Re: [PATCH 2/4] drm/msm/dsi: Fix compressed word count calculation

From: Dmitry Baryshkov
Date: Tue May 09 2023 - 07:42:14 EST

On 09/05/2023 11:54, Konrad Dybcio wrote:

On 9.05.2023 10:23, Neil Armstrong wrote:
On 09/05/2023 01:27, Dmitry Baryshkov wrote:
On 08/05/2023 23:09, Abhinav Kumar wrote:

On 5/3/2023 1:26 AM, Dmitry Baryshkov wrote:
On 03/05/2023 04:19, Jessica Zhang wrote:
Currently, word count is calculated using slice_count. This is incorrect
as downstream uses slice per packet, which is different from

Slice count represents the number of soft slices per interface, and its
value will not always match that of slice per packet. For example, it is
possible to have cases where there are multiple soft slices per interface
but the panel specifies only one slice per packet.

Thus, use the default value of one slice per packet and remove slice_count
from the word count calculation.

Fixes: bc6b6ff8135c ("drm/msm/dsi: Use DSC slice(s) packet size to compute word count")
Signed-off-by: Jessica Zhang <quic_jesszhan@xxxxxxxxxxx>
  drivers/gpu/drm/msm/dsi/dsi_host.c | 9 ++++++++-
  1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index 35c69dbe5f6f..b0d448ffb078 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -996,7 +996,14 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
          if (!msm_host->dsc)
              wc = hdisplay * dsi_get_bpp(msm_host->format) / 8 + 1;
-            wc = msm_host->dsc->slice_chunk_size * msm_host->dsc->slice_count + 1;
+            /*
+             * When DSC is enabled, WC = slice_chunk_size * slice_per_packet + 1.
+             * Currently, the driver only supports default value of slice_per_packet = 1
+             *
+             * TODO: Expand drm_panel struct to hold slice_per_packet info
+             *       and adjust DSC math to account for slice_per_packet.

slice_per_packet is not a part of the standard DSC, so I'm not sure how that can be implemented. And definitely we should not care about the drm_panel here. It should be either a part of drm_dsc_config, or mipi_dsi_device.

This is not correct.

It is part of the DSI standard (not DSC standard). Please refer to Figure 40 "One Line Containing One Packet with Data from One or More Compressed Slices" and Figure 41 "One Line Containing More than One Compressed Pixel Stream Packet".

I have reviewed section 8.8.24 and Annex D of the DSI standard.

It is not clear to me, if we can get away with always using slice_per_packet = 1. What is the DSI sink's difference between Fig. 40.(b) and Fig 41?

Are there are known panels that require slice_per_packet != 1? If so, we will have to implement support for such configurations.

This has details about this. So I still stand by my point that this should be in the drm_panel.

Note, the driver doesn't use drm_panel directly. So slices_per_packet should go to mipi_dsi_device instead (which in turn can be filled from e.g. drm_panel or from any other source).

This is a big question, where should we set those parameters ?

It's an even bigger questions for panels optionally supporting DSC in Video or Command mode (like the vtdr6130),
how to select DSC or not ? DT is not an option.
Compressed vs uncompressed modes, maybe? Would be nice to make this
togglable from userspace.. But then it may not scale for panels with e.g.
10 resolutions, all cmd/vid/dsc/nodsc

Currently the panel/panel-bridge make decision on command vs video mode. We have no way to influence that decision. If you want to make that negotiable, I'd start with adding 'cmd_supported/video_supported/dsc_supported' flags to struct mipi_dsi_hosts.


Those should tied to a panel+controller tuple.


+             */
+            wc = msm_host->dsc->slice_chunk_size + 1;
          dsi_write(msm_host, REG_DSI_CMD_MDP_STREAM0_CTRL,

With best wishes