Re: [PATCH kvmtool 2/6] arm: fdt: add stdout-path to /chosen node

[Andre Przywara <andre.przywara@arm.com>]

On Fri, 1 Feb 2019 06:26:58 +0000
Will Deacon <will.deacon@arm.com> wrote:

Hi,

> On Thu, Jan 31, 2019 at 02:57:11PM +0000, Andre Przywara wrote:
> > On Wed, 30 Jan 2019 18:20:19 +0000
> > Will Deacon <will.deacon@arm.com> wrote:  
> > > On Fri, Jan 25, 2019 at 06:07:57PM +0000, Andre Przywara wrote:  
> > > > The DT spec describes the stdout-path property in the /chosen
> > > > node to contain the DT path for a default device usable for
> > > > outputting characters. The Linux kernel uses this for earlycon
> > > > (without further parameters), other DT users might rely on this
> > > > as well.
> > > > 
> > > > Add a property containing the path to our emulated 8250 serial
> > > > device.
> > > > 
> > > > Even when we use the virtio console, the serial console is still
> > > > there and works, so we can expose this unconditionally. Putting
> > > > the virtio console path in there will not work anyway.
> > > > 
> > > > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > > > ---
> > > >  arm/fdt.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/arm/fdt.c b/arm/fdt.c
> > > > index 28ba1c2c..8cda3ded 100644
> > > > --- a/arm/fdt.c
> > > > +++ b/arm/fdt.c
> > > > @@ -143,6 +143,7 @@ static int setup_fdt(struct kvm *kvm)
> > > >  	_FDT(fdt_property_cell(fdt, "linux,pci-probe-only",
> > > > 1)); _FDT(fdt_property_string(fdt, "bootargs",
> > > > kvm->cfg.real_cmdline)); _FDT(fdt_property_u64(fdt,
> > > > "kaslr-seed", kvm->cfg.arch.kaslr_seed));
> > > > +	_FDT(fdt_property_string(fdt, "stdout-path",
> > > > "/U6_16550A@3f8"));    
> > > 
> > > Since the last string here has to match the one in hw/serial.c, I
> > > think we should be retrieving it from there rather than
> > > hardcoding it here.  
> > 
> > Are you thinking about something like setting a "char
> > *primary_console_path" in
> > hw/serial.c:serial8250_generate_fdt_node(), then using this here in
> > arm/fdt.c? Or shall the generate function directly set the
> > stdout-path?  
> 
> It's probably a bit dodgy doing it from the generate function,
> because I think we'd then be relying on the the /chosen node being
> created before the serial node (which is true, but I wouldn't like to
> rely on it).

Agreed.

> So I think either have a way to register the primary console device,
> or a way to squirrel the string away somewhere. Is there a variant of
> stdout-path which uses a phandle instead of a string?  That might end
> up being cleaner to implement.

stdout-path must be a string, so a phandle will not work. However that
string can be an alias, so I now let stdout-path always be "serial0",
then add an aliases node at the end of the FDT generation, assigning
serial0 to the path of the first instantiated serial device.

I will post a v2 with that.

Cheers,
Andre.