> -----Original Message----- > From: Uwe Kleine-König [mailto:u.kleine-koenig@xxxxxxxxxxxxxx] > Sent: 10 October 2017 11:26 > To: Han, Nandor (GE Healthcare) <nandor.han@xxxxxx> > Cc: Romain Perier <romain.perier@xxxxxxxxxxxxx>; Greg Kroah-Hartman > <gregkh@xxxxxxxxxxxxxxxxxxx>; Jiri Slaby <jslaby@xxxxxxxx>; linux- > serial@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx > Subject: EXT: Re: [PATCH] serial: imx-serial - move DMA buffer configuration > to DT > > 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. > :). It is tricky, I agree. I will try to explain it even though I think the most important part is to understand that watermark level control when data is moved from out from rx FIFO to DMA buffer (BD) and is not the trigger that returns the data from DMA buffer (BD) to serial driver (which is the one that I need). The SDMA script is using 2 triggers to move data from rx FIFO to DMA buffer (BD): 1) watermark hit and 2) aging timer. It ignores completely the "Idle line" trigger. When watermark trigger is received it will move "min(WML-1,BD count)" data from rx FIFO to BD. After this step in the rx FIFO will remain always at least 1 byte (since we move WML-1). The last byte will be moved based on the aging timer. During which SDMA will check if more data is available and move the rest. SDMA will return the BD in 2 situations: 1) BD is full 2) during aging time trigger the rx FIFO is empty. > > 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. > I agree with you on this. But my intention was to say that the driver is using the same settings as before and users are not impacted by this change, and I only add the option to control that value. > Best regards > Uwe > Regards, Nandor > -- > 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
