linux-kernel - Re: [PATCH v1] drm/mediatek: fix up 1440x900 dp display black screen issue

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Message-ID: <1574818303.15691.10.camel@mtksdaap41>
Date:   Wed, 27 Nov 2019 09:31:43 +0800
From:   CK Hu <ck.hu@...iatek.com>
To:     <yongqiang.niu@...iatek.com>
CC:     Philipp Zabel <p.zabel@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "David Airlie" <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Mark Rutland <mark.rutland@....com>,
        <dri-devel@...ts.freedesktop.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v1] drm/mediatek: fix up 1440x900 dp display black
 screen issue

Hi, Yongqiang:

On Tue, 2019-11-26 at 14:47 +0800, yongqiang.niu@...iatek.com wrote:
> From: Yongqiang Niu <yongqiang.niu@...iatek.com>
> 
> This patch fix up 1440x900 dp display black screen issue
> the computed result will overflow rdma1 fifo max size
> when external display pixel clock bigger than 74MHZ
> 
> Signed-off-by: Yongqiang Niu <yongqiang.niu@...iatek.com>
> ---
>  drivers/gpu/drm/mediatek/mtk_disp_rdma.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_disp_rdma.c b/drivers/gpu/drm/mediatek/mtk_disp_rdma.c
> index c1abde3..41143f5 100644
> --- a/drivers/gpu/drm/mediatek/mtk_disp_rdma.c
> +++ b/drivers/gpu/drm/mediatek/mtk_disp_rdma.c
> @@ -152,6 +152,10 @@ static void mtk_rdma_config(struct mtk_ddp_comp *comp, unsigned int width,
>  	 * account for blanking, and with a pixel depth of 4 bytes:
>  	 */
>  	threshold = width * height * vrefresh * 4 * 7 / 1000000;
> +
> +	if (threshold > rdma_fifo_size)
> +		threshold = rdma_fifo_size;
> +

This looks like that threshold is over spec. I think this FIFO is used
to cover the latency of reading data from DRAM. When data is read from
OVL not RDMA, OVL already has a FIFO, so RDMA FIFO is useless. So I
think you should set threshold to a special value when RDMA is in direct
link mode.

Regards,
CK

>  	reg = RDMA_FIFO_UNDERFLOW_EN |
>  	      RDMA_FIFO_PSEUDO_SIZE(rdma_fifo_size) |
>  	      RDMA_OUTPUT_VALID_FIFO_THRESHOLD(threshold);