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

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Message-ID: <20170313132701.GJ21222@n2100.armlinux.org.uk>
Date:   Mon, 13 Mar 2017 13:27:02 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        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, 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.

-- 
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