Re: [PATCH 3/5] drm/msm/dsi: Account for DSC's bits_per_pixel having 4 fractional bits

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On 05/10/2022 01:35, Marijn Suijten wrote:
On 2022-10-04 17:45:50, Dmitry Baryshkov wrote:
On Sat, 1 Oct 2022 at 22:08, Marijn Suijten
<marijn.suijten@xxxxxxxxxxxxxx> wrote:
[..]
-       bytes_in_slice = DIV_ROUND_UP(dsc->slice_width * dsc->bits_per_pixel, 8);
+       bytes_in_slice = DIV_ROUND_UP(dsc->slice_width * bpp, 8);


bytes_in_slice = DIV_ROUND_UP(dsc->slice_width * dsc->bits_per_pixel, 8 * 16); ?

Not necessarily a fan of this, it "hides" the fact that we are dealing
with 4 fractional bits (1/16th precision, it is correct though); but
since this is the only use of `bpp` I can change it and document this
fact wiht a comment on top (including referencing the validation pointed
out in dsi_populate_dsc_params()).

Alternatively we can inline the `>> 4` here?

No, I don't think so. If we shift by 4 bits, we'd loose the fractional part. DIV_ROUND_UP( .... , 8 * 16) ensures that we round it up rather than just dropping it.



         dsc->slice_chunk_size = bytes_in_slice;

@@ -913,6 +918,7 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
         u32 va_end = va_start + mode->vdisplay;
         u32 hdisplay = mode->hdisplay;
         u32 wc;
+       int ret;

         DBG("");

@@ -948,7 +954,9 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
                 /* we do the calculations for dsc parameters here so that
                  * panel can use these parameters
                  */
-               dsi_populate_dsc_params(dsc);
+               ret = dsi_populate_dsc_params(dsc);
+               if (ret)
+                       return;

                 /* Divide the display by 3 but keep back/font porch and
                  * pulse width same
@@ -1229,6 +1237,10 @@ static int dsi_cmd_dma_add(struct msm_dsi_host *msm_host,
         if (packet.size < len)
                 memset(data + packet.size, 0xff, len - packet.size);

+       if (msg->type == MIPI_DSI_PICTURE_PARAMETER_SET)
+               print_hex_dump(KERN_DEBUG, "ALL:", DUMP_PREFIX_NONE,
+                               16, 1, data, len, false);
+
         if (cfg_hnd->ops->tx_buf_put)
                 cfg_hnd->ops->tx_buf_put(msm_host);

@@ -1786,6 +1798,12 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
         int data;
         int final_value, final_scale;
         int i;
+       int bpp = dsc->bits_per_pixel >> 4;
+
+       if (dsc->bits_per_pixel & 0xf) {
+               pr_err("DSI does not support fractional bits_per_pixel\n");
+               return -EINVAL;
+       }

         dsc->rc_model_size = 8192;
         dsc->first_line_bpg_offset = 12;
@@ -1807,7 +1825,7 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
         }

         dsc->initial_offset = 6144; /* Not bpp 12 */
-       if (dsc->bits_per_pixel != 8)
+       if (bpp != 8)
                 dsc->initial_offset = 2048;     /* bpp = 12 */

         mux_words_size = 48;            /* bpc == 8/10 */
@@ -1830,16 +1848,16 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
          * params are calculated
          */
         groups_per_line = DIV_ROUND_UP(dsc->slice_width, 3);
-       dsc->slice_chunk_size = dsc->slice_width * dsc->bits_per_pixel / 8;
-       if ((dsc->slice_width * dsc->bits_per_pixel) % 8)
+       dsc->slice_chunk_size = dsc->slice_width * bpp / 8;
+       if ((dsc->slice_width * bpp) % 8)

One can use fixed point math here too:

dsc->slice_chunk_size = (dsc->slice_width * dsc->bits_per_pixel  + 8 *
16 - 1)/ (8 * 16);

Good catch, this is effectively a DIV_ROUND_UP() that we happened to
call bytes_in_slice above...

Shall I tackle this in the same patch, or insert another cleanup patch?

It's up to you. I usually prefer separate patches, even if just to ease bisecting between unrelated changes.


In fact dsi_populate_dsc_params() is called first (this comment),
followed by dsi_update_dsc_timing(), meaning that slice_chunk_size is
already provided and shouldn't be recomputed.

                 dsc->slice_chunk_size++;

         /* rbs-min */
         min_rate_buffer_size =  dsc->rc_model_size - dsc->initial_offset +
-                               dsc->initial_xmit_delay * dsc->bits_per_pixel +
+                               dsc->initial_xmit_delay * bpp +
                                 groups_per_line * dsc->first_line_bpg_offset;

-       hrd_delay = DIV_ROUND_UP(min_rate_buffer_size, dsc->bits_per_pixel);
+       hrd_delay = DIV_ROUND_UP(min_rate_buffer_size, bpp);

         dsc->initial_dec_delay = hrd_delay - dsc->initial_xmit_delay;

@@ -1862,7 +1880,7 @@ static int dsi_populate_dsc_params(struct drm_dsc_config *dsc)
         data = 2048 * (dsc->rc_model_size - dsc->initial_offset + num_extra_mux_bits);
         dsc->slice_bpg_offset = DIV_ROUND_UP(data, groups_total);

-       target_bpp_x16 = dsc->bits_per_pixel * 16;
+       target_bpp_x16 = bpp * 16;

         data = (dsc->initial_xmit_delay * target_bpp_x16) / 16;

It looks like this can be replaced with the direct multiplication
instead, maybe with support for overflow/rounding.

Thanks, Abhinav pointed out the same in patch 1/5 which originally
cleaned up most - but apparently not all! - of the math here.  I don't
think this value should ever overlow, nor does this `* 16 / 16` have any
effect on rounding (that'd be `/ 16 * 16`).

Ack


         final_value =  dsc->rc_model_size - data + num_extra_mux_bits;
--
2.37.3



--
With best wishes
Dmitry

--
With best wishes
Dmitry




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