linux-kernel - Re: [PATCH v3] drm/sun4i: tcon: fix inverted DCLK polarity

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Message-ID: <20210113094214.5ijq3inmffticym6@gilmour>
Date:   Wed, 13 Jan 2021 10:42:14 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Giulio Benetti <giulio.benetti@...ettiengineering.com>
Cc:     Marjan Pascolo <marjan.pascolo@...xom.it>, wens@...e.org,
        daniel@...ll.ch, airlied@...ux.ie, treding@...dia.com,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Giulio Benetti <giulio.benetti@...ronovasrl.com>
Subject: Re: [PATCH v3] drm/sun4i: tcon: fix inverted DCLK polarity

Hi,

On Mon, Jan 11, 2021 at 06:46:16PM +0100, Giulio Benetti wrote:
> From: Giulio Benetti <giulio.benetti@...ronovasrl.com>
> 
> During commit 88bc4178568b ("drm: Use new
> DRM_BUS_FLAG_*_(DRIVE|SAMPLE)_(POS|NEG)EDGE flags") DRM_BUS_FLAG_*
> macros have been changed to avoid ambiguity but just because of this
> ambiguity previous DRM_BUS_FLAG_PIXDATA_(POS/NEG)EDGE were used meaning
> _SAMPLE_ not _DRIVE_. This leads to DLCK inversion and need to fix but
> instead of swapping phase values, let's adopt an easier approach Maxime
> suggested:
> It turned out that bit 26 of SUN4I_TCON0_IO_POL_REG is dedicated to
> invert DCLK polarity and this makes things really easier than before. So
> let's handle DCLK polarity by adding SUN4I_TCON0_IO_POL_DCLK_POSITIVE as
> bit 26 and activating according to bus_flags the same way it is done for
> all the other signals polarity.
> 
> Fixes: 88bc4178568b ("drm: Use new DRM_BUS_FLAG_*_(DRIVE|SAMPLE)_(POS|NEG)EDGE flags")
> Suggested-by: Maxime Ripard <maxime@...no.tech>
> Signed-off-by: Giulio Benetti <giulio.benetti@...ronovasrl.com>
> ---
>  drivers/gpu/drm/sun4i/sun4i_tcon.c | 20 +-------------------
>  drivers/gpu/drm/sun4i/sun4i_tcon.h |  1 +
>  2 files changed, 2 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.c b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> index eaaf5d70e352..30171ccd87e5 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_tcon.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> @@ -569,26 +569,8 @@ static void sun4i_tcon0_mode_set_rgb(struct sun4i_tcon *tcon,
>  	if (info->bus_flags & DRM_BUS_FLAG_DE_LOW)
>  		val |= SUN4I_TCON0_IO_POL_DE_NEGATIVE;
>  
> -	/*
> -	 * On A20 and similar SoCs, the only way to achieve Positive Edge
> -	 * (Rising Edge), is setting dclk clock phase to 2/3(240°).
> -	 * By default TCON works in Negative Edge(Falling Edge),
> -	 * this is why phase is set to 0 in that case.
> -	 * Unfortunately there's no way to logically invert dclk through
> -	 * IO_POL register.
> -	 * The only acceptable way to work, triple checked with scope,
> -	 * is using clock phase set to 0° for Negative Edge and set to 240°
> -	 * for Positive Edge.
> -	 * On A33 and similar SoCs there would be a 90° phase option,
> -	 * but it divides also dclk by 2.
> -	 * Following code is a way to avoid quirks all around TCON
> -	 * and DOTCLOCK drivers.
> -	 */
>  	if (info->bus_flags & DRM_BUS_FLAG_PIXDATA_DRIVE_POSEDGE)
> -		clk_set_phase(tcon->dclk, 240);
> -
> -	if (info->bus_flags & DRM_BUS_FLAG_PIXDATA_DRIVE_NEGEDGE)
> -		clk_set_phase(tcon->dclk, 0);
> +		val |= SUN4I_TCON0_IO_POL_DCLK_POSITIVE;
>  
>  	regmap_update_bits(tcon->regs, SUN4I_TCON0_IO_POL_REG,
>  			   SUN4I_TCON0_IO_POL_HSYNC_POSITIVE |

You need to add SUN4I_TCON0_IO_POL_DCLK_POSITIVE to the mask you're
going to change here too

Maxime

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