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SubjectRe: [PATCH v3 6/6] drm/ingenic: Attach bridge chain to encoders


Le jeu., sept. 23 2021 at 20:52:23 +0200, H. Nikolaus Schaller
<hns@goldelico.com> a écrit :
> Hi Paul,
>
>> Am 23.09.2021 um 15:30 schrieb Paul Cercueil <paul@crapouillou.net>:
>>
>> Hi Nikolaus,
>>
>> Le jeu., sept. 23 2021 at 13:41:28 +0200, H. Nikolaus Schaller
>> <hns@goldelico.com> a écrit :
>>> Hi Laurent,
>>> Ah, ok.
>>> But then we still have issues.
>>> Firstly I would assume that get_edid only works properly if it is
>>> initialized
>>> through dw_hdmi_connector_create().
>>> Next, in the current code, passing DRM_BRIDGE_ATTACH_NO_CONNECTOR
>>> to
>>> dw_hdmi_bridge_attach() indeed does not call
>>> dw_hdmi_connector_create()
>>> but returns 0.
>>> This patch 6/6 makes drm/ingenic unconditionally require a
>>> connector
>>> to be attached which is defined somewhere else (device tree e.g.
>>> "connector-hdmi")
>>> unrelated to dw-hdmi. Current upstream code for drm/ingenic does
>>> not init/attach
>>> such a connector on its own so it did work before.
>>> I.e. I think we can't just use parts of dw-hdmi.
>>
>> The fact that Laurent is using dw-hdmi with
>> DRM_BRIDGE_ATTACH_NO_CONNECTOR on Renesas makes me think that it's
>> possible here as well. There's no reason why it shouldn't work with
>> ingenic-drm.
>
> That is interesting and Laurent can probably comment on differences
> between
> his setup (I wasn't able to deduce what device you are referring to)
> and dw-hdmi.
>
> For jz4780 we tried that first. I do not remember the exact reasons
> but we wasted
> weeks trying to but failed to get it working. While the dw-hdmi
> connector simply works
> on top of upstream and fails only if we apply your patch.
>
> Another issue is how you want to tell connector-hdmi to use the extra
> i2c bus driver
> for ddc which is not available directly as a standard i2c controller
> of the jz4780.
>
> hdmi-connector.yaml defines:
>
> ddc-i2c-bus:
> description: phandle link to the I2C controller used for DDC EDID
> probing
> $ref: /schemas/types.yaml#/definitions/phandle
>
> So we would need some ddc-i2c-bus = <&i2c-controller-inside-the
> dw-hdmi>.
>
> But that i2c-controller-inside-the dw-hdmi does not exist in device
> tree
> and can not be added unless someone significantly rewrites dw-hdmi to
> register and expose it as i2c controller.

No, you don't need to do that at all. Just don't set the "ddc-i2c-bus"
property.

>>
>> The ingenic-drm driver does not need to create any connector. The
>> "connector-hdmi" is connected to dw-hdmi as the "next bridge" in the
>> list.
>
> Sure. It does not *create* a connector. It expects that it can safely
> call
> drm_bridge_connector_init() to get a pointer to a newly created
> connector.
>
> But if we use the dw-hdmi connector, there is no such connector and
> "next bridge".

We don't want to use the dw-hdmi connector. Your "next bridge" is the
"hdmi-connector" that should be wired in DTS.

> Or can you tell me how to make the dw-hdmi connector created by
> dw_hdmi_connector_create() become the "next bridge" in the list for
> your driver?
> But without significantly rewriting dw-hdmi.c (and testing).

Wire it to the LCD node in DTS...

See how we do it for the IT66121 driver:
https://github.com/OpenDingux/linux/blob/jz-5.15/arch/mips/boot/dts/ingenic/rg350m.dts#L114-L134

>>
>>> If drm_bridge_attach() would return some errno if
>>> DRM_BRIDGE_ATTACH_NO_CONNECTOR
>>> is set, initialization in ingenic_drm_bind() would fail likewise
>>> with "Unable to attach bridge".
>>> So in any case dw-hdmi is broken by this drm/ingenic patch unless
>>> someone
>>> reworks it to make it compatible.
>>
>> Where would the errno be returned? Why would drm_bridge_attach()
>> return an error code?
>
> Currently dw_hdmi_bridge_attach() returns 0 if it is asked to support
> DRM_BRIDGE_ATTACH_NO_CONNECTOR.
>
> This is not treated as an error by drm_bridge_attach().
>
> Here it could return an error (-ENOTSUPP?) instead, to allow for
> error handling
> by its caller.

And why would you do that? If you don't want to attach a connector,
then drm_bridge_attach() doesn't need to do much. So it's normal that
it returns zero.

> But that raises an error message like "failed to attach bridge to
> encoder" and
> the bridge is reset and detached.
>
>>
>>> Another issue is that dw_hdmi_connector_create() does not only do
>>> dcd/edid
>>> but appears to detects hot plug and does some special
>>> initialization.
>>> So we probably loose hotplug detect if we just use
>>> drm_bridge_funcs.get_edid().
>>
>> There's drm_bridge_funcs.detect().
>
> You mean in dw-hdmi? Yes, it calls dw_hdmi_bridge_detect() which
> calls dw_hdmi_detect().
> This does some read_hpd.
>
> Anyways, this does not solve the problem that with your drm/ingenic
> proposal the
> dw-hdmi subsystem (hdmi + ddc) can no longer be initialized properly
> unless someone
> fixes either.
>
> So IMHO this should be treated as a significant blocking point for
> your patch
> because it breaks something that is working upstream and there seems
> to be no
> rationale to change it.
>
> Your commit message just says:
> "All the bridges are now attached with
> DRM_BRIDGE_ATTACH_NO_CONNECTOR."
> but gives no reason why.
>
> I fully understand that you want to change it and Laurent said that
> it will become
> standard in the far future. Therefore I suggest to find a way that
> you can find out
> if a connector has already been created by drm_bridge_attach() to
> stay compatible
> with current upstream code.

No, absolutely not. There is nothing upstream yet that can bind the
ingenic-drm driver with the dw-hdmi driver. This is your downstream
patch. I'm not breaking anything that's upstream, so there is no
blocking point.

Besides, even with your downstream patch I don't see any reason why the
dw-hdmi driver wouldn't work with this patch, provided it's wired
properly, and you never did show a proof of failure either. You come up
with "possible points where it will fail" but these are based on your
assumptions on how the drivers should be working together, and I think
you somehow miss the whole picture.

Start by wiring things properly, like in my previously linked DTS, and
*test*. If it fails, tell us where it fails. Because your "it doesn't
work" arguments have zero weight otherwise.

If I can find some time this weekend I will test it myself.

Cheers,
-Paul

> I even want to help here but I don't know how to detect the inverse of
> drm_connector_attach_encoder(), i.e.
> is_drm_encoder_attached_to_any_connector().
>
> BR and thanks,
> Nikolaus
>
>
>


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