linux-kernel - Re: [PATCH v4 23/36] media: imx: Add MIPI CSI-2 Receiver subdev driver

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Date:   Fri, 17 Feb 2017 12:38:20 +0100
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     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 23/36] media: imx: Add MIPI CSI-2 Receiver subdev
 driver

On Fri, 2017-02-17 at 11:06 +0000, Russell King - ARM Linux wrote:
> On Fri, Feb 17, 2017 at 11:47:59AM +0100, Philipp Zabel wrote:
> > On Wed, 2017-02-15 at 18:19 -0800, Steve Longerbeam wrote:
> > > +static void csi2_dphy_init(struct csi2_dev *csi2)
> > > +{
> > > +	/*
> > > +	 * FIXME: 0x14 is derived from a fixed D-PHY reference
> > > +	 * clock from the HSI_TX PLL, and a fixed target lane max
> > > +	 * bandwidth of 300 Mbps. This value should be derived
> > 
> > If the table in https://community.nxp.com/docs/DOC-94312 is correct,
> > this should be 850 Mbps. Where does this 300 Mbps value come from?
> 
> I thought you had some code to compute the correct value, although
> I guess we've lost the ability to know how fast the sensor is going
> to drive the link.

I had code to calculate the number of needed lanes from the bit rate and
link frequency. I did not actually change the D-PHY register value.
And as you pointed out, calculating the number of lanes is not useful
without input from the sensor driver, as some lane configurations might
not be supported.

> Note that the IMX219 currently drives the data lanes at 912Mbps almost
> exclusively, as I've yet to finish working out how to derive the PLL
> parameters.  (I have something that works, but it currently takes on
> the order of 100k iterations to derive the parameters.  gcd() doesn't
> help you in this instance.)

The tc358743 also currently only implements a fixed rate (of 594 Mbps).

regards
Philipp