memory to memory/devices. > > For peripherals, we cannot do a generic transfer and the transfer is > specific given a set of parameters for it to work, thus the channel is > private... These channels can be used by many users as long as that is > supported by engine. In an idle world, the dmaengine should have N > channels and provide M software channels and lets users use them and > schedule these for best throughput, we are not there yet :( > > > My understanding is also that the dmaengine subsystem assumes that > > "public" > > channels of the same type are interchangeable. > > > > In my use case I wrote a DMA engine which is specific for a subset of > > driver, and it cannot be used by others (so is sharable among a group of > > drivers). All these drivers, potentially, they can submit their transfers > > to the DMA engine in parallel; but I cannot do it because I'm forced to > > use > > dma_request_channel() in order to be able to filter the DMA channels, and > > this will make automatically the channel PRIVATE which prevents other > > drivers to use the same channel. > > Please use virtual channels for that, clients can use virtual channels > and they can be submitted to a hardware channel. > > > With the current API I do not see alternative for me. I have to use > > dma_request_channel() in order to filter DMA channels. But I am wondering > > if it make sense for you, and if it does, is there a solution today? Or, > > is it possibile to design one? > > > > Thanks (I do not know if this message is clear enough)
