drm/kmb: Enable LCD DMA for low TVDDCV

There's an undocumented dependency between LCD layer enable bits [2-5]
and the AXI pipelined read enable bit [28] in the LCD_CONTROL register.
The proper order of operation is:

1) Clear AXI pipelined read enable bit
2) Set LCD layers
3) Set AXI pipelined read enable bit

With this update, LCD can start DMA when TVDDCV is reduced down to 700mV.

Fixes: 7f7b96a8 ("drm/kmb: Add support for KeemBay Display")
Signed-off-by: Edmund Dea <edmund.j.dea@intel.com>
Signed-off-by: Anitha Chrisanthus <anitha.chrisanthus@intel.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210728003126.1425028-1-anitha.chrisanthus@intel.com

drivers/gpu/drm/kmb/kmb_drv.c

@@ -203,6 +203,7 @@ static irqreturn_t handle_lcd_irq(struct drm_device *dev)
 	unsigned long status, val, val1;
 	int plane_id, dma0_state, dma1_state;
 	struct kmb_drm_private *kmb = to_kmb(dev);
+	u32 ctrl = 0;
 
 	status = kmb_read_lcd(kmb, LCD_INT_STATUS);
 
@@ -227,6 +228,19 @@ static irqreturn_t handle_lcd_irq(struct drm_device *dev)
 				kmb_clr_bitmask_lcd(kmb, LCD_CONTROL,
 						    kmb->plane_status[plane_id].ctrl);
 
+				ctrl = kmb_read_lcd(kmb, LCD_CONTROL);
+				if (!(ctrl & (LCD_CTRL_VL1_ENABLE |
+				    LCD_CTRL_VL2_ENABLE |
+				    LCD_CTRL_GL1_ENABLE |
+				    LCD_CTRL_GL2_ENABLE))) {
+					/* If no LCD layers are using DMA,
+					 * then disable DMA pipelined AXI read
+					 * transactions.
+					 */
+					kmb_clr_bitmask_lcd(kmb, LCD_CONTROL,
+							    LCD_CTRL_PIPELINE_DMA);
+				}
+
 				kmb->plane_status[plane_id].disable = false;
 			}
 		}

drivers/gpu/drm/kmb/kmb_plane.c

@@ -427,8 +427,14 @@ static void kmb_plane_atomic_update(struct drm_plane *plane,
 
 	kmb_set_bitmask_lcd(kmb, LCD_CONTROL, ctrl);
 
-	/* FIXME no doc on how to set output format,these values are
-	 * taken from the Myriadx tests
+	/* Enable pipeline AXI read transactions for the DMA
+	 * after setting graphics layers. This must be done
+	 * in a separate write cycle.
+	 */
+	kmb_set_bitmask_lcd(kmb, LCD_CONTROL, LCD_CTRL_PIPELINE_DMA);
+
+	/* FIXME no doc on how to set output format, these values are taken
+	 * from the Myriadx tests
 	 */
 	out_format |= LCD_OUTF_FORMAT_RGB888;
 
@@ -526,6 +532,11 @@ struct kmb_plane *kmb_plane_init(struct drm_device *drm)
 		plane->id = i;
 	}
 
+	/* Disable pipeline AXI read transactions for the DMA
+	 * prior to setting graphics layers
+	 */
+	kmb_clr_bitmask_lcd(kmb, LCD_CONTROL, LCD_CTRL_PIPELINE_DMA);
+
 	return primary;
 cleanup:
 	drmm_kfree(drm, plane);