linux-kernel - Re: [PATCH 13/14] drm/bridge: synopsys: Add DW HDMI QP TX controller driver

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Date: Wed, 5 Jun 2024 13:48:09 +0200
From: Neil Armstrong <neil.armstrong@...aro.org>
To: Cristian Ciocaltea <cristian.ciocaltea@...labora.com>,
 Sam Ravnborg <sam@...nborg.org>
Cc: Andrzej Hajda <andrzej.hajda@...el.com>, Robert Foss <rfoss@...nel.org>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
 Jonas Karlman <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 Sandy Huang <hjc@...k-chips.com>, Heiko Stübner
 <heiko@...ech.de>, Andy Yan <andy.yan@...k-chips.com>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Mark Yao <markyao0591@...il.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org,
 devicetree@...r.kernel.org, kernel@...labora.com,
 Alexandre ARNOUD <aarnoud@...com>, Luis de Arquer <ldearquer@...il.com>,
 Algea Cao <algea.cao@...k-chips.com>
Subject: Re: [PATCH 13/14] drm/bridge: synopsys: Add DW HDMI QP TX controller
 driver

On 05/06/2024 12:11, Cristian Ciocaltea wrote:
> On 6/5/24 12:34 AM, Cristian Ciocaltea wrote:
>> On 6/4/24 11:41 PM, Sam Ravnborg wrote:
>>> Hi Cristian.
>>>
>>> On Tue, Jun 04, 2024 at 10:32:04PM +0300, Cristian Ciocaltea wrote:
>>>> Hi Sam,
>>>>
>>>> On 6/1/24 5:32 PM, Sam Ravnborg wrote:
>>>>> Hi Cristian,
>>>>>
>>>>> a few drive-by comments below.
>>>>>
>>>>> 	Sam
>>>>>
>>>>>
>>>>>> +
>>>>>> +static const struct drm_connector_funcs dw_hdmi_qp_connector_funcs = {
>>>>>> +	.fill_modes = drm_helper_probe_single_connector_modes,
>>>>>> +	.detect = dw_hdmi_connector_detect,
>>>>>> +	.destroy = drm_connector_cleanup,
>>>>>> +	.force = dw_hdmi_qp_connector_force,
>>>>>> +	.reset = drm_atomic_helper_connector_reset,
>>>>>> +	.atomic_duplicate_state = drm_atomic_helper_connector_duplicate_state,
>>>>>> +	.atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
>>>>>> +};
>>>>>> +
>>>>>> +static int dw_hdmi_qp_bridge_attach(struct drm_bridge *bridge,
>>>>>> +				    enum drm_bridge_attach_flags flags)
>>>>>> +{
>>>>>> +	struct dw_hdmi *hdmi = bridge->driver_private;
>>>>>> +
>>>>>> +	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR)
>>>>>> +		return drm_bridge_attach(bridge->encoder, hdmi->next_bridge,
>>>>>> +					 bridge, flags);
>>>>>> +
>>>>>> +	return dw_hdmi_connector_create(hdmi, &dw_hdmi_qp_connector_funcs);
>>>>>> +}
>>>>>
>>>>> Are there any users left that requires the display driver to create the
>>>>> connector?
>>>>> In other words - could this driver fail if DRM_BRIDGE_ATTACH_NO_CONNECTOR
>>>>> is not passed and drop dw_hdmi_connector_create()?
>>>>>
>>>>> I did not try to verify this - just a naive question.
>>>>
>>>> I've just tested this and it doesn't work - dw_hdmi_connector_create()
>>>> is still needed.
>>>
>>> Hmm, seems the display driver or some other bridge driver fails to
>>> support "DRM_BRIDGE_ATTACH_NO_CONNECTOR".
>>> what other drivers are involved?
>>
>> Could it be related to the glue driver (updated in the next patch) which
>> is also responsible for setting up the encoder?
>>
>>> Note that my comments here should be seen as potential future
>>> improvements, and do not block the patch from being used.
>>
>> Thanks for the heads up! Will try to get back to this soon and investigate.
>   
> IIUC, modern bridges should not create the connector but rely on display
> drivers to take care of, which in this case is the VOP2 driver. However,
> it also handles some of the older SoCs relying on the non-QP variant of
> DW HDMI IP. Hence the existing dw-hdmi driver would be also impacted in
> order to come up with a proper solution.
> 
> A quick check shows there are several users of this IP:
> 
> $ git grep -E '= dw_hdmi_(bind|probe)\('
> drivers/gpu/drm/bridge/imx/imx8mp-hdmi-tx.c:    hdmi->dw_hdmi = dw_hdmi_probe(pdev, plat_data);
> drivers/gpu/drm/bridge/synopsys/dw-hdmi.c:      hdmi = dw_hdmi_probe(pdev, plat_data);
> drivers/gpu/drm/imx/ipuv3/dw_hdmi-imx.c:        hdmi->hdmi = dw_hdmi_probe(pdev, match->data);
> drivers/gpu/drm/ingenic/ingenic-dw-hdmi.c:      hdmi = dw_hdmi_probe(pdev, &ingenic_dw_hdmi_plat_data);
> drivers/gpu/drm/meson/meson_dw_hdmi.c:  meson_dw_hdmi->hdmi = dw_hdmi_probe(pdev, &meson_dw_hdmi->dw_plat_data);
> drivers/gpu/drm/renesas/rcar-du/rcar_dw_hdmi.c: hdmi = dw_hdmi_probe(pdev, &rcar_dw_hdmi_plat_data);
> drivers/gpu/drm/rockchip/dw_hdmi-rockchip.c:            hdmi->hdmi = dw_hdmi_bind(pdev, encoder, plat_data);
> drivers/gpu/drm/sun4i/sun8i_dw_hdmi.c:  hdmi->hdmi = dw_hdmi_bind(pdev, encoder, plat_data);
> 
> I didn't check which display drivers would be involved, I'd guess there
> are quite a few of them as well. So it seems this ends up being a pretty
> complex task.

If this would be a brand new driver, then it should only support DRM_BRIDGE_ATTACH_NO_CONNECTOR,
so you should not create a connector from the driver.

The fact dw-hdmi accepts an attach without the flag is for legacy purpose
since some DRM drivers haven't switched to DRM_BRIDGE_ATTACH_NO_CONNECTOR yes,
but it's a requirement for new bridges so at some point you'll need to make
sure the rockchip glue and drm driver supports DRM_BRIDGE_ATTACH_NO_CONNECTOR.

This will greatly simplify the driver!

Neil

> 
> Cristian