[PATCH] drm/exynos: use 4WORD dma burst length for small fbs

[Inki Dae]

Hi Rahul,

On 2014년 05월 07일 20:25, Rahul Sharma wrote:
> From: Rahul Sharma <Rahul.Sharma at samsung.com>
> 
> In case of exynos, setting dma-burst to 16Word causes permanent
> tearing for very small buffers, e.g. cursor buffer. Burst Mode
> switching, which is based on overlay size is not recommended as
> overlay size varies a lot towards the end of the screen. This
> causes unstable DMA which results into tearing again.
> 
> Rendering small buffers with lower burst size doesn't
> cause any noticable performance overhead. 128 pixel width is
> selected based on mulitple experiments with exynos5 SoCs.
> 
> Signed-off-by: Rahul Sharma <Rahul.Sharma at samsung.com>
> Signed-off-by: Prathyush K <prathyush.k at samsung.com>
> ---
> Based on Inki Dae's exynos-drm-next branch.
> 
>  drivers/gpu/drm/exynos/exynos_drm_fimd.c |   14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/gpu/drm/exynos/exynos_drm_fimd.c b/drivers/gpu/drm/exynos/exynos_drm_fimd.c
> index 40fd6cc..ef56077 100644
> --- a/drivers/gpu/drm/exynos/exynos_drm_fimd.c
> +++ b/drivers/gpu/drm/exynos/exynos_drm_fimd.c
> @@ -38,6 +38,7 @@
>   */
>  
>  #define FIMD_DEFAULT_FRAMERATE 60
> +#define MIN_FB_WIDTH_FOR_16WORD_BURST 128
>  
>  /* position control register for hardware window 0, 2 ~ 4.*/
>  #define VIDOSD_A(win)		(VIDOSD_BASE + 0x00 + (win) * 16)
> @@ -446,6 +447,19 @@ static void fimd_win_set_pixfmt(struct fimd_context *ctx, unsigned int win)
>  
>  	DRM_DEBUG_KMS("bpp = %d\n", win_data->bpp);
>  
> +	/*
> +	 * In case of exynos, setting dma-burst to 16Word causes permanent
> +	 * tearing for very small buffers, e.g. cursor buffer. Burst Mode
> +	 * switching which is based on overlay size is not recommended as
> +	 * overlay size varies alot towards the end of the screen and rapid
> +	 * movement causes unstable DMA which results into iommu crash/tear.

FIMD has width limitation so width of hardware overlay may need to be
aligned to 16 pixels.
We had faced with similar issue and the issue had been resolved by
aligning it to 16 pixels.

So can you try to align it instead of changing burst len size?

Thanks,
Inki Dae

> +	 */
> +
> +	if (win_data->fb_width < MIN_FB_WIDTH_FOR_16WORD_BURST) {
> +		val &= ~WINCONx_BURSTLEN_MASK;
> +		val |= WINCONx_BURSTLEN_4WORD;
> +	}
> +
>  	writel(val, ctx->regs + WINCON(win));
>  }
>  
>