RE: [PATCH 1/2] video: da8xx-fb: configure FIFO threshold to reduce underflow errors

Hi Sergei,

On Thu, Jul 12, 2012 at 18:53:26, Sergei Shtylyov wrote:

Hello.

On 11-07-2012 19:14, Manjunathappa, Prakash wrote:

quoted

Patch works around the below silicon errata:
During LCDC initialization, there is the potential for a FIFO
underflow condition to occur. A FIFO underflow condition
occurs when the input FIFO is completely empty and the LCDC
raster controller logic that drives data to the output pins
attempts to fetch data from the FIFO. When a FIFO underflow
condition occurs, incorrect data will be driven out on the
LCDC data pins.

quoted

Software should poll the FUF bit field in the LCD_STAT register
to check if an error condition has occurred or service the
interrupt if FUF_EN is enabled when FUF occurs. If the FUF bit
field has been set to 1, this will indicate an underflow
condition has occurred and then the software should execute a
reset of the LCDC via the LPSC.

quoted

This problem may occur if the LCDC FIFO threshold size
(LCDDMA_CTRL[TH_FIFO_READY]) is left at its default value after
reset. Increasing the FIFO threshold size will reduce or
eliminate underflows. Setting the threshold size to 256 double
words or larger is recommended.

quoted

Above issue is described in section 2.1.3 of silicon errata
http://www.ti.com/lit/er/sprz313e/sprz313e.pdf

quoted

Signed-off-by: Rajashekhara, Sudhakar <redacted>
Signed-off-by: Manjunathappa, Prakash <redacted>
---
  drivers/video/da8xx-fb.c |   15 +++++++++++----
  include/video/da8xx-fb.h |    3 +++
  2 files changed, 14 insertions(+), 4 deletions(-)

quoted

diff --git a/drivers/video/da8xx-fb.c b/drivers/video/da8xx-fb.c
index 47118c7..2010dd7 100644
--- a/drivers/video/da8xx-fb.c
+++ b/drivers/video/da8xx-fb.c

@@ -344,8 +344,8 @@ static void lcd_blit(int load_mode, struct da8xx_fb_par *par)
  	lcd_enable_raster();
  }

-/* Configure the Burst Size of DMA */
-static int lcd_cfg_dma(int burst_size)
+/* Configure the Burst Size and fifo threhold of DMA */
+static int lcd_cfg_dma(int burst_size,  int fifo_th)

    One space too much.

Yes, I will correct this. Don't know how checkpatch missing it.

quoted

  {
  	u32 reg;

@@ -719,8 +722,10 @@ static irqreturn_t lcdc_irq_handler_rev02(int irq, void *arg)

  	if ((stat & LCD_SYNC_LOST) && (stat & LCD_FIFO_UNDERFLOW)) {
  		lcd_disable_raster();
+		clk_disable(par->lcdc_clk);
  		lcdc_write(stat, LCD_MASKED_STAT_REG);

    Will LCDC register wrtite work without LCDC clock?

Yes it cannot happen. I will remove it.

Thanks,
Prakash