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

[Thomas Huth <thuth@redhat.com>]

On 11.07.2018 20:43, Eduardo Habkost wrote:
> On Wed, Jul 11, 2018 at 07:00:54PM +0200, Sebastian Bauer wrote:
>> 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.
> 
> What do you mean by "machines that are not inside the QEMU tree"?
> MachineClass registration is not an API for external use.
> 
>>
>>>> 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).
> 
> I'm not sure yet if most machines with default_display==NULL
> don't want Cirrus.  I'm also unsure if any of the machines from
> that list will break if we start using VGA by default.

I think most machines with default_display == NULL simply do not want
any graphic cards at all. So we likely document that default_display ==
NULL means no graphic card, and fix the machines that have other
assumptions if necessary.

>>                     [...]  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.
> 
> This is doable too, if we have a clear consensus on what would be
> a reasonable default on TYPE_MACHINE.  Personally I prefer the
> default on TYPE_MACHINE to be "none".

Yes, please, no magic default hardware for all machines. That will only
cause confusion and problems in the future. People who add new boards
will keep forgetting to set a value in their new boards and then you
later wonder whether that was on purpose or by accident ==> If there is
no default display set, this should simply mean "no display".

 Thomas