Re: [virtio-dev] virtio-console: initial size retrieval, port parameter reporting

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Oct 13, 2019 at 07:48:09PM +0200, Jan Kiszka wrote:
> Hi all,
> 
> while implementing a basic virtio-console backend, I noticed some grey
> area regarding when and how the initial console size should be read by
> the driver.
> 
> Linux currently only does that sizing when the device signals a config
> change, either via a config interrupt (non-multiport) or by sending a
> VIRTIO_CONSOLE_RESIZE control message. But the spec is not clear whether
> a device is supposed to do exactly that.
> 
> Let's look at the two cases, first without VIRTIO_CONSOLE_F_MULTIPORT:
> I would have expected that the driver can find the console size in the
> configuration early, even before FEATURES_OK. And I would have expected
> that Linux evaluates that value during probe. Again, the spec does not
> tell us whether the device must send that initial config update to
> declare the size information valid.
> 
> Now with VIRTIO_CONSOLE_F_MULTIPORT:
> Here, the device can only signal the dimensions for ports > 0 via a
> control message. This suggests (but the spec does not state it
> explicitly) that the device will likely send VIRTIO_CONSOLE_RESIZE
> message after VIRTIO_CONSOLE_PORT_READY. And that also for port 0,
> although its size is available also via the config. BTW, is a device in
> that case also supposed to signal a config change? Or should a device
> never offer F_MULTIPORT and F_SIZE at the same time?
> 
> In general, the spec does not state when a device should (or rather
> must) send any of the _CONSOLE_PORT, _RESIZE and _PORT_NAME messages. I
> assume they are meant to be sent after _PORT_READY from the driver
> because the spec says:
> 
> "After creating and initializing each port, a VIRTIO_CONSOLE_PORT_READY
> control message is sent to the device for that port so the device can
> let the driver know of any additional configuration options set for that
> port."
> 
> If my assumption is correct, that ordering should be stated more
> explicitly in the spec.
> 
> Jan

Makes sense to me.


> -- 
> Siemens AG, Corporate Technology, CT RDA IOT SES-DE
> Corporate Competence Center Embedded Linux
> 
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