Re: [PATCH] platform/raspberrypi: vchiq: register bus at subsys_initcall

["Gaston Gonzalez" <gascoar@gmail.com>]

On Mon Oct 19, 2026 at 9:26 AM -03, Martin Wetterwald wrote:
> On Sun, Oct 18, 2026 at 9:53 PM Gaston Gonzalez <gascoar@gmail.com> wrote:
>
>> Running 'speaker-test -t wav -D plughw:1,0 -c 2' crash the system, so
>> thinks seem not to be working with that interface.
>>
>> Im trying to debug what happens in that condition; I will need a couple
>> of days I guess. So far, this is what I could check.
>>
>> Any feedback on the test or debugging would be appreciated.
>
> Hi Gaston,
> Thanks for digging into this.
>

Hi Martin,


> The patch only changes when the VCHIQ bus and driver are registered during
> boot. So it's enough to check these two cases to validate the patch:
> - without my patch: dmesg shows the "unable to register with bus_type
>   'vchiq-bus'" error, and bcm2835_hdmi is absent from /proc/asound/cards.
> - with my patch: the error is gone and bcm2835_hdmi is present in
>   /proc/asound/cards.
>

Understood. Of the two cases, the second one is already tested if I'm not
mistaken. I will check the 'without your patch' case first, and then
I'll rerun the test with your patch.

> (Since the Pi Zero has no Headphones jack, you need to test both cases with
> snd_bcm2835.enable_hdmi=1 in the command line. Otherwise the card never shows
> up, even when everything works.)
>
> Could you check these two cases?
>

Yes, will do.

>
> Testing that you can hear the sound going through bcm2835-audio is a full
> functional test that goes even further than the patch: it does not only test
> bus registration but also that the audio driver is working.
>
> On my side, without vc4-drm, I can successfully hear sound through both bcm2835
> Headphones and bcm2835 HDMI:
>
>   # aplay -l
>   **** List of PLAYBACK Hardware Devices ****
>   card 0: HDMI [bcm2835 HDMI], device 0: bcm2835 HDMI [bcm2835 HDMI]
>   [...]
>   card 0: HDMI [bcm2835 HDMI], device 1: IEC958 [IEC958]
>   [...]
>   card 1: Headphones [bcm2835 Headphones], device 0: bcm2835
> Headphones [bcm2835 Headphones]
>   [...]
>
>   # speaker-test -D plughw:0,0 # (HDMI)
>
>   # speaker-test -D plughw:1,0 # (Headphones, not on the Pi Zero)
>
>
> If you still want to output sound through bcm2835_hdmi (optional):
>
> There can only be one owner of the HDMI port: either the firmware or the
> kernel.
> When the Raspberry Pi boots, the firmware first owns the HDMI and displays a
> rainbow splash. Then it launches Linux.
>
> There are 2 ways for the kernel to deal with the video output:
>
> 1) With vc4-drm (CONFIG_DRM_VC4), the kernel takes over the HDMI controller
>    from the firmware and handles everything itself (this is your case).
> 2) With simple-framebuffer (CONFIG_DRM_SIMPLEDRM or CONFIG_FB_SIMPLE), the
>    kernel leaves the HDMI to the firmware and just writes pixels to a
> memory area
>    (the framebuffer) set up by the firmware.
>
> I get a similar problem on my side when sending sound through bcm2835_hdmi with
> vc4-drm loaded: my screen shuts down (but the kernel is still alive).
> It most likely comes from the firmware and the kernel both trying to drive the
> same HDMI port: with vc4-drm, the kernel owns the HDMI controller, and the
> firmware (which bcm2835-audio talks to through VCHIQ) is not supposed to touch
> it.
>

Ah ok, I did not know that. Thank you for the explanation.

> A simple way to output sound through bcm2835_hdmi is to not use the vc4-drm
> driver at all, and use simple-framebuffer if you still want a display.
>
> To remove vc4-drm:
> - Do not compile the driver into the kernel (CONFIG_DRM_VC4=n) or blacklist the
>   module.
> - Remove any vc4 overlay in config.txt file (e.g. remove dtoverlay=vc4-kms-v3d).
>
> To enable simple-framebuffer, use one of the two configs:
>
>   CONFIG_DRM=y
>   CONFIG_DRM_SIMPLEDRM=y
>   CONFIG_DRM_FBDEV_EMULATION=y
>   CONFIG_FRAMEBUFFER_CONSOLE=y
>
> or, the older way:
>
>   CONFIG_FB=y
>   CONFIG_FB_SIMPLE=y
>   CONFIG_FRAMEBUFFER_CONSOLE=y
>
> (FRAMEBUFFER_CONSOLE=y, and DRM_FBDEV_EMULATION in the DRM case, are only
> needed to see the boot messages on the screen, with console=tty1 on the command
> line.)
>
> If you have remote access to your Pi Zero (either through network+SSH or
> through serial console), you actually don't even need the simple-framebuffer:
> you can just remove vc4-drm, and let the firmware display its rainbow splash on
> the HDMI port.
> Then you can login to your Pi Zero through SSH for example and launch
> speaker-test.
> As long as the HDMI video output is active, it's enough for the sound to be
> able to go through, even if the image was not drawn by Linux.
>
> But again, this full functional test is optional.
>

Ok, I’ll test the patch first with the current configuration, then
with simple-framebuffer enabled in a second step.

Regards,

Gaston

> Regards,
> Martin