ESP32-S3: Continuous DMA from PSRAM via. LCD produces garbage.

[faptastic]

Hi.

I have created a sketch that allocates memory and then links up DMA descriptors so that it loops forever. When I try to do this using PSRAM as the DMA buffer, I get garbage output.

Refer to live example here:

https://github.com/mrfaptastic/esp32s3-hub75-psram

Any ideas? Is this some cache issue again? How do I work around this when outputting continuously?

[ESP_Sprite]

Might very well be the case indeed; if the CPU writes to those PSRAM buffers, you need to poke the cache to have it write back that to the PSRAM first.

[faptastic]

Could I write to the PSRAM using DMA from SRAM?

Would that get around the issue of trying to change bytes directly from the PSRAM malloc?

[ESP_Sprite]

You can, but it's probably easier to writeback the cache using Cache_WriteBack_Addr() (from rom/cache.h) after you're don writing it.

[faptastic]

Thanks for this. Got it to work after calling this function.

[faptastic]

I notice now that if I try use a clock divider less than 4 with PSRAM it fails to provide an output, but can use a clock divider of up down to 2 with the SRAM when using the LCD peripheral.

Any ideas why? I have an S3 with Octal SPIRAM.

Code: Select all

    // LCD_CAM_LCD_CLK_SEL Select LCD module source clock. 0: clock source is disabled. 1: XTAL_CLK. 2: PLL_D2_CLK. 3: PLL_F160M_CLK. (R/W)
    LCD_CAM.lcd_clock.lcd_clk_sel = 2;        
    LCD_CAM.lcd_clock.lcd_ck_out_edge = 0;    // PCLK low in 1st half cycle
    LCD_CAM.lcd_clock.lcd_ck_idle_edge = 0;   // PCLK low idle
    LCD_CAM.lcd_clock.lcd_clk_equ_sysclk = 0; // PCLK = CLK / (CLKCNT_N+1)


    if (_cfg.psram_clk_hack) // fastest speed I can get PSRAM to work before nothing shows
    {
      LCD_CAM.lcd_clock.lcd_clkm_div_num = 4; 
    }
    else
    {
      //LCD_CAM.lcd_clock.lcd_clkm_div_num = lcd_clkm_div_num;      
      LCD_CAM.lcd_clock.lcd_clkm_div_num = 3;      
    }
     ESP_LOGI(TAG, "Clock divider is %d", LCD_CAM.lcd_clock.lcd_clkm_div_num);     

    
    LCD_CAM.lcd_clock.lcd_clkm_div_a = 1;     // 0/1 fractional divide
    LCD_CAM.lcd_clock.lcd_clkm_div_b = 0;
    

[ESP_Sprite]

I don't have the calculations at hand here, but it's possibly a bandwidth issue; PSRAM (even octal) has less bandwidth than internal RAM and you may be saturating the bus.