Re: Serial port driver usage and status

From: Austin Schuh <austin.linux@gmail.com>
To: Michel Macena Oliveira <michel@austral-dynamics.com>
Cc: Itai Handler <itai.handler@gmail.com>, linux-rt-users@vger.kernel.org
Subject: Re: Serial port driver usage and status
Date: Thu, 16 Jul 2020 13:40:39 -0700	[thread overview]
Message-ID: <CABsbf=E9D_NvXHZ5z1aLFN4Jcg+s+ADVFZVJi=jkpPOknTN7cA@mail.gmail.com> (raw)
In-Reply-To: <CAKkbJ-wDsGO+ZHKZFkPfUL113ONc=NNNeShQkLf+ZH82iaBTEQ@mail.gmail.com>

On Thu, Jul 16, 2020 at 1:21 PM Michel Macena Oliveira
<michel@austral-dynamics.com> wrote:
>
> Em qui., 16 de jul. de 2020 às 16:26, Itai Handler
> <itai.handler@gmail.com> escreveu:
> >
> > Hi Michel,
> >
> > On 7/16/20, Michel Macena Oliveira <michel@austral-dynamics.com> wrote:
> > > Hi, sorry for my newbie question,
> > > What is the status of the serial port driver?
> > > I've written some code to use serial ports  for the generic kernel linux,
> > > And now I want to write something similar for the real time kernel, do
> > > I have to use some specific library, flags or something or is it just
> > > the same settings?
> > >
> > > I'm asking because now I need to take into account the serial access
> > > in the scheduler to ensure a real time behavior .
> > >
> > > Any help, advice, hint or tips would be appreciated.
> > >
> >
> > Do you mean that you wrote some code that works with a /dev/tty* serial port?
> >
> > In some kernel versions (e.g 4.19) we saw high latencies in the rx
> > path of serial drivers.
> > I think that this is due to rx handled in the context of a workqueue
> > which also handles other system activities.
> > We found a workaround to this problem. If I remember correctly we
> > create a dedicated thread for the port and assign it a high priority.
> >
> > Itai Handler
>
> Thanks for the answer,
> Yes I wrote a code that works with  /dev/tty* serial using termios.h.
> I thought exactly what you suggested, to use a dedicated thread, but
> how to properly implement it?
> I mean in a simple way, just need to put open and write calls inside
> the high priority (RT) thread?
>
> Current kernel: 4.4.208-rt198 #1 SMP PREEMPT RT

Itai is talking about the kernel work to handle the serial data, not
the userspace work.  We found the same thing, that the work goes
through a work queue in the kernel, which adds unbounded latency.

https://www.spinics.net/lists/linux-serial/msg07341.html looks
relevant and matches my memory of what we did to solve this.  There's
either a flag in userspace or patch to apply to the kernel (and then a
flag) to skip the work queue.

Austin