linux-kernel - Re: [PATCH] tty: Don't force RISCV SBI console as preferred console

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Message-ID: <CAAhSdy3z1aDdVZ3dM1bec0z_pNtmfdJ0XukX_0YWwy7Q90G9AA@mail.gmail.com>
Date:   Fri, 26 Apr 2019 11:51:18 +0530
From:   Anup Patel <anup@...infault.org>
To:     Atish Patra <atish.patra@....com>
Cc:     Anup Patel <Anup.Patel@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christoph Hellwig <hch@...radead.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] tty: Don't force RISCV SBI console as preferred console

On Fri, Apr 26, 2019 at 10:11 AM Atish Patra <atish.patra@....com> wrote:
>
> On 4/25/19 6:35 AM, Anup Patel wrote:
> > The Linux kernel will auto-disables all boot consoles whenever it
> > gets a preferred real console.
> >
> > Currently on RISC-V systems, if we have a real console which is not
> > RISCV SBI console then boot consoles (such as earlycon=sbi) are not
> > auto-disabled when a real console (ttyS0 or ttySIF0) is available.
> > This results in duplicate prints at boot-time after kernel starts
> > using real console (i.e. ttyS0 or ttySIF0) if "earlycon=" kernel
> > parameter was passed by bootloader.
> >
> > The reason for above issue is that RISCV SBI console always adds
> > itself as preferred console which is causing other real consoles
> > to be not used as preferred console.
> >
>
> Do we even need HVC_SBI console to be enabled by default? Disabling
> CONFIG_HVC_RISCV_SBI seems to be fine while running in QEMU.

Actually, HVC_SBI console is useful on boards (such as SiFive Unleashed)
lacking upstream serial driver. It allows us to boot upstream kernel to prompt
on such boards with just timer driver (and probably irqchip driver).

Also, we should be able to use same kernel image on QEMU and SiFive
Unleashed board so disabling CONFIG_HVC_RISCV_SBI for QEMU is
a temporary solution.

>
> If we don't need it, I suggest we should remove the config option from
> defconfig in addition to this patch.

Like mentioned above, HVC_SBI is useful for newer SOCs and boards
where serial driver is not yet up-streamed.

Regards,
Anup

>
> Regards,
> Atish
> > Ideally "console=" kernel parameter passed by bootloaders should
> > be the one selecting a preferred real console.
> >
> > This patch fixes above issue by not forcing RISCV SBI console as
> > preferred console.
> >
> > Fixes: afa6b1ccfad5 ("tty: New RISC-V SBI console driver")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Anup Patel <anup.patel@....com>
> > ---
> >   drivers/tty/hvc/hvc_riscv_sbi.c | 1 -
> >   1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/tty/hvc/hvc_riscv_sbi.c b/drivers/tty/hvc/hvc_riscv_sbi.c
> > index 75155bde2b88..31f53fa77e4a 100644
> > --- a/drivers/tty/hvc/hvc_riscv_sbi.c
> > +++ b/drivers/tty/hvc/hvc_riscv_sbi.c
> > @@ -53,7 +53,6 @@ device_initcall(hvc_sbi_init);
> >   static int __init hvc_sbi_console_init(void)
> >   {
> >       hvc_instantiate(0, 0, &hvc_sbi_ops);
> > -     add_preferred_console("hvc", 0, NULL);
> >
> >       return 0;
> >   }
> >
>