Re: [PATCH] serial: imx: Improve PIO prevention if TX DMA has been started

[Uwe Kleine-König]

On Mon, Aug 14, 2017 at 08:38:04PM +0200, Clemens Gruber wrote:
> Hello Uwe,
> 
> On Mon, Aug 14, 2017 at 08:51:49AM +0200, Uwe Kleine-König wrote:
> > Hello Clemens,
> > 
> > On Sun, Aug 13, 2017 at 12:07:56AM +0200, Clemens Gruber wrote:
> > > On Sat, Aug 12, 2017 at 09:54:51PM +0200, Uwe Kleine-König wrote:
> > > > On Sat, Aug 12, 2017 at 05:12:10PM +0200, Clemens Gruber wrote:
> > > > > The imx_transmit_buffer function should return if TX DMA has already
> > > > > been started and not just skip over the buffer PIO write loop. (Which
> > > > > did fix the initial problem, but could have unintentional side-effects)
> > > > > 
> > > > > Tested on an i.MX6Q board with half-duplex RS-485 and with RS-232.
> > > > > 
> > > > > Cc: Ian Jamison <ian.dev@xxxxxxxxxx>
> > > > > Cc: Uwe-Kleine König <u.kleine-koenig@xxxxxxxxxxxxxx>
> > > > > Fixes: 514ab34dbad6 ("serial: imx: Prevent TX buffer PIO write when a
> > > > > DMA has been started")
> > > > 
> > > > AFAIK no newline in the Fixes: line.
> > > 
> > > Thanks. A checkpatch warning for this would be great.
> > 
> > I assume that is a note to yourself to look into that? :-)
> 
> It's on my TODO list ;)
> 
> > 
> > > > > diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> > > > > index 80934e7bd67f..fce538eb8c77 100644
> > > > > --- a/drivers/tty/serial/imx.c
> > > > > +++ b/drivers/tty/serial/imx.c
> > > > > @@ -452,13 +452,14 @@ static inline void imx_transmit_buffer(struct imx_port *sport)
> > > > >  		if (sport->dma_is_txing) {
> > > > >  			temp |= UCR1_TDMAEN;
> > > > >  			writel(temp, sport->port.membase + UCR1);
> > > > > +			return;
> > > > >  		} else {
> > > > >  			writel(temp, sport->port.membase + UCR1);
> > > > >  			imx_dma_tx(sport);
> > > > >  		}
> > > > 
> > > > Shouldn't the return go here?
> > > 
> > > Yes, it can also go here (and probably should). The problem of
> > > xmit->tail jumping over xmit->head occurs only if we are already DMA
> > > txing and then go into the PIO loop, but not the first time after
> > > calling imx_dma_tx. That's why the v1 passed the tests too.
> > > I'll have to conduct a few more tests and if they succeed I'll send a
> > > v2 where we return in both cases (already txing and starting to).
> > > 
> > > > Did you understand the problem? Can you say why this only hurts in RS485
> > > > half-duplex but not (as it seems) in regular rs232 mode?
> > > 
> > > I am not sure anyone understands (yet) why it a) only hurts RS-485 and
> > > b) only occurs on SMP systems.
> > > If you have more insight, please share it. :)
> > 
> > I asked because I thought you might have understood it before patching
> > it ...
> 
> Yeah, this patch went out way too early, sorry for that! :/
> 
> @gregkh: Please ignore this patch!
> 
> About the underlying problem (b) why it only occurs on SMP systems:
> I think Ian's theory is correct:
> DMA is started, then the PIO is done until the xmit buffer is empty and
> immediately after that, DMA is stopped.
> On SMP systems, where the DMA TX thread can run on another core, it is
> already too late.
> 
> Regarding problem (a) why it only hurts RS-485: One possibility could be
> the timing difference / additional delay due to for example toggling the
> transmit-enable GPIO via mctrl_gpio_set.
> Meaning that with RS-232 on SMP systems DMA is also stopped just early
> enough to not bork the circular xmit buffer.
> 
> If this is true then the imx driver did not really use TX DMA in
> practice before.
> 
> Thoughts?

This matches my theory about this driver and its (not-)usage of DMA.
 
> I'll try to trace this next week to verify these hypotheses.

Great. That's earlier than I would find time for that.

Best regards
Uwe

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