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

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Message-ID: <ffc15dce-a98b-97ab-8667-e8bf031181c8@denx.de>
Date:   Fri, 9 Dec 2016 05:24:50 +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: use bus_format to determine LCD bus width

On 12/09/2016 04:44 AM, Stefan Agner wrote:
> 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.

Ah, can we then create some function, something like mxsfb_set_bus_fmt()
to be explicit about this bus (the wires coming out of the CPU) and
pixel (memory layout of the framebuffer) format difference ? I think
that'd help with readability.

> Also, if you have a
> controller with multiple framebuffers/layers, mxsfb_set_pixel_fmt would
> get called per layer...

Which in this case, you don't and cannot have, right ?

> 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...

My feeling is it should be part of the mode_set_nofb, because we're
setting all the controller parameters there, so that should be all
kept in one place, which for a simple controller like this might be
the approach to take.

>>>  	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?

I mean caching the content of the register in struct mxsfb_drm_private,
so you always program content which you have under control into the
register. Heck, maybe I should've used regmap for this driver ...

>>> +	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!

NP, thanks for the fixed and keeping up with my ranting :)

-- 
Best regards,
Marek Vasut