[PATCH] serial/sifive: select SERIAL_EARLYCON

[PATCH] serial/sifive: select SERIAL_EARLYCON

[Christoph Hellwig]

The sifive serial driver implements earlycon support, but unless
another driver is built in that supports earlycon support it won't
be usable.  Explicitly select SERIAL_EARLYCON instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 drivers/tty/serial/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index 530cb966092f..6b77a72278e3 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig
@@ -1075,6 +1075,7 @@ config SERIAL_SIFIVE_CONSOLE
 	bool "Console on SiFive UART"
 	depends on SERIAL_SIFIVE=y
 	select SERIAL_CORE_CONSOLE
+	select SERIAL_EARLYCON
 	help
 	  Select this option if you would like to use a SiFive UART as the
 	  system console.
-- 
2.20.1


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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Andreas Schwab]

On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:

> The sifive serial driver implements earlycon support,

It should probably be documented in admin-guide/kernel-parameters.txt.

Andreas.

-- 
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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Christoph Hellwig]

On Tue, Sep 10, 2019 at 08:57:37AM +0200, Andreas Schwab wrote:
> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
> 
> > The sifive serial driver implements earlycon support,
> 
> It should probably be documented in admin-guide/kernel-parameters.txt.

How so?  Wіth OF and a stdout path you just set earlycon on the
command line without any arguments and it will be found.

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Andreas Schwab]

On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:

> On Tue, Sep 10, 2019 at 08:57:37AM +0200, Andreas Schwab wrote:
>> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
>> 
>> > The sifive serial driver implements earlycon support,
>> 
>> It should probably be documented in admin-guide/kernel-parameters.txt.
>
> How so?  Wіth OF and a stdout path you just set earlycon on the
> command line without any arguments and it will be found.

Doesn't work for me.

[    0.000000] Malformed early option 'earlycon'

Andreas.

-- 
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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Christoph Hellwig]

On Tue, Sep 10, 2019 at 10:18:15AM +0200, Andreas Schwab wrote:
> > How so?  Wіth OF and a stdout path you just set earlycon on the
> > command line without any arguments and it will be found.
> 
> Doesn't work for me.
> 
> [    0.000000] Malformed early option 'earlycon'

That functionality is implemented by param_setup_earlycon and
early_init_dt_scan_chosen_stdout.  Check why those aren't built into
your kernel.

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Andreas Schwab]

On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:

> On Tue, Sep 10, 2019 at 10:18:15AM +0200, Andreas Schwab wrote:
>> > How so?  Wіth OF and a stdout path you just set earlycon on the
>> > command line without any arguments and it will be found.
>> 
>> Doesn't work for me.
>> 
>> [    0.000000] Malformed early option 'earlycon'
>
> That functionality is implemented by param_setup_earlycon and
> early_init_dt_scan_chosen_stdout.  Check why those aren't built into
> your kernel.

They are.

Andreas.

-- 
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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Palmer Dabbelt]

On Tue, 10 Sep 2019 07:36:30 PDT (-0700), Christoph Hellwig wrote:
> On Tue, Sep 10, 2019 at 10:18:15AM +0200, Andreas Schwab wrote:
>> > How so?  Wіth OF and a stdout path you just set earlycon on the
>> > command line without any arguments and it will be found.
>> 
>> Doesn't work for me.
>> 
>> [    0.000000] Malformed early option 'earlycon'
>
> That functionality is implemented by param_setup_earlycon and
> early_init_dt_scan_chosen_stdout.  Check why those aren't built into
> your kernel.

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.

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Christoph Hellwig]

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.

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Palmer Dabbelt]

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.

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Andreas Schwab]

On Sep 16 2019, Palmer Dabbelt <palmer@sifive.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.

Andreas.

-- 
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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Palmer Dabbelt]

On Mon, 16 Sep 2019 12:40:10 PDT (-0700), schwab@suse.de wrote:
> On Sep 16 2019, Palmer Dabbelt <palmer@sifive.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.

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Andreas Schwab]

On Sep 16 2019, Palmer Dabbelt <palmer@sifive.com> wrote:

> 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.

If that's how it's supposed to work, ok.  Other platforms add it in
u-boot.

Andreas.

