Re: [Qemu-devel] [PATCH] Realview/Versatile: Export LED state

[Luiz Capitulino <lcapitulino@redhat.com>]

On Fri, 11 Jan 2013 16:44:49 +0100
Wolfgang Mauerer <wolfgang.mauerer@siemens.com> wrote:

> On 11/01/13 16:23, Andreas Färber wrote:
> > Am 11.01.2013 16:00, schrieb Peter Maydell:
> >> On 11 January 2013 13:42, Wolfgang Mauerer <wolfgang.mauerer@siemens.com> wrote:
> >>> The configuration register for the onboard LEDs is
> >>> emulated, but the state is not exported, which makes
> >>> the feature not particularly useful. Create a character
> >>> device to make status changes accessible to the host.
> >>>
> >>> For example, use the command line argument
> >>>
> >>> -chardev socket,id=leds,host=localhost,port=12345,server,nowait
> >>>
> >>> to observe status changes via a socket.
> >>
> >> This isn't the only board we emulate which has LEDs. I'd
> >> rather see this problem tackled with a general plan for "this
> >> is how we handle LEDs in QEMU boards" rather than a
> >> versatile board specific patch.
> > 
> > Hm, mips_malta.c does use a CharDriverState... just a more "structured"
> > one using ASCII art.
> 
> that's one of the reaons why I used a character device to export the
> status information.
> 
> However, a simple custom protocol (or maybe the same information in
> JSON) seemed more appripriate since I assume that graphical frontends
> will eventually visualise the LED status. Having to parse some ASCII
> art output for this seems suboptimal.
> 
> Exporting a binary QOM property (as you suggested in another email)
> might be an option, although I'm not really sure if LED status changes
> should be communicated this way -- to me, using a chardev feels nicer.
> Do the QMP maintainers have an opinion?

I'm not completely sure I understand the problem here, but if this is
some kind of status to be queried then exporting it through a QOM
property seems the right thing to do.

If you go for a custom protocol, then using QMP syntax might be a good
idea, as you can use our infra and clients which already know QMP will
also appreciate this (this is what qemu-ga did, btw).