tegra-camera-platform device tree settings

Robotics & Edge Computing Jetson & Embedded Systems Jetson TX1
1

I’m a bit confused by the options in the tegra-camera-platform device tree.

/**
* Physical settings to calculate max ISO BW
*
* num_csi_lanes = <>;
* Total number of CSI lanes when all cameras are active
*
* max_lane_speed = <>;
* Max lane speed in Kbit/s
*
* min_bits_per_pixel = <>;
* Min bits per pixel
*
* vi_peak_byte_per_pixel = <>;
* Max byte per pixel for the VI ISO case
*
* vi_bw_margin_pct = <>;
* Vi bandwidth margin in percentage
*
* max_pixel_rate = <>;
* Max pixel rate in Kpixel/s for the ISP ISO case
*
* isp_peak_byte_per_pixel = <>;
* Max byte per pixel for the ISP ISO case
*
* isp_bw_margin_pct = <>;
* Isp bandwidth margin in percentage
*/

num_csi_lanes makes sense, if I have two cameras that each use one lane then it would be 2. Or if both of those cameras used 2 lanes, it would be 4.

max_lane_speed is how fast the TX should talk to the camera over mipi. What was this value in r24.2?

min_bits_per_pixel I assume is your bayer pixel format bits per pixel. So MEDIA_BUS_FMT_SRGGB12_1X12 would be 12, or MEDIA_BUS_FMT_SRGGB12_1X10 would be 10.

As for the rest, what are “VI ISO case” and “ISP ISO case?” And how do we calculate the ISP bandwidth margin?

2

Found the default values from r24.2:

#define CAMERA_PEAK_BPP 2
#define LANE_SPEED_1_GBPS 1000000000
#define LANE_SPEED_1_5_GBPS 1500000000

#if defined(CONFIG_ARCH_TEGRA_21x_SOC)
    num_csi_lanes = 2;
    max_num_streams = 6;
    max_lane_speed = LANE_SPEED_1_5_GBPS;
    min_bits_per_pixel = 10;
#elif defined(CONFIG_ARCH_TEGRA_12x_SOC) || defined(CONFIG_ARCH_TEGRA_13x_SOC)
    num_csi_lanes = 2;
    max_num_streams = 2;
    max_lane_speed = LANE_SPEED_1_GBPS;
    min_bits_per_pixel = 10;

Then there’s this formula

/*
     * Let's go with simple registering max dedicated BW
     * approach for now.
     *
     * The formula is:
     * Camera's max total ISO BW =
     * ((max_num_streams *
     * num_csi_lanes * max_lane_speed) /
     * min_bits_per_pixel) * max_peak_BPP
     *
     * Above considered cap is CSI link cap, but we need to
     * consider real sensor-on-board cap also, DT based approach
     * should handle it.
     *
     * Only VI out is considered, because in case of
     * max # of cameras running, only VI is in ISO mode.
     *
     * TODO: Try renegotiate approach later.
     */

So Camera’s max total ISO bandwidth in r24.2 would be (((2 * 1500000000 * 6) / 10) * 2) / 1000 = 3600000 Kbit/s

3

hello Atrer,

please check the [Sensor Driver Programming Guide] from L4T documentation.
there are descriptions about these properties,
thanks

4

Which version? I’m looking at r28.2 and I don’t even see a mention that those are required for tegra-camera-platform. I had to find out about them from the dtsi.

Here’s the example it gives:

tegra-camera-platform {
    compatible = "nvidia, tegra-camera-platform";
    modules {
        module0 {
            badge = "imx185_bottom_liimx185";
		   position = "bottom";
		   orientation = "0";
		   drivernode0 {
                pcl_id = "v4l2_sensor";
                devname = "imx185 30-001a";
                proc-device-tree = "/proc/device-tree/i2c@3180000/tca9546@70/i2c@0/imx185_a@1a";
            };        };
    };
5

hello Atrer,

please check [L4T R28.2 DP Documentation]-> [Release 28.2 Development Guide]-> [Camera Development]-> [Sensor Driver Programming Guide]
searching for the “Module Properties” for the descriptions.
thanks

6

Hello JerryChang,

Is there really a description of ‘max_lane_speed’ in the documentation ? I don’t find it.

What’s the unit of that value ? bits/s or Kbits/s ?

7

hello phdm,

it is used to calculate ISO bandwidth, you could found the formula in the kernel sources,
thanks

sources/kernel/kernel-4.4/drivers/video/tegra/camera/tegra_camera_platform.c