Re: [Qemu-devel] [PATCH v2 2/2] vga: don't pick cirrus by default

[Sebastian Bauer <mail@sebastianbauer.info>]

Am 2018-07-11 17:48, schrieb Eduardo Habkost:
> "none" looked like a false positive when I first looked, but now
> I think it's not.  Shouldn't it set default_display="none"?

I think that there is some other logic burried that these machine 
doesn't get a graphics display. But overall it is indeed not clearly 
defined.

But see below.

>> If the patch is applied to 3.1 then I think there is enough time to 
>> fix
>> issues caused by the patch. Additionally, a warning could be put in 
>> the
>> ChangeLog for 3.0 that in 3.1 that the default mode will be std unless
>> machines define an own default. This is should be enough time for 
>> people to
>> complain or to fix things.
> I don't think we will really make user-visible changes: we can
> simply work to keep existing behavior, but the difference is that
> this will be implemented by setting default_display explicitly on
> all machines.

Even if all machines were using explicit default settings the patch will 
affect machines that are not inside the QEMU tree. If the patch is to be 
applied as it is these are affected. To warn users (or devs in this 
case) about this, an entry in the ChangeLog would be appropriate.

>> If the patch is to be applied to 3.0 then all non-ppc ones need to be
>> reconsidered.
>> The "important" ppc machines have been fixed already. I can do the 
>> remaining
>> if this is wanted.
> This part worries me: do we have other machines that are broken
> right now?

I don't know which of them are broken or how this can be elaborated, but 
they are potentially affected. For instance, the sam460ex platform 
doesn't care about this setting, I can say that it is not broken. Other 
platforms like the mac apparently were broken (and fixed in the 
meantime). It is hard to tell which of them are really broken without 
someone that knows the platform trying it and telling it. 'Make check' 
did catch only one single case. It could also be that nobody cares about 
other affected machines.

Overall I think the patch is an improvement over the previous state as 
preferring the Cirrus doesn't make much sense if most machines don't 
prefer it. The more I think over it, the more I think that the concept 
needs further fine-tuning though (not necessarily in this patch).

If OTOH it would become a requirement for machines to set a default 
display then this fallback logic could removed. Instead, qemu could 
simply bailout on machines that define no default display (including 
"none") and also bailout when the requested default display is not 
available. This would be checkable by 'make check'. I still think that 
the most common value can be a default (strictly bailing out when it is 
not available unlike it is done now), but this is a matter of taste I 
guess.

Bye
Sebastian