[PATCH 1/2] drm/bridge: Add virtual display DT bindings

[Andrzej Hajda]

On 28.08.2018 15:45, Linus Walleij wrote:
> On Mon, Aug 27, 2018 at 1:53 PM Andrzej Hajda <a.hajda at samsung.com> wrote:
>
>> On 24.08.2018 14:23, Linus Walleij wrote:
>>> This adds bindings for a virtual display to be used with displays
>>> inside entirely virtual environments which do not emulate things
>>> like monitors but just need timing information to be supplied to
>>> its display controller.
>>>
>>> This is inspired by earlier work by Liviu Dudau.
>> If this is pure virtual then it should be connected to other pure
>> virtual components.
> OK it's a bit of half-half but outside of my grasp, I am just trying
> to support legacy systems.
>
> The device tree is there:
> arch/arm64/boot/dts/arm/rtsm_ve-aemv8a.dts
> arch/arm64/boot/dts/arm/rtsm_ve-motherboard.dtsi
>
> The latter contains the CLCD which is the display driver.
>
> In RTSM, which is an ARM product I do not develop and
> cannot make changes to, there is an emulated CLCD. So
> that part appear as if it was real hardware. Like in QEMU.
>
> But the display does not have any emulation. The raw
> output from the CLCD is latched out to the screen.
>
> I do not know exactly know how the RTSM emulator
> actually works, but I suspect that the little graphic window
> on the screeen adapts to what gets written into the
> CLCD control registers, so anything goes, more or less.
>
> To satisfy the CLCD with some timings and resolution,
> this bridge gives it that, from the device tree, in a way
> that clearly conveys that "this is not a real thing".
>
> The old code uses DPI. This is not DPI, not even close
> to it. It worked simply because all the DPI really does is
> what this patch does: provide timings.
>
> By contrast on QEMU I have patches the emulator to
> properly represent the I2C DDC link and provide EDID
> so that is all fine.
>
> I cannot patch RTSM to emulate I2C and DDC. It's not
> open source. But the device tree in the kernel supports
> this "machine" and so, I have to maintain it when modernizng
> the fbdev driver to a DRM driver.
>
> I'm sorry RTSM is half/half. Not my fault. I can't fix...

I do not know the platform, so I  I have dug little bit, but I wan't
call it  thorough research. Just please be kind if I wrote sth stupid.
What I have found:
1. DTS shows CLCD is pl111.
2. pl111 documentation says it's output interface supports STN and TFT
up to 24-bit bus. I do not know STN, but TFT seems to be compatible with
DPI.

If it is correct, dpi panel seems to be OK. And I think it is less
important how the emulator works, more important is that it should
emulate pl111, including it's output interfaces.

Regards
Andrzej
>
>> And one more thing, you are defining virtual panel but you are using
>> drm_bridge framework, why not drm_panel?
> This was discussed before in the previous patch set:
> https://lists.freedesktop.org/archives/dri-devel/2018-July/183516.html
>
> Essentially, it's because the display driver is just connected
> to "something" not a panel and a bridge is likely the best I
> can come up with - a bridge over to a virtual display.
>
> A patch to add "VGA", "SGA" and "XGA" to the simple panels
> was NACKed (sort of, politely):
> https://lists.freedesktop.org/archives/dri-devel/2018-July/183698.html
>
> Also it was noted that it would be nice to have something that
> would make it easy to change resolutions on these virtual
> display things. Voila.



>
> Yours,
> Linus Walleij
>
>