Hello, On Tue, Oct 10, 2017 at 07:58:31AM +0000, Han, Nandor (GE Healthcare) wrote: > > > > That doesn't sound like a good fix but more like a work around. > > > > Which other options did you test to fix your problem? > > > > > > > > > > I haven't tried any other, because except using maybe, ioctl I haven't > > > got anything better. > > > > My question didn't target where to configure the buffer size instead of dts. I > > wonder if it would help to change the fifo watermark limits for example. > > > > Ok. Sorry I didn't understand correct. Watermark will not help in this case because it is used only to trigger > the moving of the data from RX FIFO to DMA buffer. The iMX DMA (check the iMX6 RM A.3.1.2.4 since is missing > from iMX53 RM) will only return data to serial driver when no more data exist in the RX FIFO (is using aging timer for this check) > or BD (DMA buffer) is full. If data is sent continuously DMA will return data to driver only when BD is full. I read once over the description but I failed to understand it. At least it talks about water mark levels and min(WML-1,Count) which makes me think that the DMA script makes use of water marking too and my idea to play with that instead of buffer sizes might be sensible. > So what we need here is a trigger that will force DMA to return data > faster to serial driver. The only one that I could find was the size > of the buffer. Of course another way will be to create different SDMA > scripts that can do all kinds of handling -- but dint' want to go on > that route :) :-) > > > Our problem is that in our system some serial ports needs to have > > > really low data latency, where others trade more bytes over data > > > latency. This situation results in a need of beeing able to have > > > different DMA buffer size for different ports. > > > > > > How can DMA buffer size affect latency? > > > DMA works like this: (To answer to your question DMA buffer is not > > > FIFO) 1. Transfer the data from HW FIFO to DMA buffer based on some > > > interrupts (character received, etc) 2. Transfer the DMA buffer back > > > to serial port based on some events (buffer full, aging timer, etc) 3. Serial > > port forwards to tty buffer. > > > > BTW In the past I saw the serial core introduce latency, too. Are you sure > > that's not your bottle neck? > > Can you be more specific? http://www.spinics.net/lists/linux-serial/msg17767.html > > > Data availability to consumer depends on: DMA buffer size, baud rate > > > and communication pattern. By communication patter I'm refering that > > > we send data continuoselly (serial line is never idle) or packet by > > > packet (serial line is idle in between) > > > Example: > > > Baud: 19200 (1Byte = 0.52 ms) > > > DMA buffer size: 100 bytes > > > Communication pattern: continuously > > > => DMA will return data to serial port only when DMA buffer is > > > full, since the communication is continuously. This result in a > > > data latency of 0.52 ms* 100bytes = 52ms. In case the buffer > > > will be 200bytes the letency will be double. > > > > > > I agree with you, this is not directly a hw property but a DMA configuration > > item. > > > But I've found this to be the best way to configure this comparing with using > > ioctl. > > > > > > Let me know if you need more clarification and I would really be open > > > to other options that will solve our problem. > > > > > > <snip> > > > > > > > > +- fsl,dma-size : Indicate the size of the DMA buffer and its > > > > > +periods > > > > > > > > This is a sparse description, just from reading that I don't > > > > understand what it does. > > > > > > > > > > Serial driver configures a circular ring of buffers for DMA. Here we > > > can configure the size and the number of buffers. > > > > The problem is: How should a person, who wants to make available a port on a > > machine via dts, choose what value to use for fsl,dma-size? > > It doesn't have too. If this configuration is not provided the serial port will have a default value. > The user has the possibility to tweak this settings in case is needed. "The default value works in general" is no good justification to not provide easy-to-grasp and well documented knobs. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
