Re: [PATCH 0/4] drm/meson: Module removal fixes

[Marc Zyngier]

Hi Neil,

On 2020-11-17 08:49, Neil Armstrong wrote:
> Hi Marc,
>
> On 16/11/2020 21:07, Marc Zyngier wrote:
> > Hi all,
> >
> > Having recently moved over to a top-of-the-tree u-boot on one of my
> > VIM3L systems in order to benefit from unrelated improvements
> > (automatic PCIe detection, EFI...), I faced the issue that my kernel
> > would hang like this:
> >
> > [  OK  ] Finished Helper to synchronize boot up for ifupdown.
> > [  OK  ] Started Rule-based Manager for Device Events and Files.
> > [    7.114516] VDDCPU: supplied by regulator-dummy
> > [  OK  ] Found device /dev/ttyAML0.
> > [    7.146862] meson-drm ff900000.vpu: Queued 2 outputs on vpu
> > [    7.169630] fb0: switching to meson-drm-fb from simple
> > [    7.169944] Console: switching to colour dummy device 80x25
> > [    7.179250] meson-drm ff900000.vpu: CVBS Output connector not 
> > available
> >
> > and that's it.
> >
> > After some poking around, I figured out that it is in the
> > meson-dw-hdmi module that the CPU was hanging...
>
> I'll be interested in having your kernel config, I never had such 
> report
> since I enabled HDMI support in U-Boot a few years ago.

Yeah, I was pretty surprised too. I have a hunch that this is caused
by u-boot DT exposing an extra MMIO region (dubbed "hhi") that gets
picked up by the kernel driver. *Not* having the region in the DT
(as in the kernel's version of the same DT) makes the driver work
exactly once:

Decompiled u-boot DT:

        hdmi-tx@0 {
                compatible = "amlogic,meson-g12a-dw-hdmi";
                reg = <0x00 0x00 0x00 0x10000 0x00 0x3c000 0x00 0x1000>;
                [...]
                reg-names = "hdmitx\0hhi";

Decompiled kernel DT:

        hdmi-tx@0 {
                compatible = "amlogic,meson-g12a-dw-hdmi";
                reg = <0x00 0x00 0x00 0x10000>;

There seem to be some complex interactions between the HDMI driver
and the DRM driver, both using this MMIO region at any given time.
But I admit not having tried very hard to follow the DRM maze of
intricate callbacks. All I needed was this box to reliably boot with
the firmware-provided DT.

You can find a reasonably recent version of my config at [1].

        M.

[1] http://www.loen.fr/tmp/Config.full-arm64
--
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