linux-kernel - Re: [PATCH] tty: serial: Use fifo in 8250 console driver

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Date:   Tue, 25 Jan 2022 18:53:48 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     Wander Costa <wcosta@...hat.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Wander Lairson Costa <wander@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Johan Hovold <johan@...nel.org>,
        Andrew Jeffery <andrew@...id.au>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH] tty: serial: Use fifo in 8250 console driver

On Tue, Jan 25, 2022 at 12:40:27PM +0000, Jon Hunter wrote:
> 
> On 25/01/2022 10:29, Wander Costa wrote:
> > On Tue, Jan 25, 2022 at 7:06 AM Jon Hunter <jonathanh@...dia.com> wrote:
> > > 
> > > 
> > > On 25/01/2022 09:36, Jiri Slaby wrote:
> > > 
> > > ...
> > > 
> > > > > The test is bogus:
> > > > >           use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> > > > >                   port->fifosize > 1 &&
> > > > >                   (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO)
> > > > > 
> > > > > FCR is write only. Reading it, one gets IIR contents.
> > > > 
> > > > In particular, the test is checking whether there is no interrupt
> > > > pending (UART_FCR_ENABLE_FIFO == UART_IIR_NO_INT). So it oscillates
> > > > between use_fifo and not, depending on the interrupt state of the chip.
> > > > 
> > > > Could you change it into something like this:
> > > > --- a/drivers/tty/serial/8250/8250_port.c
> > > > +++ b/drivers/tty/serial/8250/8250_port.c
> > > > @@ -3396,7 +3396,7 @@ void serial8250_console_write(struct
> > > > uart_8250_port *up, const char *s,
> > > > 
> > > >           use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> > > >                   port->fifosize > 1 &&
> > > > -               (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) &&
> > > > +               (up->fcr & UART_FCR_ENABLE_FIFO) &&
> > > >                   /*
> > > >                    * After we put a data in the fifo, the controller will
> > > > send
> > > >                    * it regardless of the CTS state. Therefore, only use
> > > > fifo
> > > > 
> > > > 
> > > > And see whether it fixes the issue. Anyway, of what port type is the
> > > > serial port (what says dmesg/setserial about that)?
> > > 
> > > 
> > > Thanks. Unfortunately, this did not fix it. The port type is PORT_TEGRA ...
> > > 
> > >    70006000.serial: ttyS0 at MMIO 0x70006000 (irq = 72, base_baud = 25500000) is a Tegra
> > 
> > I see PORT_TEGRA has different values for fifosize and tx_loadsz.
> > Maybe we should use tx_loadsz.
> > Could you please give a try to this patch:
> > 
> > diff --git a/drivers/tty/serial/8250/8250_port.c
> > b/drivers/tty/serial/8250/8250_port.c
> > index 2abb3de11a48..d3a93e5d55f7 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -3343,7 +3343,7 @@ static void serial8250_console_fifo_write(struct
> > uart_8250_port *up,
> >   {
> >          int i;
> >          const char *end = s + count;
> > -       unsigned int fifosize = up->port.fifosize;
> > +       unsigned int fifosize = up->tx_loadsz;
> >          bool cr_sent = false;
> > 
> >          while (s != end) {
> > @@ -3409,8 +3409,8 @@ void serial8250_console_write(struct
> > uart_8250_port *up, const char *s,
> >          }
> > 
> >          use_fifo = (up->capabilities & UART_CAP_FIFO) &&
> > -               port->fifosize > 1 &&
> > -               (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) &&
> > +               up->tx_loadsz > 1 &&
> > +               (up->fcr & UART_FCR_ENABLE_FIFO) &&
> >                  /*
> >                   * After we put a data in the fifo, the controller will send
> >                   * it regardless of the CTS state. Therefore, only use fifo
> > 
> 
> 
> Thanks. Yes that does fix it.
> 
> Andy, does this work for X86?

Reported-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

No, it does NOT fix an issue (I see it on a handful x86) with the legacy UART
(means the 8250_pnp is in use). And I believe the same will be the case on LPSS
ones (8250_dw / 8250_lpss) and HSU (8250_mid), because the patch influences on
all of them.

-- 
With Best Regards,
Andy Shevchenko