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

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Date:   Tue, 25 Jan 2022 07:29:58 -0300
From:   Wander Costa <wcosta@...hat.com>
To:     Jon Hunter <jonathanh@...dia.com>
Cc:     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>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH] tty: serial: Use fifo in 8250 console driver

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



>
> Jon
>
> --
> nvpublic
>