Questions about the pixel clock of HDMI

Robotics & Edge Computing Jetson & Embedded Systems Jetson TK1
1

Hi All,
I notice that the pixel clock of HDMI can only be a multiple of 12000000 HZ.

In kernel/drivers/video/tegra/dc/hdmi.c

static unsigned long  tegra12x_hdmi_determine_parent(
        const struct tegra_dc *dc, struct clk *parent_clk, int pclk)
{
        /* T124 hdmi pclk:
         *   parentClk = pclk * m  (m=1,1.5,2,2.5,...,128.5)
         *   (refclk * n) = pclk * m  (n=1,1.5,2,2.5,...,128.5)
         *     (no half resolutions for m due to uneven out duty cycle)
         *   (refclk * N / 2) = pclk * m  (N=2,3,4,...,257)
         *   m = (refclk / 2 * N) / pclk  (m=1,2,3,...,128)
         *     looking for N to make m whole number
         */
        int  n, m;
        int  b, fr, f;

        /* following parameters should come from parent clock */
        const int  ref  = 12000000;   /* reference clock to parent */
        const int  pmax = 600000000;  /* max freq of parent clock */
        b = 0;
        fr = 1000;
        
        for (n = 4; (ref / 2 * n) <= pmax; n++) {
                if ((ref / 2 * n) < pclk)  /* too low */
                        continue;
                m = DIV_ROUND_UP((ref / 2 * n), (pclk / 1000));
                f = m % 1000;  /* fractional parts */
                f = (0 == f) ? f : (1000 - f);  /* round-up */
                if (0 == f) {  /* exact match */
                        if ((ref / 2 * b) < 100000000) {
                                /* parent clock runs at a minumum of 100MHz */
                                continue;
                        }
                        b = n;
                        fr = f;
                        break;
                } else if (f < fr) {
                        b = n;
                        fr = f;
                }
        }

        return (unsigned long)(ref / 2 * b);
}

I would like to know if this is the hardware limitation?
Can we set a specific pixel clock something like 275330000HZ?

2

FrankPCP,

I just went through the history of this code and it is indeed a hw limitation of TK1. As I understand, even TX1/TX2 cannot give out accurate pclk due to the rounding up and clk limitation.

3

Hi

If so, what is the case with this release of Xavier? Experience has shown that TX1 / TX2 can set a pixel clock relatively more precisely than TK1, but I wonder if Xavier has improved a bit more. I wonder if it is comparable to the graphics card used for PC or Notebook.

4

We have a updated driver for xavier and shall be in next release.

5

When is the next release about?

And if I buy Xavier now, can not I see the HDMI PIXEL CLOCK improvements in the currently released Jetpack compared to TX1 / TX2?