Re: [Qemu-devel] [PATCH 0/3] vga: add endianness switching support

[Alexander Graf]

On 23.09.14 22:24, Benjamin Herrenschmidt wrote:
> On Tue, 2014-09-23 at 14:30 +0200, Gerd Hoffmann wrote:
>>   Hi,
> 
> (Adding Mike)
> 
>> Series applies on top of the vga patches posted yesterday.
>> It adds a mmio register to the pci stdvga cards allowing
>> to switch framebuffer endiannness.  Ben's patches 1+2 prepare
>> for runtime-swicthable endianness and patch #3 actually
>> implements the new register.  It has been placed in a new
>> range to avoid conflicts in case the bochs dispi interface
>> will be extended in the future.  The separate range also
>> makes it easier to turn off on old machine types.
> 
> Thanks. I'll try to produce patches for offb and bochsdrmfb to
> adjust endianness & test some time this week or next week.
> 
> Now the remaining point of contention is the "auto switch"
> functionality which, sadly, is needed for existing LE guests to work
> properly.
> 
> I understand the reticence, on the other hand, our "pseries" machine is
> really a paravirtualized environment where qemu implements the
> hypervisor hcall interface, so in that context, it does make some sense
> that the "hypervisor" affects some HW configuration/state on endian
> switch (which is such an hcall).
> 
> The problem is how to do that in a reasonably clean way from an
> implementation perspective.
> 
> The patch at
> https://github.com/ozbenh/qemu/commit/778c92cebcd12fdafd2050dc4d7ca2521c462e46
>  is one attempt to do it, for whatever it's worth, but I'm open to 
> alternatives.

Apart from stylistic problems I think the approach is reasonable. The
only alternative I could think of would be to simply loop through every
device in the device tree we can find and search for VGA objects.


Alex