Hi Marek,

Thank you for the patch.

On Mon, Jun 21, 2021 at 12:47:01AM +0200, Marek Vasut wrote:

There is some sort of corner case behavior of the controller, which could rarely be triggered at least on i.MX6SX connected to 800x480 DPI panel and i.MX8MM connected to DPI->DSI->LVDS bridged 1920x1080 panel (and likely on other setups too), where the image on the panel shifts to the right and wraps around. This happens either when the controller is enabled on boot or even later during run time. The condition does not correct itself automatically, i.e. the display image remains shifted.

It seems this problem is known and is due to sporadic underflows of the LCDIF FIFO. While the LCDIF IP does have underflow/overflow IRQs, neither of the IRQs trigger and neither IRQ status bit is asserted when this condition occurs.

All known revisions of the LCDIF IP have CTRL1 RECOVER_ON_UNDERFLOW bit, which is described in the reference manual since i.MX23 as " Set this bit to enable the LCDIF block to recover in the next field/frame if there was an underflow in the current field/frame. " Enable this bit to mitigate the sporadic underflows.

Fixes: 45d59d704080 ("drm: Add new driver for MXSFB controller") Signed-off-by: Marek Vasut marex@denx.de Cc: Daniel Abrecht public@danielabrecht.ch Cc: Emil Velikov emil.l.velikov@gmail.com Cc: Laurent Pinchart laurent.pinchart@ideasonboard.com Cc: Lucas Stach l.stach@pengutronix.de Cc: Stefan Agner stefan@agner.ch

---

drivers/gpu/drm/mxsfb/mxsfb_kms.c | 29 +++++++++++++++++++++++++++++ drivers/gpu/drm/mxsfb/mxsfb_regs.h | 1 + 2 files changed, 30 insertions(+)

diff --git a/drivers/gpu/drm/mxsfb/mxsfb_kms.c b/drivers/gpu/drm/mxsfb/mxsfb_kms.c index 300e7bab0f43..01e0f525360f 100644 --- a/drivers/gpu/drm/mxsfb/mxsfb_kms.c +++ b/drivers/gpu/drm/mxsfb/mxsfb_kms.c @@ -115,6 +115,35 @@ static void mxsfb_enable_controller(struct mxsfb_drm_private *mxsfb) reg |= VDCTRL4_SYNC_SIGNALS_ON; writel(reg, mxsfb->base + LCDC_VDCTRL4);

• /*

• * Enable recovery on underflow.
  

• *
  

• * There is some sort of corner case behavior of the controller,
  

• * which could rarely be triggered at least on i.MX6SX connected
  

• * to 800x480 DPI panel and i.MX8MM connected to DPI->DSI->LVDS
  

• * bridged 1920x1080 panel (and likely on other setups too), where
  

• * the image on the panel shifts to the right and wraps around.
  

• * This happens either when the controller is enabled on boot or
  

• * even later during run time. The condition does not correct
  

• * itself automatically, i.e. the display image remains shifted.
  

• *
  

• * It seems this problem is known and is due to sporadic underflows
  

• * of the LCDIF FIFO. While the LCDIF IP does have underflow/overflow
  

• * IRQs, neither of the IRQs trigger and neither IRQ status bit is
  

• * asserted when this condition occurs.
  

• *
  

• * All known revisions of the LCDIF IP have CTRL1 RECOVER_ON_UNDERFLOW
  

• * bit, which is described in the reference manual since i.MX23 as
  

• * "
  

• *   Set this bit to enable the LCDIF block to recover in the next
  

• *   field/frame if there was an underflow in the current field/frame.
  

• * "
  

• * Enable this bit to mitigate the sporadic underflows.
  

• */
  

• reg = readl(mxsfb->base + LCDC_CTRL1);
• reg |= CTRL1_RECOVER_ON_UNDERFLOW;
• writel(reg, mxsfb->base + LCDC_CTRL1);

Looks good to me. Thanks for the detailed explanation.

Reviewed-by: Laurent Pinchart laurent.pinchart@ideasonboard.com

• writel(CTRL_RUN, mxsfb->base + LCDC_CTRL + REG_SET);

}

diff --git a/drivers/gpu/drm/mxsfb/mxsfb_regs.h b/drivers/gpu/drm/mxsfb/mxsfb_regs.h index 55d28a27f912..df90e960f495 100644 --- a/drivers/gpu/drm/mxsfb/mxsfb_regs.h +++ b/drivers/gpu/drm/mxsfb/mxsfb_regs.h @@ -54,6 +54,7 @@ #define CTRL_DF24 BIT(1) #define CTRL_RUN BIT(0)

+#define CTRL1_RECOVER_ON_UNDERFLOW BIT(24) #define CTRL1_FIFO_CLEAR BIT(21) #define CTRL1_SET_BYTE_PACKAGING(x) (((x) & 0xf) << 16) #define CTRL1_GET_BYTE_PACKAGING(x) (((x) >> 16) & 0xf)