linux-kernel - Re: [PATCH 4/5] drm/vc4: Add DPI driver

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Date:	Thu, 24 Mar 2016 17:02:34 -0700
From:	Eric Anholt <eric@...olt.net>
To:	Rob Herring <robh@...nel.org>
Cc:	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Stephen Warren <swarren@...dotorg.org>,
	Lee Jones <lee@...nel.org>, Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>
Subject: Re: [PATCH 4/5] drm/vc4: Add DPI driver

Rob Herring <robh@...nel.org> writes:

> On Fri, Mar 18, 2016 at 07:42:45PM -0700, Eric Anholt wrote:
>> The DPI interface involves taking a ton of our GPIOs to be used as
>> outputs, and routing display signals over them in parallel.
>> 
>> Signed-off-by: Eric Anholt <eric@...olt.net>
>> ---
>>  .../devicetree/bindings/display/brcm,bcm-vc4.txt   |  67 +++
>>  drivers/gpu/drm/vc4/Kconfig                        |   1 +
>>  drivers/gpu/drm/vc4/Makefile                       |   1 +
>>  drivers/gpu/drm/vc4/vc4_debugfs.c                  |   1 +
>>  drivers/gpu/drm/vc4/vc4_dpi.c                      | 518 +++++++++++++++++++++
>>  drivers/gpu/drm/vc4/vc4_drv.c                      |   1 +
>>  drivers/gpu/drm/vc4/vc4_drv.h                      |   5 +
>>  7 files changed, 594 insertions(+)
>>  create mode 100644 drivers/gpu/drm/vc4/vc4_dpi.c
>> 
>> diff --git a/Documentation/devicetree/bindings/display/brcm,bcm-vc4.txt b/Documentation/devicetree/bindings/display/brcm,bcm-vc4.txt
>> index 56a961a..1782c3f 100644
>> --- a/Documentation/devicetree/bindings/display/brcm,bcm-vc4.txt
>> +++ b/Documentation/devicetree/bindings/display/brcm,bcm-vc4.txt
>> @@ -35,6 +35,44 @@ Optional properties for HDMI:
>>  		  as an interrupt/status bit in the HDMI controller
>>  		  itself).  See bindings/pinctrl/brcm,bcm2835-gpio.txt
>>  
>> +Required properties for DPI:
>> +- compatible:		Should be "brcm,bcm2835-dpi"
>> +- reg:			Physical base address and length of the registers
>> +- clocks:		a) core: The core clock the unit runs on
>> +			b) pixel: The pixel clock that feeds the pixelvalve
>> +- port:			Port node with a single endpoint connecting to the
>> +			  panel device, as defined in [1]
>> +- brcm,output-format:	Output data format, must be one of:
>> +			0) disabled
>> +			1) 00000000rrrrrggggggbbbbb
>> +			2) 000rrrrr00gggggg000bbbbb
>> +			3) 00rrrrr000gggggg00bbbbb0
>> +			4) 000000rrrrrrggggggbbbbbb
>> +			5) 00rrrrrr00gggggg00bbbbbb
>> +			6) rrrrrrrrggggggggbbbbbbbb
>> +
>> +Optional properties for DPI:
>> +- brcm,rgb-order:		RGB reordering, must be one of:
>> +				0) RGB
>> +				1) BGR
>> +				2) GRB
>> +				3) BRG
>
>> +- brcm,hsync-disable:		Disables the hsync signal
>> +- brcm,vsync-disable:		Disables the vsync signal
>> +- brcm,output-enable-disable:	Disables the output enable signal
>> +- brcm,hsync-falling:		Outputs the hsync signal on the falling clk edge
>> +- brcm,vsync-falling:		Outputs the vsync signal on the falling clk edge
>> +- brcm,output-enable-falling:	Outputs the output enable signal on the
>> +				  falling clk edge
>> +- brcm,output-enable-invert:	Inverts the polarity of the output enable
>> +				  signal
>> +- brcm,pixel-clk-invert:	Inverts the polarity of the pixel clk signal
>> +- brcm,output-enable-mode:	Sets output enable when (vsync | hsync)
>> +				  instead of (hactive & vactive)
>
> These are all really properties of what the panel requires and we 
> already have video timings binding that would cover some of these.
>
> Also, do you have actual users? Some of these seem like they would be 
> rare or never. I've not seen panels caring about which clock edge the 
> sync signals are on.

I was using output-format, rgb_order, output-enable-mode to get my panel
to work.  I'm suspicious that the !output_enable_mode is not useful,
though (Note: I had documented it backwards: false is hsync|vsync, true
is hactive&vactive).  That just left me with output-format.  I think
.bus_format in the panel_desc can cover that, so I've now dropped all of
these brcm properties.

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