Re: [RFC PATCH] v4l2-dv-timings: add support for reduced blanking v2

Hans Verkuil
2015-06-08T10:44:08Z

On 06/05/2015 10:52 AM, Prashant Laddha wrote:
> Added support for reduced blanking version 2 (RB v2) in cvt timings.
> Standard specifies a fixed vsync pulse of 8 lines to indicate RB v2
> timings. Vertical back porch is fixed at 6 lines and vertical front
> porch is remainder of vertical blanking time.
>
> For Rb v2, horizontal blanking is fixed at 80 pixels. Horizontal sync
> is fixed at 32. All horizontal timing counts (active pixels, front,
> back porches) can be specified upto a precision of 1.
>
> To Do: Pass aspect ratio information to v4l2_detect_cvt()
> RB v2 allows for non standard aspect ratios. In RB v2 vsync does not
> indicate aspect ratio. In the absence of aspect ratio information,
> v4l2_detect_cvt() cannot calculate image width from image height.

Reading the CVT spec I see that RB v2 assumes that the pixel aspect ratio
is always square (section 3.4.3, item 7 says that the aspect ratio is
to be derived from the active video resolution, which is only possible
if the pixel aspect ratio is square.

So I would suggest adding a 'frame_width' argument to v4l2_detect_cvt: this
will be used in the v2 case. If RB v2 is detected and frame_width is 0, then
v4l2_detect_cvt will return false.

Since RB v2 is something we'll only see with digital video interfaces where
we always know the horizontal active video, this shouldn't be a problem.

Some code comments below:

>
> Cc: Hans Verkuil <hans.verkuil@cisco.com>
> Signed-off-by: Prashant Laddha <prladdha@cisco.com>
> ---
> drivers/media/v4l2-core/v4l2-dv-timings.c | 72 +++++++++++++++++++++++--------
> 1 file changed, 54 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/v4l2-dv-timings.c b/drivers/media/v4l2-core/v4l2-dv-timings.c
> index 0d849fc..4efc6f6 100644
> --- a/drivers/media/v4l2-core/v4l2-dv-timings.c
> +++ b/drivers/media/v4l2-core/v4l2-dv-timings.c
> @@ -309,6 +309,7 @@ EXPORT_SYMBOL_GPL(v4l2_print_dv_timings);
> */
>
> #define CVT_PXL_CLK_GRAN 250000 /* pixel clock granularity */
> +#define CVT_PXL_CLK_GRAN_RB_V2 1000 /* granularity for reduced blanking v2*/
>
> /* Normal blanking */
> #define CVT_MIN_V_BPORCH 7 /* lines */
> @@ -328,10 +329,14 @@ EXPORT_SYMBOL_GPL(v4l2_print_dv_timings);
> /* Reduced Blanking */
> #define CVT_RB_MIN_V_BPORCH 7 /* lines */
> #define CVT_RB_V_FPORCH 3 /* lines */
> -#define CVT_RB_MIN_V_BLANK 460 /* us */
> +#define CVT_RB_MIN_V_BLANK 460 /* us */
> #define CVT_RB_H_SYNC 32 /* pixels */
> -#define CVT_RB_H_BPORCH 80 /* pixels */
> #define CVT_RB_H_BLANK 160 /* pixels */
> +/* Reduce blanking Version 2 */
> +#define CVT_RB_V2_H_BLANK 80 /* pixels */
> +#define CVT_RB_MIN_V_FPORCH 3 /* lines */
> +#define CVT_RB_V2_MIN_V_FPORCH 1 /* lines */
> +#define CVT_RB_V_BPORCH 6 /* lines */
>
> /** v4l2_detect_cvt - detect if the given timings follow the CVT standard
> * @frame_height - the total height of the frame (including blanking) in lines.
> @@ -356,9 +361,10 @@ bool v4l2_detect_cvt(unsigned frame_height, unsigned hfreq, unsigned vsync,
> int v_fp, v_bp, h_fp, h_bp, hsync;
> int frame_width, image_height, image_width;
> bool reduced_blanking;
> + bool rb_v2 = false;
> unsigned pix_clk;
>
> - if (vsync < 4 || vsync > 7)
> + if (vsync < 4 || vsync > 8)
> return false;
>
> if (polarities == V4L2_DV_VSYNC_POS_POL)
> @@ -368,17 +374,32 @@ bool v4l2_detect_cvt(unsigned frame_height, unsigned hfreq, unsigned vsync,
> else
> return false;
>
> + if (reduced_blanking && vsync == 8)
> + rb_v2 = true;
> +
> + if (!rb_v2 && vsync > 7)
> + return false;
> +
> if (hfreq == 0)
> return false;
>
> /* Vertical */
> if (reduced_blanking) {
> - v_fp = CVT_RB_V_FPORCH;
> - v_bp = (CVT_RB_MIN_V_BLANK * hfreq) / 1000000 + 1;
> - v_bp -= vsync + v_fp;
> -
> - if (v_bp < CVT_RB_MIN_V_BPORCH)
> - v_bp = CVT_RB_MIN_V_BPORCH;
> + if (rb_v2) {
> + v_bp = CVT_RB_V_BPORCH;
> + v_fp = (CVT_RB_MIN_V_BLANK * hfreq) / 1000000 + 1;
> + v_fp -= vsync + v_bp;
> +
> + if (v_fp < CVT_RB_V2_MIN_V_FPORCH)
> + v_fp = CVT_RB_V2_MIN_V_FPORCH;
> + } else {
> + v_fp = CVT_RB_V_FPORCH;
> + v_bp = (CVT_RB_MIN_V_BLANK * hfreq) / 1000000 + 1;
> + v_bp -= vsync + v_fp;
> +
> + if (v_bp < CVT_RB_MIN_V_BPORCH)
> + v_bp = CVT_RB_MIN_V_BPORCH;
> + }
> } else {
> v_fp = CVT_MIN_V_PORCH_RND;
> v_bp = (CVT_MIN_VSYNC_BP * hfreq) / 1000000 + 1 - vsync;
> @@ -415,22 +436,40 @@ bool v4l2_detect_cvt(unsigned frame_height, unsigned hfreq, unsigned vsync,
> else
> return false;
> break;
> + case 8:
> + /* To Do:
> + * For Reduced Blanking v2, vsync does not indicate aspect ratio and
> + * hence can not be used to derive image width. In such a case, either
> + * aspect ratio information or image width should be supplied to
> + * v4l2_detect_cvt(). This would need API change. As of now assuming
> + * 16:9 as default aspect ratio.
> + * */
> + image_width = (image_height * 16) / 9;
> + break;
> default:
> return false;
> }
>
> - image_width = image_width & ~7;
> + if (!rb_v2)
> + image_width = image_width & ~7;
>
> /* Horizontal */
> if (reduced_blanking) {
> - pix_clk = (image_width + CVT_RB_H_BLANK) * hfreq;
> - pix_clk = (pix_clk / CVT_PXL_CLK_GRAN) * CVT_PXL_CLK_GRAN;
> + int h_blank;
>
> - h_bp = CVT_RB_H_BPORCH;
> + if (rb_v2)
> + h_blank = CVT_RB_V2_H_BLANK;
> + else
> + h_blank = CVT_RB_H_BLANK;

