> -----Original Message----- > From: linux-serial-owner@xxxxxxxxxxxxxxx [mailto:linux-serial- > owner@xxxxxxxxxxxxxxx] On Behalf Of Han, Nandor (GE Healthcare) > Sent: 10 October 2017 13:17 > To: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>; Jiri Slaby > <jslaby@xxxxxxxx>; linux-serial@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx; > Martyn Welch <martyn.welch@xxxxxxxxxxxxxxx> > Subject: EXT: [PATCH] serial: imx-serial - move DMA buffer configuration to > DT > > > > > -----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 > > Anything that I could still clarify here I'll be happy to explain :) > > 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 -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
