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

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Date:   Tue, 25 Jan 2022 12:40:27 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Wander Costa <wcosta@...hat.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 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?

Jon

-- 
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