linux-kernel - Re: [PATCH] serial/liteuart; Add support for earlycon

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Message-ID: <d17a43ab-facf-8ffb-5147-7492fb85b0b5@kernel.org>
Date:   Mon, 17 May 2021 10:25:46 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Stafford Horne <shorne@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Florent Kermarrec <florent@...oy-digital.fr>,
        Mateusz Holenko <mholenko@...micro.com>,
        Joel Stanley <joel@....id.au>,
        "Gabriel L . Somlo" <gsomlo@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Karol Gugala <kgugala@...micro.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-doc@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH] serial/liteuart; Add support for earlycon

On 17. 05. 21, 10:23, Stafford Horne wrote:
> On Mon, May 17, 2021 at 10:16:43AM +0200, Jiri Slaby wrote:
>> On 15. 05. 21, 10:45, 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@...il.com>
>>> Cc: Florent Kermarrec <florent@...oy-digital.fr>
>>> Cc: Mateusz Holenko <mholenko@...micro.com>
>>> Cc: Joel Stanley <joel@....id.au>
>>> Cc: Gabriel L. Somlo <gsomlo@...il.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);
>>
>> Hi,
>>
>> am I missing something or this doesn't differ to liteuart_putchar?
> 
> Hi, No you are right, I missed that thanks for catching it.
> 
> I should be able to remove this function and use putchar below.

OK.

I've just noticed you add an ifdef CONFIG_SERIAL_EARLYCON here, but you 
select SERIAL_EARLYCON in Kconfig above. So the ifdef is sort of 
pointless? (Or you shouldn't select SERIAL_EARLYCON.)

thanks,
-- 
js
suse labs