[Openpvrsgx-devgroup] CI20/JZ4780 HDMI (was Re: Status & HW to bring)

[H. Nikolaus Schaller]

Hi Paul,

> Am 10.11.2019 um 17:45 schrieb Paul Boddie <paul at boddie.org.uk>:
> 
> On Saturday 9. November 2019 14.16.26 H. Nikolaus Schaller wrote:
>> 
>> CI20 / JZ4780 loads the driver but there is sadly no mainline HDMI driver.
> 
> As far as I can tell, the Synopsys DesignWare HDMI driver in...

Yes, with these keywords I have found more hints.

> 
> drivers/gpu/drm/bridge/synopsys
> 
> ...could be suitable. Certainly, comparing the following files from the 3.18 
> CI20 kernel and the Letux kernel...
> 
> drivers/gpu/drm/jz4780/dwc_hdmi_regs.h
> drivers/gpu/drm/bridge/synopsys/dw-hdmi.h
> 
> ...indicates that the same hardware appears to be involved, and some brief 
> investigations into the actual code indicates some close similarities.
> 
> More documentation for the Synopsys driver and some chipsets using the 
> peripheral can be found here:
> 
> Documentation/devicetree/bindings/display/bridge/dw_hdmi.txt
> Documentation/devicetree/bindings/display/rockchip/dw_hdmi-rockchip.txt
> Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
> 
> (Searching for "dw-hdmi" yields these and other results.)
> 
> Some example device tree files can be found, too:
> 
> arch/arm/boot/dts/rk3288.dtsi
> arch/arm/boot/dts/sun8i-r40.dtsi
> arch/arm64/boot/dts/rockchip/rk3328.dtsi
> 
> However, a quick look suggests that these bindings and drivers may be distinct 
> from the bridge driver:
> 
> drivers/gpu/drm/rockchip/dw_hdmi-rockchip.c
> drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> drivers/gpu/drm/sun4i/sun8i_dw_hdmi.c
> 
> So, maybe a suitable definition for the JZ4780 would still largely resemble 
> that found in the 3.18 kernel:
> 
> arch/mips/boot/dts/jz4780.dtsi
> 
> Documentation is rather lacking, though, so I don't have any immediate 
> suggestions about how the bridge driver would be activated.
> 
> Paul
> 
> P.S. Online references:
> 
> https://www.kernel.org/doc/html/latest/gpu/bridge/dw-hdmi.html
> 
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/bridge/synopsys
> 
> https://elixir.bootlin.com/linux/latest/source/Documentation/gpu/bridge/dw-hdmi.rst

Thanks for all this valuable background information!

It helped me to find

	https://lore.kernel.org/patchwork/patch/549314/

which also confirms that it is a Synopsis controller.
And there is already code... But I have not found what happened to it and why
it did not find its way into mainline.

I'll try if it can be rebased and integrated into the 5.4-rc kernels...

BR,
Nikolaus