From mboxrd@z Thu Jan  1 00:00:00 1970
From: p.zabel@pengutronix.de (Philipp Zabel)
Date: Mon, 13 Mar 2017 14:55:01 +0100
Subject: [PATCH v4 29/36] media: imx: mipi-csi2: enable setting and
 getting of frame rates
In-Reply-To: <20170313132701.GJ21222@n2100.armlinux.org.uk>
References: <1487211578-11360-1-git-send-email-steve_longerbeam@mentor.com>
 <1487211578-11360-30-git-send-email-steve_longerbeam@mentor.com>
 <20170220220409.GX16975@valkosipuli.retiisi.org.uk>
 <20170221001332.GS21222@n2100.armlinux.org.uk>
 <25596b21-70de-5e46-f149-f9ce3a86ecb7@gmail.com>
 <1487667023.2331.8.camel@pengutronix.de>
 <20170313131647.GB10701@valkosipuli.retiisi.org.uk>
 <20170313132701.GJ21222@n2100.armlinux.org.uk>
Message-ID: <1489413301.2288.53.camel@pengutronix.de>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Mon, 2017-03-13 at 13:27 +0000, Russell King - ARM Linux wrote:
> On Mon, Mar 13, 2017 at 03:16:48PM +0200, Sakari Ailus wrote:
> > The vast majority of existing drivers do not implement them nor the user
> > space expects having to set them. Making that mandatory would break existing
> > user space.
> > 
> > In addition, that does not belong to link validation either: link validation
> > should only include static properties of the link that are required for
> > correct hardware operation. Frame rate is not such property: hardware that
> > supports the MC interface generally does not recognise such concept (with
> > the exception of some sensors). Additionally, it is dynamic: the frame rate
> > can change during streaming, making its validation at streamon time useless.
> 
> So how do we configure the CSI, which can do frame skipping?
> 
> With what you're proposing, it means it's possible to configure the
> camera sensor source pad to do 50fps.  Configure the CSI sink pad to
> an arbitary value, such as 30fps, and configure the CSI source pad to
> 15fps.
> 
> What you actually get out of the CSI is 25fps, which bears very little
> with the actual values used on the CSI source pad.
> 
> You could say "CSI should ask the camera sensor" - well, that's fine
> if it's immediately downstream, but otherwise we'd need to go walking
> down the graph to find something that resembles its source - there may
> be mux and CSI2 interface subdev blocks in that path.  Or we just accept
> that frame rates are completely arbitary and bear no useful meaning what
> so ever.

Which would include the frame interval returned by VIDIOC_G_PARM on the
connected video device, as that gets its information from the CSI output
pad's frame interval.

regards
Philipp