Message-ID: <7e9ac0e8e6a8c6b2059246ee9e5b543d@agner.ch>
Date:   Tue, 13 Dec 2016 15:56:42 -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 20:24, Marek Vasut wrote:
> 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.

Sure we can, we just have to read the value of the register once more...

> 
>> 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 ?
> 

I think the controller has basically support for one additional surface.
They call it LCDIFx_AS (Alpha Surface). But those register have a
slightly different layout, so I guess we can't really reuse
mxsfb_set_pixel_fmt...

>> 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.
> 

It kind of feels wrong in any CRTC callback, since it is a property of
an encoder. But since we only have one CRTC and one encoder it really
does not matter.

My point was that we don't touch that register in mode_set_nofb. But
when we anyway move the code in a separate function anyway, than we
might as well call it from mxsfb_crtc_mode_set_nofb.

>>>>  	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 ...
> 

The FSL DCU driver (used for Vybrid/LS1021a) which I currently maintain
uses regmap. But it doesn't feel quite right either, the DRM subsystem
actually holds current state already, just on a higher level. I first
tried to use it for suspend and resume (before the DRM suspend resume
helper were available), but it was messy, stuff got async between
DRM/actual controller settings or have been written twice
(unnecessarily).


Quite often you actually have to touch a register from one subsystem
only too (e.g. in DCU, the layer registers maps nicely with the DRM
plane support). I guess this one register is a bit a catch all
configuration register... Thinking about it, IMHO reading/writing that
one register multiple times is really not a big deal...

--
Stefan

>>>> +	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 :)

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