RE: [PATCH] serial: imx-serial - move DMA buffer configuration to DT

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> -----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/  |
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