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

[Liviu Dudau <liviu.dudau@arm.com>]

On Wed, Aug 29, 2018 at 11:58:20AM +0200, Andrzej Hajda wrote:
> On 28.08.2018 15:45, Linus Walleij wrote:
> > On Mon, Aug 27, 2018 at 1:53 PM Andrzej Hajda <a.hajda@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.

Yeah, unfortunately that ship has sailed a long time ago. The emulator
people thought emulating the register interface is good enough and took
liberties on how the behaviour was "emulated". End result: output
interfaces are not the same.

Best regards,
Liviu

> 
> 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
> >
> >
> 

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯
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