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Date:   Sat, 15 May 2021 08:17:17 -0400
From:   "Gabriel L. Somlo" <gsomlo@gmail.com>
To:     Stafford Horne <shorne@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Florent Kermarrec <florent@enjoy-digital.fr>,
        Mateusz Holenko <mholenko@antmicro.com>,
        Joel Stanley <joel@jms.id.au>,
        Jonathan Corbet <corbet@lwn.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Karol Gugala <kgugala@antmicro.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-doc@vger.kernel.org, linux-serial@vger.kernel.org
Subject: Re: [PATCH] serial/liteuart; Add support for earlycon
Message-ID: <YJ+7zTmMOxlCbaRf@errol.ini.cmu.edu>
References: <20210515084519.167343-1-shorne@gmail.com>
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In-Reply-To: <20210515084519.167343-1-shorne@gmail.com>
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On Sat, May 15, 2021 at 05:45:18PM +0900, Stafford Horne wrote:
> Most litex boards using RISC-V soft cores us the sbi earlycon, however
> this is not available for non RISC-V litex SoC.  This patch enables
> earlycon for liteuart which is available on all Litex SoC's making
> support for earycon debugging more widely available.
> 
> Signed-off-by: Stafford Horne <shorne@gmail.com>
> Cc: Florent Kermarrec <florent@enjoy-digital.fr>
> Cc: Mateusz Holenko <mholenko@antmicro.com>
> Cc: Joel Stanley <joel@jms.id.au>
> Cc: Gabriel L. Somlo <gsomlo@gmail.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  5 +++
>  drivers/tty/serial/Kconfig                    |  1 +
>  drivers/tty/serial/liteuart.c                 | 31 +++++++++++++++++++
>  3 files changed, 37 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 04545725f187..2d4a43af8de2 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1084,6 +1084,11 @@
>  			the driver will use only 32-bit accessors to read/write
>  			the device registers.
>  
> +		liteuart,<addr>
> +			Start an early console on a litex serial port at the
> +			specified address. The serial port must already be
> +			setup and configured. Options are not yet supported.
> +
>  		meson,<addr>
>  			Start an early, polled-mode console on a meson serial
>  			port at the specified address. The serial port must
> diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
> index 0c4cd4a348f4..9ceffe6ab6fd 100644
> --- a/drivers/tty/serial/Kconfig
> +++ b/drivers/tty/serial/Kconfig
> @@ -1531,6 +1531,7 @@ config SERIAL_LITEUART
>  	depends on OF || COMPILE_TEST
>  	depends on LITEX
>  	select SERIAL_CORE
> +	select SERIAL_EARLYCON
>  	help
>  	  This driver is for the FPGA-based LiteUART serial controller from LiteX
>  	  SoC builder.
> diff --git a/drivers/tty/serial/liteuart.c b/drivers/tty/serial/liteuart.c
> index 64842f3539e1..38c472487e68 100644
> --- a/drivers/tty/serial/liteuart.c
> +++ b/drivers/tty/serial/liteuart.c
> @@ -372,6 +372,37 @@ static int __init liteuart_console_init(void)
>  console_initcall(liteuart_console_init);
>  #endif /* CONFIG_SERIAL_LITEUART_CONSOLE */
>  
> +#ifdef CONFIG_SERIAL_EARLYCON
> +static void early_liteuart_putc(struct uart_port *port, int c)
> +{
> +	while (litex_read8(port->membase + OFF_TXFULL))
> +		cpu_relax();
> +
> +	litex_write8(port->membase + OFF_RXTX, c);
> +}
> +
> +static void early_liteuart_write(struct console *console, const char *s,
> +				    unsigned int count)
> +{
> +	struct earlycon_device *device = console->data;
> +	struct uart_port *port = &device->port;
> +
> +	uart_console_write(port, s, count, early_liteuart_putc);
> +}
> +
> +static int __init early_liteuart_setup(struct earlycon_device *device,
> +				       const char *options)
> +{
> +	if (!device->port.membase)
> +		return -ENODEV;
> +
> +	device->con->write = early_liteuart_write;
> +	return 0;
> +}
> +
> +OF_EARLYCON_DECLARE(liteuart, "litex,liteuart", early_liteuart_setup);
> +#endif /* CONFIG_SERIAL_EARLYCON */
> +
>  static int __init liteuart_init(void)
>  {
>  	int res;
> -- 
> 2.31.1
 
FWIW:
Reviewed-and-tested-by: Gabriel Somlo <gsomlo@gmail.com>

Thanks,
--Gabriel