[PATCH] drm: lcdif: change burst size to 256B

[Marek Vasut]

On 7/26/22 11:43, Marco Felsch wrote:
> FIFO underruns are seen if a AXI bus master with a higher priority do a
> lot of memory access. Increase the burst size to 256B to avoid such
> underruns and to improve the memory access efficiency.

Sigh, this again ...

> diff --git a/drivers/gpu/drm/mxsfb/lcdif_kms.c b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> index 1bec1279c8b5..1f22ea5896d5 100644
> --- a/drivers/gpu/drm/mxsfb/lcdif_kms.c
> +++ b/drivers/gpu/drm/mxsfb/lcdif_kms.c
> @@ -143,8 +143,20 @@ static void lcdif_set_mode(struct lcdif_drm_private *lcdif, u32 bus_flags)
>   	       CTRLDESCL0_1_WIDTH(m->crtc_hdisplay),
>   	       lcdif->base + LCDC_V8_CTRLDESCL0_1);
>   
> -	writel(CTRLDESCL0_3_PITCH(lcdif->crtc.primary->state->fb->pitches[0]),
> -	       lcdif->base + LCDC_V8_CTRLDESCL0_3);
> +	/*
> +	 * Undocumented P_SIZE and T_SIZE bit fields but according the
> +	 * downstream kernel they control the AXI burst size. As of now there
> +	 * are two known values:
> +	 *  1 - 128Byte
> +	 *  2 - 256Byte
> +	 *
> +	 * Downstream has set the burst size to 256Byte to improve the memory
> +	 * efficiency so set it here too. This also reduces the FIFO underrun
> +	 * possibility.
> +	 */
> +	ctrl = CTRLDESCL0_3_P_SIZE(2) | CTRLDESCL0_3_T_SIZE(2) |
> +	       CTRLDESCL0_3_PITCH(lcdif->crtc.primary->state->fb->pitches[0]);
> +	writel(ctrl, lcdif->base + LCDC_V8_CTRLDESCL0_3);
>   }

Sometimes I wonder whether this might be some successor of MXSFB 
LCDIF_CTRL2n OUTSTANDING_REQS and BURST_LEN_B fields.

+CC Liu, who seems to have a lot of knowledge about this IP.

Reviewed-by: Marek Vasut <marex at denx.de>