Date:   Fri, 4 Oct 2019 21:45:54 +0000
From:   Atish Patra <Atish.Patra@....com>
To:     "palmer@...ive.com" <palmer@...ive.com>,
        "schwab@...e.de" <schwab@...e.de>
CC:     "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "hch@....de" <hch@....de>, "jslaby@...e.com" <jslaby@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

On Mon, 2019-09-16 at 14:38 -0700, Palmer Dabbelt wrote:
> On Mon, 16 Sep 2019 12:40:10 PDT (-0700), schwab@...e.de wrote:
> > On Sep 16 2019, Palmer Dabbelt <palmer@...ive.com> wrote:
> > 
> > > On Sun, 15 Sep 2019 23:42:53 PDT (-0700), Christoph Hellwig
> > > wrote:
> > > > On Fri, Sep 13, 2019 at 01:40:27PM -0700, Palmer Dabbelt wrote:
> > > > > OpenEmbedded passes "earlycon=sbi", which I can find in the
> > > > > doumentation.
> > > > > I can't find anything about just "earlycon".  I've sent a
> > > > > patch adding sbi
> > > > > to the list of earlycon arguments.
> > > > 
> > > > earlycon without arguments is documented, although just for
> > > > ARM64.
> > > > I can send a patch to update it to properly cover all DT
> > > > platforms
> > > > in addition.
> > > 
> > > Thanks.  I've kind of lost track of the thread, but assuming that
> > > does the
> > > "automatically pick an earlycon" stuff then that's probably what
> > > we should
> > > be using in the distros.
> > 
> > Except that it doesn't work.
> 
> Sorry, once again I've lost track of the thread.
> 
> The code looks generic.  The device tree in arch/riscv for the HiFive
> Unleashed 
> doesn't have a stdout-path set, which if I understand correctly is
> used by the 
> automatic earlycon stuff to pick a console.  I gave this a quick test
> on QEMU, 
> which finds a 16550 earlycon for me.  I use openembedded's
> qemuriscv64 target, 
> the following diff to make sure I'm getting an earlycon
> 
> diff --git a/drivers/tty/serial/8250/8250_early.c
> b/drivers/tty/serial/8250/8250_early.c
> index 5cd8c36c8fcc..61290714bbcb 100644
> --- a/drivers/tty/serial/8250/8250_early.c
> +++ b/drivers/tty/serial/8250/8250_early.c
> @@ -106,6 +106,7 @@ static void early_serial8250_write(struct console
> *console,
>         struct earlycon_device *device = console->data;
>         struct uart_port *port = &device->port;
> 
> +       uart_console_write(port, "_e_", 3, serial_putc);
>         uart_console_write(port, s, count, serial_putc);
>  }
> 
> and run this command line
> 
>     /home/palmer/work/linux/openembedded-riscv64/build/tmp-
> glibc/work/x86_64-linux/qemu-helper-native/1.0-r1/recipe-sysroot-
> native/usr/bin/qemu-system-riscv64 -device virtio-net-
> device,netdev=net0,mac=52:54:00:12:35:02 -netdev
> user,id=net0,hostfwd=tcp::2222-:22,hostfwd=tcp::2323-
> :23,tftp=/home/palmer/work/linux/openembedded-riscv64/build/tmp-
> glibc/deploy/images/qemuriscv64 -drive
> id=disk0,file=/home/palmer/work/linux/openembedded-riscv64/build/tmp-
> glibc/deploy/images/qemuriscv64/core-image-full-cmdline-qemuriscv64-
> 20190711162644.rootfs.ext4,if=none,format=raw -device virtio-blk-
> device,drive=disk0 -object rng-random,filename=/dev/urandom,id=rng0
> -device virtio-rng-device,rng=rng0 -show-cursor -monitor null -device
> loader,file=/home/palmer/work/linux/linux/arch/riscv/boot/Image,addr=
> 0x80200000  -nographic -machine virt  -m 512 -serial mon:stdio
> -serial null -kernel /home/palmer/work/linux/openembedded-
> riscv64/build/tmp-glibc/deploy/images/qemuriscv64/fw_jump.elf -append
> 'root=/dev/vda rw highres=off  console=ttyS0 mem=512M ip=dhcp
> earlycon '
> 
> which gives me some early stuff and then some non-early stuff
> 
> _e_[    0.407579] printk: console [ttyS0] disabled
> _e_[    0.409205] 10000000.uart: ttyS0 at MMIO 0x10000000 (irq = 10,
> base_baud = 230400) is a 16550A
> [    0.410720] printk: console [ttyS0] enabled
> _e_[    0.410720] printk: console [ttyS0] enabled
> [    0.411391] printk: bootconsole [ns16550a0] disabled
> _e_[    0.411391] printk: bootconsole [ns16550a0] disabled
> [    0.420664] [drm] radeon kernel modesetting enabled.
> [    0.428086] random: fast init done
> [    0.429331] random: crng init done
> [    0.440678] loop: module loaded
> [    0.447607] virtio_blk virtio1: [vda] 262830 512-byte logical
> blocks (135 MB/128 MiB)
> [    0.469483] libphy: Fixed MDIO Bus: probed
> 
> If you don't have something like "/chosen/stdout-path = &uart0;" in
> your device 
> tree, then that's probably the issue.  Here's where it's set in
> Christoph's 
> k210:
> 
>     
> http://git.infradead.org/users/hch/riscv.git/blob/f10e64873eafc68516b8884c06b9290b9887633b:/arch/riscv/boot/dts/kendryte/kd210.dts#l20
> 
> but we don't set it for the HiFive Unleashed.  I'd call that a bug,
> something 
> like this
> 
> diff --git a/arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dts
> b/arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dts
> index 93d68cbd64fe..6d0ec76d93fe 100644
> --- a/arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dts
> +++ b/arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dts
> @@ -13,6 +13,7 @@
>         compatible = "sifive,hifive-unleashed-a00", "sifive,fu540-
> c000";
>  
>         chosen {
> +               stdout-path = &uart0;
>         };
>  
>         cpus {
> 
> should fix it.  LMK if I've misunderstood something.
> 

OpenSBI already adds this node to DT for U-Boot serial console.
I have tested that just adding "earlycon" to the commandline works as
long as you are using OpenSBI and SERIAL_EARLYCON is enabled.

If this entry can be added to dt residing in kernel, we can remove the
code that adds the "stdout-path" from OpenSBI.

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-- 
Regards,
Atish

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