Re: [Xen-devel] Question on hvc console init

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-10-28 at 17:32 +0000, Julien Grall wrote:
> Hi Konrad,
> 
> On 10/28/2014 04:53 PM, Konrad Rzeszutek Wilk wrote:
> > On Tue, Oct 28, 2014 at 05:53:25PM +0200, Iurii Konovalenko wrote:
> >> Hello, all!
> >>
> >> I try to bring up Xen on Renesas Lager board (r8a7790 SoC - R-Car H2).
> >> Xen revision is 4.4.
> >> I try to run Linux (kernel 3.14 + LTSI patches) as Dom0.
> >> In kernel I've found strange behaviour in hvc console init function.
> >> In file drivers/tty/hvc/hvc_xen.c in function xen_cons_init(void) sources 
> >> are:
> >>
> >>     if (!xen_domain())
> >>         return 0;
> >>
> >>     if (xen_initial_domain())
> >>         ops = &dom0_hvc_ops;
> >>     else {
> >>
> >> xen_domain() and xen_initial_domain() are defined to check
> >> xen_domain_type variable. This variable is defined and initialized to
> >> XEN_NATIVE in arch/arm/xen/enlighten.c. The real value of this
> >> variable is set in same file function xen_guest_init(), that is
> >> early_initcall. But eraly_initcall is called later, than
> >> console_initcall, that's why in time of running xen_cons_init(void)
> >> xen_domain_type is not initialized to correct value and
> >> xen_cons_init() does not initialize console, as returns on first check
> >> "if (!xen_domain())".
> >> It is not critical in normal operation, because we have
> >> device_initcall xen_hvc_init() that is called after xen_guest_init(),
> >> it initialize hvc. But in case of kernel falls before
> >> device_initcall's, we can't see any printouts, that could be useful.
> >>
> >> Could you please explain, may be using some configs or arguments in
> >> command line for kernel, how to enable this early console?
> > 
> > 
> > It is explained in the kernel-parameters.txt 
> > (https://www.kernel.org/doc/Documentation/kernel-parameters.txt)
> > :
> > 
> > earlyprintk=        [X86,SH,BLACKFIN,ARM,M68k]
> >                     earlyprintk=vga
> >                     earlyprintk=efi
> >                     earlyprintk=xen
> > ...
> 
> AFAIK, earlyprintk=xen has never work on ARM. This is because we don't
> know that we running on Xen until late.

earlyprintk on arm doesn't parse it's argument at all, it is a boolean
option which just enables printk via the hardcoded (via .config) debug
UART. Because of that it's incompatible with multiplatform kernels as
well.

Some platforms (e.g. arm64) have replace earlyprintk with a more
flexible earlycon, I don't know if arm is one of those (yet?). It would
probably be nice to integrate the xen early print stuff with earlycon at
some point.

Ian.


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