Re: DRM interaction problems on Ingenic CI20 / jz4780 with dw-hdmi and ingenic-drm

[Paul Cercueil]

Hi Nikolaus,

So I just wrote a HDMI driver for a different chip, I guess I can 
answer some of your questions now.


Le lun. 13 avril 2020 à 13:25, Paul Cercueil <paul@xxxxxxxxxxxxxxx> a 
écrit :
> Hi Nikolaus,
>
>
> Le sam. 11 avril 2020 à 16:14, H. Nikolaus Schaller 
> <hns@xxxxxxxxxxxxx> a écrit :
> > Hi,
> > we (Paul Boddie and me) are working to get HDMI functional on the
> > Ingenic CI20 board with jz4780 SoC which uses a specialization of
> > the dw-hdmi driver.
> >
> >
> > So far we have identified two issues.
> >
> > The first is that HPD interrupts are not properly processed.
> >
> > drm_helper_hpd_irq_event() is called by HPD events but
> > dev->mode_config.poll_enabled is false.

This is to be used when there's no hardware interrupt. I believe you 
have one, right? Then call drm_kms_helper_hotplug_event() from the 
interrupt handler instead.

> > Therefore the interrupt is ignored and nothing happens.
> >
> > Now I wonder about the logic behind checking for poll_enabled.
> > I understand that a driver can do either polling or irq or both.
> >
> > Therefore handling the irq_event shouldn't be disabled by 
> > poll_enabled
> > being false. Otherwise we can only do: nothing, polling, polling+irq
> > but not irq alone.
> >
> > The jz4780 hdmi subsystem (drm/bridge/dw-hdmi.c) uses
> >
> > 	connector->polled = DRM_CONNECTOR_POLL_HPD;
> >
> > but shouldn't this enable polling? Note that there seems to be
> > no (direct) call to drm_kms_helper_poll_init().
> >
> > If we set dev->mode_config.poll_enabled = true in
> > drm_helper_hpd_irq_event() things start to work.
> >
> > Please can you clarify what would be best practise here to
> > get HPD event handling working.

Remove that - this stuff is for hardware without interrupts, where 
everything has to be polled.

> > The other issue is in dw-hdmi.c:
> >
> > We found out that ingenic_drm_encoder_atomic_check() fails because
> >
> > info->num_bus_formats == 0
> >
> > and not 1. This blocks further initialization.
> >
> > The reason seems to be that dw_hdmi_bridge_attach() does not call
> > drm_display_info_set_bus_formats() with a proper format like
> > other drivers (e.g. drm/bridge/ti-tfp410.c) are doing.
> >
> > We have patched to set a single bus format MEDIA_BUS_FMT_RGB888_1X24
> > and then DRM setup seems to work (although we still have no valid
> > HDMI signal but that is likely something else).
> >
> > Please can you explain how setting the bus format should be fixed
> > in dw-hdmi.c.

I'm not sure, but that information may come from EDID data. Are you 
able to obtain video modes from the connected monitor?

-Paul

> > If these questions should be forwarded to other specialists, please
> > do so.
>
> It should be sent to the DRI mailing list, you missed the most 
> important one.
>
> -Paul
>
> > BR and thanks,
> > Nikolaus Schaller