Re: [Xen-devel] [PATCH RFC 3/9] console: add EHCI debug port based serial console

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.07.12 at 22:51, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
>> +    set_fixmap_nocache(FIX_EHCI_DBGP, dbgp->bar_val);
> 
> Should this have dbgp->bar_val & PAGE_MASK ?

No, the set_fixmap_nocache() implementation shifts out the low
bits.

>> +    case PHYSDEVOP_DBGP_RESET_PREPARE:
> 
> Oh, you have a handoff protocol!
> 
> So how does this work without using this? Meaning if one uses Xen hypervisor
> EHCI with a pvops kernel that does not implement this?

The hypervisor will lose access to its console (and the next console
access will cause a brief hang that does no other harm). That's the
same behavior as for native Linux if you ripped out the handoff
protocol (and the effect is even specified that way by the
specification). Plus - no-one is required to use the new functionality
when (s)he's aware that Dom0 doesn't cope.

>> +        return ehci_dbgp_external_startup(&ehci_dbgp) ?: 1;
> 
> Oh, this is new. With the ?: it just passes on the return value from
> ehci_dbgp_external_startup?

Yes, but it's not new. Just grep for the construct in Xen or Linux -
it's being used in a number of places, and has been supported by
gcc virtually forever.

Jan


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