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

[H. Nikolaus Schaller]

> Am 11.11.2019 um 10:08 schrieb H. Nikolaus Schaller <hns at goldelico.com>:
> 
> 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.

Ah, here is the relevant comment:

https://lore.kernel.org/patchwork/patch/549312/#731123

Unfortunately I have no idea what they are talking about :)
Maybe the drivers/gpu/drm/bridge/synopsys gives hints how to improve the
jz4780 driver - or the synopsys driver.

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