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In-Reply-To: <ded0b788f5896621cffd8f64c679ef375231b6d1.camel@ndufresne.ca>
From:   Ricardo Ribalda <ribalda@chromium.org>
Date:   Fri, 24 Nov 2023 10:58:01 +0100
Message-ID: <CANiDSCsctSyfgXxH3iroJPOE7R_9jB7todiEMBCU_5OG3EOCKQ@mail.gmail.com>
Subject: Re: [PATCH v2] media: uvcvideo: Implement V4L2_EVENT_FRAME_SYNC
To:     Nicolas Dufresne <nicolas@ndufresne.ca>
Cc:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Esker Wong <esker@google.com>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>,
        Sakari Ailus <sakari.ailus@iki.fi>,
        Esker Wong <esker@chromium.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
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Precedence: bulk

> >
> > For bulk devices this is a maximum of 0.05 msec (32KiB/600MBps)
>
> I lack a bit of knowledge on how to scale this to different devices, with
> different speed/framesize. My only bulk device is:
>
> https://inogeni.com/product/4k2usb3/
>
> Which is USB 3.0, and have raw (NV12) resolution from 640x480 (max 60pfs) to 4K
> (max 30fps). What would the error look like with that ?

For bulk devices the maximum delay from packing multiple packets into
a urb is 0.05 msec (32KiB/600MBps)

Laurent's message <20231109000327.GE21616@pendragon.ideasonboard.com>
explains where those numbers come from :).

Regards!

>
> > For 1MiB transfer isoc devices (which is the biggest we have seen),
> > that is 1.8 msec.
> > In both cases, this is smaller than the jitter to process the event
> > itself by userspace.
> >
> > The time from V4L2_EVENT_FRAME_SYNC to DQBUF is around 30 msec.
> >
> > I do not know how much delay is considered acceptable... but if we
> > take the delay argument to the extreme, we could say that
> > V4L2_EVENT_FRAME_SYNC can never be implemented, because the event will
> > always be delayed by something.
>
> We have v4l2_event.timestamp for all events, so the jitter to process the event
> by userpace can be removed reliably already.
>
> Nicolas
>
> p.s. missed it earlier
>
> >
> > >
> > > --
> > > Regards,
> > >
> > > Laurent Pinchart
> >
> >
> >
>


--
Ricardo Ribalda