Message-ID: <20230828082009.kehgwi4fxtj3bfze@CAB-WSD-L081021>
Date:   Mon, 28 Aug 2023 11:20:09 +0300
From:   Dmitry Rokosov <ddrokosov@...rdevices.ru>
To:     Lucas Tanure <tanure@...ux.com>
CC:     Neil Armstrong <neil.armstrong@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Kevin Hilman <khilman@...libre.com>,
        <linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-amlogic@...ts.infradead.org>
Subject: Re: [PATCH v2] Revert "tty: serial: meson: Add a earlycon for the T7
 SoC"

On Sun, Aug 27, 2023 at 11:31:35AM +0100, Lucas Tanure wrote:
> On 27-08-2023 11:23, Neil Armstrong wrote:
> > Hi Lucas,
> > 
> > Le 27/08/2023 à 12:04, Lucas Tanure a écrit :
> > > On 27-08-2023 10:48, Neil Armstrong wrote:
> > > > Hi Lucas,
> > > > 
> > > > Le 27/08/2023 à 10:29, Lucas Tanure a écrit :
> > > > > This reverts commit 6a4197f9763325043abf7690a21124a9facbf52e.
> > > > > New SoC will use ttyS0 instead of ttyAML, so T7 SoC doesn't
> > > > > need a OF_EARLYCON_DECLARE.
> > > > > 
> > > > > Signed-off-by: Lucas Tanure <tanure@...ux.com>
> > > > > ---
> > > > > Since V1:
> > > > > - add Signed-off-by:
> > > > > 
> > > > >   drivers/tty/serial/meson_uart.c | 2 --
> > > > >   1 file changed, 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/tty/serial/meson_uart.c
> > > > > b/drivers/tty/serial/meson_uart.c
> > > > > index c4f61d82fb727..790d910dafa5d 100644
> > > > > --- a/drivers/tty/serial/meson_uart.c
> > > > > +++ b/drivers/tty/serial/meson_uart.c
> > > > > @@ -648,8 +648,6 @@ meson_serial_early_console_setup(struct
> > > > > earlycon_device *device, const char *opt
> > > > >   OF_EARLYCON_DECLARE(meson, "amlogic,meson-ao-uart",
> > > > >               meson_serial_early_console_setup);
> > > > > -OF_EARLYCON_DECLARE(meson, "amlogic,t7-uart",
> > > > > -            meson_serial_early_console_setup);
> > > > >   #define MESON_SERIAL_CONSOLE_PTR(_devname)
> > > > > (&meson_serial_console_##_devname)
> > > > >   #else
> > > > 
> > > > How do you use earlycon with this removed ?
> > > > 
> > > > Neil
> > > This is my kernel cmdline:
> > > console=ttyS0,921600 no_console_suspend earlycon=ttyS0,0xfe078000
> > > 
> > > And I can see my log:
> > > boot 64bit kernel
> > > [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd092]
> > > [    0.000000] Linux version 6.5.0-rc7-next-20230825+
> > > (tanureal@...en) (aarch64-none-linux-gnu-gcc (GNU Toolchain for the
> > > A-profile Architecture 10.3-2021.07 (arm-10.29)) 10.3.1 20210621,
> > > GNU ld (GNU Toolchain fo3
> > > [    0.000000] KASLR disabled due to lack of seed
> > > [    0.000000] Machine model: Khadas vim4
> > > [    0.000000] OF: reserved mem:
> > > 0x0000000005000000..0x00000000052fffff (3072 KiB) nomap non-reusable
> > > secmon@...0000
> > > [    0.000000] OF: reserved mem:
> > > 0x0000000005300000..0x00000000072fffff (32768 KiB) nomap
> > > non-reusable secmon@...0000
> > > ...
> > > [    0.079368] io scheduler mq-deadline registered
> > > [    0.079374] io scheduler kyber registered
> > > [    0.079549] io scheduler bfq registered
> > > [    0.083373] fe078000.serial: ttyS0 at MMIO 0xfe078000 (irq = 14,
> > > base_baud = 1500000) is a meson_uart
> > > [    0.083403] printk: console [ttyS0] enabled
> > > 
> > > As the log is OK, and T7 for now is binding against S4 code we
> > > should drop this patch and add a S4 one if needed.
> > > 
> > > But just having this log is not enough for testing earlycon?
> > > I am assuming that by just having the log since 0.0000 is a good
> > > sign that earlycon is working.
> > > Could you give further guidance?
> > 
> > First the kernel argument is simply "earlycon" with no other options,
> > the earlycon code will fetch the first uart using the DT
> > /chosen/stdout-path property.
> > 
> > Then you should see something like right after "Machine model":
> > [    0.000000] earlycon: meson0 at MMIO 0x00000000ff803000 (options
> > '115200n8')
> > 
> > The boot log you share doesn't use earlycon, it waits until all
> > depedency of the UART
> > has been probed before probing the UART driver and prints the whole boot
> > log.
> > 
> > Earlycon is a mechanism to use the UART HW as configured by the
> > bootloader in
> > a minimal way very early in the boot in order to detect very early lockups.
> > 
> > Keeping OF_EARLYCON_DECLARE for t7 would enable this.
> > 
> > Neil
> > 
> > > 
> > > Lucas Tanure
> > > 
> > 
> But then we would need to add Earlycon for S4 too.
> How can we add one Earlycon for all future SoCs, like S4 and T7?
> 

Per my understanding, you just need to declare UART dts node with the
following compatible string: "amlogic,meson-ao-uart". It will enabled
earlycon. There is no neccessary to declare separate
OF_EARLYCON_DECLARE(). It's working okay for A1 dts:

meson-a1.dtsi uart declaration:
	uart_AO_B: serial@...0 {
		compatible = "amlogic,meson-a1-uart",
			     "amlogic,meson-ao-uart";
		reg = <0x0 0x2000 0x0 0x18>;
		interrupts = <GIC_SPI 26 IRQ_TYPE_EDGE_RISING>;
		clocks = <&xtal>, <&xtal>, <&xtal>;
		clock-names = "xtal", "pclk", "baud";
		status = "disabled";
	};

earlycon declaration from dts:
	aliases {
		serial0 = &uart_AO_B;
	};

	chosen {
		stdout-path = "serial0:115200n8";
	};

> 
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-- 
Thank you,
Dmitry

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