Re: [PATCH] drm/msm/dp: fix the max supported bpp logic

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Hi Stephen

On 7/26/2024 5:24 PM, Stephen Boyd wrote:
Quoting Abhinav Kumar (2024-07-25 15:03:19)
diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c b/drivers/gpu/drm/msm/dp/dp_panel.c
index a916b5f3b317..56ce5e4008f8 100644
--- a/drivers/gpu/drm/msm/dp/dp_panel.c
+++ b/drivers/gpu/drm/msm/dp/dp_panel.c
@@ -423,8 +424,10 @@ int dp_panel_init_panel_info(struct dp_panel *dp_panel)
                                 drm_mode->clock);
         drm_dbg_dp(panel->drm_dev, "bpp = %d\n", dp_panel->dp_mode.bpp);

-       dp_panel->dp_mode.bpp = max_t(u32, 18,
-                               min_t(u32, dp_panel->dp_mode.bpp, 30));
+       max_supported_bpp = dp_panel_get_mode_bpp(dp_panel, dp_panel->dp_mode.bpp,
+                                                 dp_panel->dp_mode.drm_mode.clock);
+       dp_panel->dp_mode.bpp = max_t(u32, 18, max_supported_bpp);

Is the max_t() usage still required once 'max_supported_bpp' is also a
u32? Also, what is 18? Shouldn't that be some sort of define so we know
what it represents?

Or maybe none of that is required? From what I can tell,
dp_panel_get_mode_bpp() calls dp_panel_get_supported_bpp() which will
essentially clamp the bpp range between 18 and 30, unless
dp_panel->dp_mode.bpp is between 30 and 18 but not divisible by 6, e.g.
29. Perhaps this patch can be included and the max_t above dropped.

---8<--
diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c
b/drivers/gpu/drm/msm/dp/dp_panel.c
index 07db8f37cd06..5cd7c138afd3 100644
--- a/drivers/gpu/drm/msm/dp/dp_panel.c
+++ b/drivers/gpu/drm/msm/dp/dp_panel.c
@@ -90,22 +90,22 @@ static int dp_panel_read_dpcd(struct dp_panel *dp_panel)
  static u32 dp_panel_get_supported_bpp(struct dp_panel *dp_panel,
  		u32 mode_edid_bpp, u32 mode_pclk_khz)
  {
-	struct dp_link_info *link_info;
+	const struct dp_link_info *link_info;
  	const u32 max_supported_bpp = 30, min_supported_bpp = 18;
-	u32 bpp = 0, data_rate_khz = 0;
+	u32 bpp, data_rate_khz;

  	bpp = min_t(u32, mode_edid_bpp, max_supported_bpp);

  	link_info = &dp_panel->link_info;
  	data_rate_khz = link_info->num_lanes * link_info->rate * 8;

-	while (bpp > min_supported_bpp) {
+	do {
  		if (mode_pclk_khz * bpp <= data_rate_khz)
-			break;
+			return bpp;
  		bpp -= 6;
-	}
+	} while (bpp > min_supported_bpp);

-	return bpp;
+	return min_supported_bpp;
  }


Thanks for the feedback.

Your change looks valid. We can use this and drop the max_t usage.

Let me push this with your Suggested-by credits.

  static int dp_panel_update_modes(struct drm_connector *connector,




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