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

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Date:   Thu, 08 Dec 2016 19:44:56 -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] drm/mxsfb: use bus_format to determine LCD bus width

On 2016-12-08 15:33, Marek Vasut wrote:
> On 12/08/2016 11:52 PM, 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>
>> ---
>>  drivers/gpu/drm/mxsfb/mxsfb_crtc.c | 28 +++++++++++++++++++++++++---
>>  drivers/gpu/drm/mxsfb/mxsfb_regs.h |  1 +
>>  2 files changed, 26 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_crtc.c b/drivers/gpu/drm/mxsfb/mxsfb_crtc.c
>> index 4bcc8a3..00fa244 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)
> 
> You should probably drop the WARNING: comment in this function too.
> 
>>  	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);
>> @@ -89,6 +87,9 @@ static int mxsfb_set_pixel_fmt(struct mxsfb_drm_private *mxsfb)
>>
>>  static void mxsfb_enable_controller(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;
> 
> So why do you move the bus width configuration from
> mxsfb_set_pixel_fmt() to here ? Is there any reason
> for it?
> 

To emphasize that it is not related to pixel format. Also, if you have a
controller with multiple framebuffers/layers, mxsfb_set_pixel_fmt would
get called per layer...

In a full DRM driver it probably would be part of a encoder function, I
feel here it seems to map best to enable controller. It could probably
also be in mxsfb_crtc_enable (or even mxsfb_crtc_mode_set_nofb I guess),
but we do not touch LCDC_CTRL there...

>>  	u32 reg;
>>
>>  	if (mxsfb->clk_disp_axi)
>> @@ -97,7 +98,28 @@ static void mxsfb_enable_controller(struct mxsfb_drm_private *mxsfb)
>>  	mxsfb_enable_axi_clk(mxsfb);
>>
>>  	/* If it was disabled, re-enable the mode again */
>> -	writel(CTRL_DOTCLK_MODE, mxsfb->base + LCDC_CTRL + REG_SET);
>> +	reg = readl(mxsfb->base + LCDC_CTRL);
> 
> Wouldn't it make more sense to cache the LCDC_CTRL content rather than
> doing R-M-W on it ?
> 

Not sure what you mean by cache? Isn't the variable reg counting as a
cache?

>> +	reg |= CTRL_DOTCLK_MODE;
>> +
>> +	if (mxsfb->connector.display_info.num_bus_formats)
>> +		bus_format = mxsfb->connector.display_info.bus_formats[0];
>> +
>> +	reg &= ~CTRL_BUS_WIDTH_MASK;
>> +	switch (bus_format) {
>> +	case MEDIA_BUS_FMT_RGB565_1X16:
>> +		reg |= CTRL_SET_BUS_WIDTH(STMLCDIF_16BIT);
>> +		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);
> 
> On MX6SX:
> Tested-by: Marek Vasut <marex@...x.de>

Thx!

--
Stefan