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

[Philipp Zabel]

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