linux-kernel - Re: [PATCH v2] Revert "tty: serial: meson: Add a earlycon for the T7 SoC"

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Message-ID: <b0eb409f-a437-4054-9072-a6ed093e88b7@linaro.org>
Date:   Mon, 28 Aug 2023 09:58:48 +0200
From:   Neil Armstrong <neil.armstrong@...aro.org>
To:     Lucas Tanure <tanure@...ux.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Kevin Hilman <khilman@...libre.com>
Cc:     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 27/08/2023 12:31, 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?
> 

As of today, S4 has an UART with amlogic,meson-ao-uart as fallback compatibl,
but if there's no AO UART on S4, then this is wrong and yes a new OF_EARLYCON_DECLARE
with amlogic,meson-s4-uart should be added.

Neil