-- 
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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Atish Patra]

On Mon, 2019-09-16 at 14:38 -0700, Palmer Dabbelt wrote:
> On Mon, 16 Sep 2019 12:40:10 PDT (-0700), schwab@suse.de wrote:
> > On Sep 16 2019, Palmer Dabbelt <palmer@sifive.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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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Paul Walmsley]

[-- Attachment #1: Type: text/plain, Size: 830 bytes --]

On Tue, 10 Sep 2019, Andreas Schwab wrote:

> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
> 
> > On Tue, Sep 10, 2019 at 08:57:37AM +0200, Andreas Schwab wrote:
> >> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
> >> 
> >> > The sifive serial driver implements earlycon support,
> >> 
> >> It should probably be documented in admin-guide/kernel-parameters.txt.
> >
> > How so?  Wіth OF and a stdout path you just set earlycon on the
> > command line without any arguments and it will be found.
> 
> Doesn't work for me.
> 
> [    0.000000] Malformed early option 'earlycon'

Try:

    earlycon=sifive,0x10010000

If it works, you should see this message on the kernel console very early 
in the boot:

[    0.000000] earlycon: sifive0 at MMIO 0x0000000010010000 (options '')


- Paul

[-- Attachment #2: Type: text/plain, Size: 161 bytes --]

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Andreas Schwab]

On Sep 16 2019, Paul Walmsley <paul.walmsley@sifive.com> wrote:

> On Tue, 10 Sep 2019, Andreas Schwab wrote:
>
>> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
>> 
>> > On Tue, Sep 10, 2019 at 08:57:37AM +0200, Andreas Schwab wrote:
>> >> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
>> >> 
>> >> > The sifive serial driver implements earlycon support,
>> >> 
>> >> It should probably be documented in admin-guide/kernel-parameters.txt.
>> >
>> > How so?  Wіth OF and a stdout path you just set earlycon on the
>> > command line without any arguments and it will be found.
>> 
>> Doesn't work for me.
>> 
>> [    0.000000] Malformed early option 'earlycon'
>
> Try:
>
>     earlycon=sifive,0x10010000

That's not what Christoph wants.

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
GPG Key fingerprint = 0196 BAD8 1CE9 1970 F4BE  1748 E4D4 88E3 0EEA B9D7
"And now for something completely different."

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Christoph Hellwig]

On Mon, Sep 16, 2019 at 02:00:39PM +0200, Andreas Schwab wrote:
> > Try:
> >
> >     earlycon=sifive,0x10010000
> 
> That's not what Christoph wants.

Well, what I want is auto selection if the device tree uses the
stdout-path.  My Kendryte DT uses that and it works like a charm.  If
it doesn't work for you on the U54 board chances are that your DT
doesn't have the right settings and you have to use the line that
Paul posted.

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Paul Walmsley]

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On Mon, 16 Sep 2019, Andreas Schwab wrote:

> On Sep 16 2019, Paul Walmsley <paul.walmsley@sifive.com> wrote:
> 
> > On Tue, 10 Sep 2019, Andreas Schwab wrote:
> >
> >> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
> >> 
> >> > On Tue, Sep 10, 2019 at 08:57:37AM +0200, Andreas Schwab wrote:
> >> >> On Sep 10 2019, Christoph Hellwig <hch@lst.de> wrote:
> >> >> 
> >> >> > The sifive serial driver implements earlycon support,
> >> >> 
> >> >> It should probably be documented in admin-guide/kernel-parameters.txt.
> >> >
> >> > How so?  Wіth OF and a stdout path you just set earlycon on the
> >> > command line without any arguments and it will be found.
> >> 
> >> Doesn't work for me.
> >> 
> >> [    0.000000] Malformed early option 'earlycon'
> >
> > Try:
> >
> >     earlycon=sifive,0x10010000
> 
> That's not what Christoph wants.

I support Christoph's plan to add generic implicit earlycon support.

In the meantime, once v5.4-rc1 is released, I'll send a patch to add a 
sifive driver section to the earlycon documentation in 
admin-guide/kernel-parameters.txt.  Thanks for the (earlier) suggestion.


- Paul

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Christoph Hellwig]

On Mon, Sep 16, 2019 at 05:18:57AM -0700, Paul Walmsley wrote:
> I support Christoph's plan to add generic implicit earlycon support.

I'm not planning to add it, it exist already.  You just need a DT that
sets the stdout node up properly, and a earlycon driver using
OF_EARLYCON_DECLARE like the sifive uart driver.

The DT here for example works perfectly fine:

http://git.infradead.org/users/hch/riscv.git/commitdiff/f10e64873eafc68516b8884c06b9290b9887633b

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Re: [PATCH] serial/sifive: select SERIAL_EARLYCON

[Paul Walmsley]

On Tue, 10 Sep 2019, Christoph Hellwig wrote:

> The sifive serial driver implements earlycon support, but unless
> another driver is built in that supports earlycon support it won't
> be usable.  Explicitly select SERIAL_EARLYCON instead.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>

- Paul

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