linux-kernel - Re: [PATCH v4 29/36] media: imx: mipi-csi2: enable setting and getting of frame rates

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Message-ID: <1489413301.2288.53.camel@pengutronix.de>
Date:   Mon, 13 Mar 2017 14:55:01 +0100
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Sakari Ailus <sakari.ailus@....fi>,
        Steve Longerbeam <slongerbeam@...il.com>, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, kernel@...gutronix.de,
        fabio.estevam@....com, mchehab@...nel.org, hverkuil@...all.nl,
        nick@...anahar.org, markus.heiser@...marIT.de,
        laurent.pinchart+renesas@...asonboard.com, bparrot@...com,
        geert@...ux-m68k.org, arnd@...db.de, sudipm.mukherjee@...il.com,
        minghsiu.tsai@...iatek.com, tiffany.lin@...iatek.com,
        jean-christophe.trotin@...com, horms+renesas@...ge.net.au,
        niklas.soderlund+renesas@...natech.se, robert.jarzmik@...e.fr,
        songjun.wu@...rochip.com, andrew-ct.chen@...iatek.com,
        gregkh@...uxfoundation.org, shuah@...nel.org,
        sakari.ailus@...ux.intel.com, pavel@....cz,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org,
        devel@...verdev.osuosl.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>
Subject: Re: [PATCH v4 29/36] media: imx: mipi-csi2: enable setting and
 getting of frame rates

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