Hello, On Mon, Oct 02, 2017 at 01:17:41PM +0000, Han, Nandor (GE Healthcare) wrote: > > -----Original Message----- > > From: Uwe Kleine-König [mailto:u.kleine-koenig@xxxxxxxxxxxxxx] > > Sent: 29 June 2017 21:26 > > To: Romain Perier <romain.perier@xxxxxxxxxxxxx> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>; Jiri Slaby > > <jslaby@xxxxxxxx>; linux-serial@xxxxxxxxxxxxxxx; Han, Nandor (GE > > Healthcare) <nandor.han@xxxxxx>; devicetree@xxxxxxxxxxxxxxx > > Subject: EXT: Re: [PATCH] serial: imx-serial - move DMA buffer configuration > > to DT > > > > Hello, > > > > Cc: += devicetree@xxxxxxxxxxxxxxx > > > > On Wed, Jun 28, 2017 at 12:15:14PM +0200, Romain Perier wrote: > > > From: Nandor Han <nandor.han@xxxxxx> > > > > > > The size of the DMA buffer can affect the delta time between data > > > being produced and data being consumed. Basically the DMA system will > > > move data to tty buffer when a) DMA buffer is full b) serial line is idle. > > > The situation is visible when producer generates data continuously and > > > there is no possibility for idle line. At this point the DMA buffer is > > > directly affecting the delta time. > > Hi Uwe, > Maybe I can explain a bit better the situation. At least I've tried to explain well enough > the problem and the fix. :) > > > > > This doesn't look like a hw property but a configuration item. Also I don't > > understand the problematic case. The i.MX is sending continuously and then > > doesn't receive bytes until the DMA buffer is full? > > Yes > > > What is the DMA buffer? > > You don't mean the FIFO here, do you? > > > > DMA buffer is not HW FIFO. Is the buffer provided by serial driver to DMA to store data. > > > 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. > 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? > 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? What you want (for a low latency port) is that also small amounts of data received are passed quickly to the upper layer. The knob you identified to be available for that is the dma buffer size. I'd prefer to talk about low latency instead of buffer sizes when setting parameters for the port. That this influences the buffer size (and maybe watermark settings) under the hood shouldn't matter for the person configuring the low latency property. 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
