linux-kernel - Re: [PATCH v2] drm/mxsfb: use bus_format to determine LCD bus width

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Message-ID: <1958f3a8-1536-0d47-838e-7a0810076f78@denx.de>
Date:   Wed, 14 Dec 2016 08:52:19 +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 v2] drm/mxsfb: use bus_format to determine LCD bus width

On 12/14/2016 02:02 AM, Stefan Agner wrote:
> The LCD bus width does not need to align with the pixel format. The
> LCDIF controller automatically converts between pixel formats and
> bus width by padding or dropping LSBs.
> 
> The DRM subsystem has the notion of bus_format which allows to
> determine what bus_formats are supported by the display. Choose the
> first available or fallback to 24 bit if none are available.
> 
> Signed-off-by: Stefan Agner <stefan@...er.ch>

Minor nits below.

> ---
> Changes in v2:
> - Use seperate function mxsfb_set_bus_fmt
> 
>  drivers/gpu/drm/mxsfb/mxsfb_crtc.c | 33 +++++++++++++++++++++++++++++++--
>  drivers/gpu/drm/mxsfb/mxsfb_regs.h |  1 +
>  2 files changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_crtc.c b/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
> index 4bcc8a3..956769b 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
> @@ -65,13 +65,11 @@ static int mxsfb_set_pixel_fmt(struct mxsfb_drm_private *mxsfb)
>  	switch (format) {
>  	case DRM_FORMAT_RGB565:
>  		dev_dbg(drm->dev, "Setting up RGB565 mode\n");
> -		ctrl |= CTRL_SET_BUS_WIDTH(STMLCDIF_16BIT);
>  		ctrl |= CTRL_SET_WORD_LENGTH(0);
>  		ctrl1 |= CTRL1_SET_BYTE_PACKAGING(0xf);
>  		break;
>  	case DRM_FORMAT_XRGB8888:
>  		dev_dbg(drm->dev, "Setting up XRGB8888 mode\n");
> -		ctrl |= CTRL_SET_BUS_WIDTH(STMLCDIF_24BIT);
>  		ctrl |= CTRL_SET_WORD_LENGTH(3);
>  		/* Do not use packed pixels = one pixel per word instead. */
>  		ctrl1 |= CTRL1_SET_BYTE_PACKAGING(0x7);
> @@ -87,6 +85,35 @@ static int mxsfb_set_pixel_fmt(struct mxsfb_drm_private *mxsfb)
>  	return 0;
>  }
>  
> +static void mxsfb_set_bus_fmt(struct mxsfb_drm_private *mxsfb)
> +{
> +	struct drm_crtc *crtc = &mxsfb->pipe.crtc;
> +	struct drm_device *drm = crtc->dev;
> +	u32 bus_format = MEDIA_BUS_FMT_RGB888_1X24;
> +	u32 reg;
            ^^^ this is uninitialized

> +	writel(CTRL_BUS_WIDTH_MASK, mxsfb->base + LCDC_CTRL + REG_CLR);
> +
> +	if (mxsfb->connector.display_info.num_bus_formats)
> +		bus_format = mxsfb->connector.display_info.bus_formats[0];
> +
> +	switch (bus_format) {
> +	case MEDIA_BUS_FMT_RGB565_1X16:
> +		reg |= CTRL_SET_BUS_WIDTH(STMLCDIF_16BIT);
                ^^^
                here you're doing operation on uninited value.

> +		break;
> +	case MEDIA_BUS_FMT_RGB666_1X18:
> +		reg |= CTRL_SET_BUS_WIDTH(STMLCDIF_18BIT);
> +		break;
> +	case MEDIA_BUS_FMT_RGB888_1X24:
> +		reg |= CTRL_SET_BUS_WIDTH(STMLCDIF_24BIT);
> +		break;
> +	default:
> +		dev_err(drm->dev, "Unknown media bus format %d\n", bus_format);
> +		break;
> +	}
> +	writel(reg, mxsfb->base + LCDC_CTRL + REG_SET);

What I was thinking might be better is to save the calculated LCD_CRTC
value in mxsfb_drm_private , but maybe R-M-W is good enough after all?
Two writes are IMO not great because it unnecessarily reconfigures the
controller twice.

> +}
> +
>  static void mxsfb_enable_controller(struct mxsfb_drm_private *mxsfb)
>  {
>  	u32 reg;
> @@ -182,6 +209,8 @@ static void mxsfb_crtc_mode_set_nofb(struct mxsfb_drm_private *mxsfb)
>  
>  	writel(vdctrl0, mxsfb->base + LCDC_VDCTRL0);
>  
> +	mxsfb_set_bus_fmt(mxsfb);
> +
>  	/* Frame length in lines. */
>  	writel(m->crtc_vtotal, mxsfb->base + LCDC_VDCTRL1);
>  
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_regs.h b/drivers/gpu/drm/mxsfb/mxsfb_regs.h
> index 31d62cd..66a6ba9 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_regs.h
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_regs.h
> @@ -44,6 +44,7 @@
>  #define CTRL_DATA_SELECT		(1 << 16)
>  #define CTRL_SET_BUS_WIDTH(x)		(((x) & 0x3) << 10)
>  #define CTRL_GET_BUS_WIDTH(x)		(((x) >> 10) & 0x3)
> +#define CTRL_BUS_WIDTH_MASK		(0x3 << 10)
>  #define CTRL_SET_WORD_LENGTH(x)		(((x) & 0x3) << 8)
>  #define CTRL_GET_WORD_LENGTH(x)		(((x) >> 8) & 0x3)
>  #define CTRL_MASTER			(1 << 5)
> 


-- 
Best regards,
Marek Vasut