From: Gerd Hoffmann <kraxel@redhat.com>
To: Alexandre Courbot <acourbot@chromium.org>
Cc: "Keiichi Watanabe" <keiichiw@chromium.org>,
virtio-dev@lists.oasis-open.org,
"Linux Media Mailing List" <linux-media@vger.kernel.org>,
"Alex Lau" <alexlau@chromium.org>,
"Daniel Vetter" <daniel@ffwll.ch>,
"Dylan Reid" <dgreid@chromium.org>,
"David Staessens" <dstaessens@chromium.org>,
"Dmitry Sepp" <dmitry.sepp@opensynergy.com>,
"Enrico Granata" <egranata@google.com>,
"Frediano Ziglio" <fziglio@redhat.com>,
"Hans Verkuil" <hverkuil@xs4all.nl>,
"Stéphane Marchesin" <marcheu@chromium.org>,
"Pawel Osciak" <posciak@chromium.org>,
spice-devel@lists.freedesktop.org,
"David Stevens" <stevensd@chromium.org>,
"Tomasz Figa" <tfiga@chromium.org>,
uril@redhat.com,
"Samiullah Khawaja" <samiullah.khawaja@opensynergy.com>,
"Kiran Pawar" <kiran.pawar@opensynergy.com>
Subject: Re: [PATCH v3 1/2] virtio-video: Add virtio video device specification
Date: Wed, 4 Mar 2020 07:42:51 +0100 [thread overview]
Message-ID: <20200304064251.zzkhevqgth6uets6@sirius.home.kraxel.org> (raw)
In-Reply-To: <CAPBb6MWwBbNULCfMxN_KLt_Zd8kmmNy2JPi6XjLF1YgxxCPydw@mail.gmail.com>
Hi,
> > With a feature flag both driver and device can choose whenever they want
> > support v1 or v2 or both. With a version config field this is more
> > limited, the device can't decide to support both. So the bonus points
> > for a smooth transition go to the feature flags not the version field ;)
>
> I agree that feature flags would be preferable in general, but I'm
> concerned by the fact that there is (IIUC) a limited number of them.
We have 64 total, some reserved, 24 are available to devices right now,
see https://www.kraxel.org/virtio/virtio-v1.1-cs01-video-v3.html#x1-130002
> Video tends to change significantly over time, and to have optional
> features that would also be presented as feature flags. After a while
> we may run out of them, while a new protocol version would allow us to
> extend the config struct with some new flags. Or am I missing
> something?
Not everything needs a feature flag. For example we have
VIRTIO_VIDEO_CMD_QUERY_CAPABILITY, and we can add new video formats
without needing a feature flag. Maybe it is a good idea to explicitly
say in the specs that this can happen and that the driver should simply
ignore any unknown format returned by the device.
> I also wonder how "support v1 or v2 or both" would work with feature
> flags. In order to make it possible to opt out of v1, I guess we would
> need "v1 supported" flag to begin with?
The driver can ignore any device without v2 feature flag set.
The device can refuse to accept a driver without v2 support (don't allow
setting the FEATURES_OK bit).
A explicit v1 feature flag would allow the guest driver print a more
specific error message ("device doesn't support v1" instead of "feature
negotiation failed"), but that's it.
cheers,
Gerd
next prev parent reply other threads:[~2020-03-04 6:43 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-06 10:20 [PATCH v3 0/2] Virtio video device specification Keiichi Watanabe
2020-02-06 10:20 ` [PATCH v3 1/2] virtio-video: Add virtio " Keiichi Watanabe
2020-02-25 9:59 ` Gerd Hoffmann
2020-02-27 7:24 ` Keiichi Watanabe
2020-02-27 9:28 ` Gerd Hoffmann
2020-03-04 4:31 ` Alexandre Courbot
2020-03-04 6:42 ` Gerd Hoffmann [this message]
2020-03-04 10:07 ` Alexandre Courbot
2020-03-23 12:07 ` Keiichi Watanabe
2020-03-23 13:28 ` Dmitry Sepp
2020-03-23 15:48 ` Keiichi Watanabe
2020-03-25 9:47 ` Dmitry Sepp
2020-03-27 3:35 ` Keiichi Watanabe
2020-03-30 9:53 ` Dmitry Sepp
2020-04-06 9:32 ` Alexandre Courbot
2020-04-06 11:46 ` Keiichi Watanabe
2020-04-07 9:21 ` Dmitry Sepp
2020-04-09 10:46 ` Keiichi Watanabe
2020-04-17 8:08 ` Dmitry Sepp
2020-04-20 9:57 ` Keiichi Watanabe
2020-04-21 8:38 ` Dmitry Sepp
2020-04-24 11:42 ` Keiichi Watanabe
2020-04-27 14:28 ` Dmitry Sepp
2020-04-07 14:49 ` Dmitry Sepp
2020-04-09 10:46 ` Keiichi Watanabe
2020-04-09 13:13 ` Dmitry Sepp
2020-04-24 11:45 ` Keiichi Watanabe
2020-04-27 9:33 ` Dmitry Sepp
2020-05-18 5:17 ` Keiichi Watanabe
2020-05-27 12:12 ` Dmitry Sepp
2020-05-29 14:21 ` Keiichi Watanabe
2020-06-01 7:19 ` Alexandre Courbot
2020-02-06 10:20 ` [PATCH v3 2/2] virtio-video: Define a feature for exported objects from different virtio devices Keiichi Watanabe
2020-02-25 10:01 ` Gerd Hoffmann
2020-02-27 7:24 ` Keiichi Watanabe
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