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

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Date:   Wed, 14 Dec 2016 12:51:03 -0800
From:   Stefan Agner <stefan@...er.ch>
To:     Marek Vasut <marex@...x.de>
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 2016-12-13 23:52, Marek Vasut wrote:
> 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?

Yeah I see what you are saying, however I would prefer to not cache if
there is no real reason to...

It is now a write to the clear and one write to the set variant of the
register, not sure if that counts as "two writes".

> Two writes are IMO not great because it unnecessarily reconfigures the
> controller twice.

Does that matter? At this point, the RUN bit is guaranteed not to be
set... 

--
Stefan

> 
>> +}
>> +
>>  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)
>>