linux-kernel - Re: [Letux-kernel] [PATCH v16 1/4] drm/bridge: dw-hdmi: introduce dw_hdmi_enable

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Date:   Fri, 4 Mar 2022 14:30:46 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     "H. Nikolaus Schaller" <hns@...delico.com>,
        Paul Cercueil <paul@...pouillou.net>
Cc:     Paul Boddie <paul@...die.org.uk>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Maxime Ripard <maxime@...no.tech>,
        Jonas Karlman <jonas@...boo.se>,
        David Airlie <airlied@...ux.ie>,
        Robert Foss <robert.foss@...aro.org>,
        linux-mips <linux-mips@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Subject: Re: [Letux-kernel] [PATCH v16 1/4] drm/bridge: dw-hdmi: introduce
 dw_hdmi_enable_poll()

Hi,

On 03/03/2022 18:59, H. Nikolaus Schaller wrote:
> Hi Paul, Neil,
> 
>> Am 03.03.2022 um 18:20 schrieb Paul Cercueil <paul@...pouillou.net>:
>>
>> Hi Nikolaus,
>>
>> [snip]
>>
>>>> Well he said "the Ingenic DRM core" aka ingenic-drm-drv.c. You do have access to the main drm_device in the ingenic_drm_bind() function, so you can add it there (with a cleanup function calling drm_kms_helper_poll_fini() registered with drmm_add_action_or_reset()).
>>> Well, do you really want to mix HPD detection between connector, Synopsys bridge and Ingenic DRM core? These are independent...
>>> Or should be accessed only through the bridge chain pointers.
>>> IMHO we should keep separate functions separate.
>>
>> The drm_kms_helper_poll_init() just says "this DRM device may have connectors that need to be polled" so it very well fits inside the main driver, IMHO.
> 
> As far as I understand, it has the side-effect to always set dev->mode_config.poll_enabled and
> schedule_delayed_work() for all devices.
> I am not sure if this is intended for arbitrary ingenic-drm devices. But you know better than me.
> 
> 
> Hm. But wait, I think I now finally remember why I have proposed it the way it is!
> It is always better to go back to requirements and find the least invasive solution.
> 
> - HPD IRQ works and calls dw_hdmi_irq() [as can be shown by adding printk()]
> - it is just that the udevd is only notified if poll_enabled = true (but no polling takes place!).
> 
> An earlier version (v4) to fix this proposed to add an explicit call to drm_kms_helper_hotplug_event()
> in dw_hdmi_irq() but that was rejected a while ago because drm_helper_hpd_irq_event() will already call it:
> 
> 	https://www.spinics.net/lists/dri-devel/msg316846.html
> 
> Since this did not take into account that dev->mode_config.poll_enabled must be set true, I then proposed the
> enable_poll() mechanism just to set this bit for the ingenic-dw-hdmi specialization.
> 
> So a HPD event is delivered to the dw-hdmi driver as dw_hdmi_irq() and that calls drm_helper_hpd_irq_event()
> but not drm_kms_helper_hotplug_event() and user-space is not getting aware.
> 
> It is all a hack because we mix the dw-hdmi driver which originally did register its own connector
> with an explicit connector...
> 
> In summary I now thing that the v4 patch is the simplest and least invasive solution.
> 
> We neither have to introduce a dw_hdmi_enable_poll() function or call drm_kms_helper_poll_init() anywhere.
> 
> It is just a single line to add to dw-hdmi. And neither touches ingenic-dw-hdmi nor ingenic-drm-drv.
> 
> So let's go back to v4 version (just modify commit message to better describe why we have to call
> drm_kms_helper_hotplug_event() explicitly) and forget about alternatives.

Please don't and add drm_kms_helper_poll_init() from the ingenic-drm-drv.c like every other DRM driver.

Adding drm_kms_helper_hotplug_event() in dw-hdmi will impact other drivers using dw-hdmi but correctly
calling drm_kms_helper_poll_init().

Neil

> 
> BR,
> Nikolaus