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: 7f7b96a8a0a1 ("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

1 files changed, 14 insertions, 0 deletions

diff --git a/drivers/gpu/drm/kmb/kmb_drv.c b/drivers/gpu/drm/kmb/kmb_drv.c
index 96ea1a2c11dd..c0b1c6f99249 100644
--- a/drivers/gpu/drm/kmb/kmb_drv.c
+++ b/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;
 			}
 		}