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

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Date:   Tue, 25 Jan 2022 10:06:34 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Jiri Slaby <jirislaby@...nel.org>, wander@...hat.com
Cc:     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 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

Jon

-- 
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