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

[Steve Longerbeam]

On 02/22/2017 03:38 PM, Steve Longerbeam wrote:
> On 02/17/2017 03:38 AM, Philipp Zabel wrote:
> > 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).
>
> I've analyzed the OV5640 video modes, and they generate the following
> "sysclk"'s in Mhz:
>
> 210
> 280
> 420
> 560
> 840
>
> But I don't know whether this is equivalent to bit rate. Is it the same
> as Mbps-per-lane? If so, this could be indicated to the sink by
> implementing V4L2_CID_LINK_FREQ in the ov5640.c sensor driver.

er, rather, if I knew what this "sysclk" value was, I could use it
to convert to a link frequency and return it in V4L2_CID_LINK_FREQ.

Steve

> The Mbps-per-lane value would then be link_freq * 2, and then
> Mbps-per-lane could be used to lookup the correct "hsfreqsel"
> value to program the D-PHY.
>
> I've added this to imx6-mipi-csi2.c. If the source didn't implement
> V4L2_CID_LINK_FREQ then it uses a default 849 Mbps-per-lane.
>
>
> Steve