linux-kernel - Re: [PATCH] drm/mxsfb: fix pixel clock polarity

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Message-ID: <c43446fb-5359-8d4c-cb0f-b37716cd73b8@denx.de>
Date:   Thu, 8 Dec 2016 01:49:03 +0100
From:   Marek Vasut <marex@...x.de>
To:     Stefan Agner <stefan@...er.ch>
Cc:     airlied@...ux.ie, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/mxsfb: fix pixel clock polarity

On 12/08/2016 01:27 AM, Stefan Agner wrote:
> The DRM subsystem specifies the pixel clock polarity from a
> controllers perspective: DRM_BUS_FLAG_PIXDATA_NEGEDGE means
> the controller drives the data on pixel clocks falling edge.
> That is the controllers DOTCLK_POL=0 (Default is data launched
> at negative edge).
> 
> Also change the data enable logic to be high active by default
> and only change if explicitly requested via bus_flags. With
> that defaults are:
> - Data enable: high active
> - Pixel clock polarity: controller drives data on negative edge
> 
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
> Hi Marek,

Hi, that was quick, thanks for checking this.

> I discovered this while testing on a i.MX 7 eLCDIF IP. Particularly the
> non-standard DE polarity was causing issues. I was using a EDT display
> which is part of simple panel driver since a while now and does not
> specify any bus_flags currently... Of course I could (and probably should)
> add the proper bus_flags there too, but there are several displays
> which do not specify any polarity and likely rely on sensible driver
> standards (which is afact high active for the DE signal).

I actually use a panel which requires correct settings of the flags, see
e0932f9d7ba9a16f99a84943b720f109de8e3e06 in mainline , so this patch
would break things for me. So I wonder whether you should fix the panel
driver or whether the mxsfb should be fixed ?

> --
> Stefan
> 
>  drivers/gpu/drm/mxsfb/mxsfb_crtc.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_crtc.c b/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
> index 0818903..4bcc8a3 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
> @@ -168,9 +168,16 @@ static void mxsfb_crtc_mode_set_nofb(struct mxsfb_drm_private *mxsfb)
>  		vdctrl0 |= VDCTRL0_HSYNC_ACT_HIGH;
>  	if (m->flags & DRM_MODE_FLAG_PVSYNC)
>  		vdctrl0 |= VDCTRL0_VSYNC_ACT_HIGH;
> -	if (bus_flags & DRM_BUS_FLAG_DE_HIGH)
> +	/* Data Enable should be high active by default */
> +	if (!(bus_flags & DRM_BUS_FLAG_DE_LOW))
>  		vdctrl0 |= VDCTRL0_ENABLE_ACT_HIGH;
> -	if (bus_flags & DRM_BUS_FLAG_PIXDATA_NEGEDGE)
> +	/*
> +	 * Note: DRM_BUS_FLAG_PIXDATA defines are controller centric,
> +	 * controllers VDCTRL0_DOTCLK is display centric.
> +	 * Drive on positive edige      -> display samples on falling edge
> +	 * DRM_BUS_FLAG_PIXDATA_POSEDGE -> VDCTRL0_DOTCLK_ACT_FALLING
> +	 */
> +	if (bus_flags & DRM_BUS_FLAG_PIXDATA_POSEDGE)
>  		vdctrl0 |= VDCTRL0_DOTCLK_ACT_FALLING;
>  
>  	writel(vdctrl0, mxsfb->base + LCDC_VDCTRL0);
> 


-- 
Best regards,
Marek Vasut