I prefer: h_blank = rb_v2 ? CVT_RB_V2_H_BLANK : CVT_RB_H_BLANK

> +
> + pix_clk = (image_width + h_blank) * hfreq;
> + pix_clk = (pix_clk / CVT_PXL_CLK_GRAN_RB_V2) * CVT_PXL_CLK_GRAN_RB_V2;

Shouldn't the granularity be dependent on rb_v2?

> +
> + h_bp = h_blank / 2;
> hsync = CVT_RB_H_SYNC;
> - h_fp = CVT_RB_H_BLANK - h_bp - hsync;
> + h_fp = h_blank - h_bp - hsync;
>
> - frame_width = image_width + CVT_RB_H_BLANK;
> + frame_width = image_width + h_blank;
> } else {
> unsigned ideal_duty_cycle_per_myriad =
> 100 * CVT_C_PRIME - (CVT_M_PRIME * 100000) / hfreq;
> @@ -438,11 +477,9 @@ bool v4l2_detect_cvt(unsigned frame_height, unsigned hfreq, unsigned vsync,
>
> if (ideal_duty_cycle_per_myriad < 2000)
> ideal_duty_cycle_per_myriad = 2000;
> -
> h_blank = image_width * ideal_duty_cycle_per_myriad /
> (10000 - ideal_duty_cycle_per_myriad);
> h_blank = (h_blank / (2 * CVT_CELL_GRAN)) * 2 * CVT_CELL_GRAN;
> -

Why the spurious whitespace changes?

> pix_clk = (image_width + h_blank) * hfreq;
> pix_clk = (pix_clk / CVT_PXL_CLK_GRAN) * CVT_PXL_CLK_GRAN;
>
> @@ -483,7 +520,6 @@ bool v4l2_detect_cvt(unsigned frame_height, unsigned hfreq, unsigned vsync,
>
> if (reduced_blanking)
> fmt->bt.flags |= V4L2_DV_FL_REDUCED_BLANKING;
> -

Ditto.

> return true;
> }
> EXPORT_SYMBOL_GPL(v4l2_detect_cvt);
>

Regards,

Hans