linux-kernel - Re: [PATCH v6 09/10] drm/vc4: hdmi: stop rereading EDID in get

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Message-ID: <73xdxb575n4ncmadffkwqzczoewbadq3forpvqt4vp7zfln2nq@o2wmbbbepwgg>
Date: Sun, 8 Dec 2024 13:06:46 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Maxime Ripard <mripard@...nel.org>
Cc: Andrzej Hajda <andrzej.hajda@...el.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman <jonas@...boo.se>, 
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Subject: Re: [PATCH v6 09/10] drm/vc4: hdmi: stop rereading EDID in
 get_modes()

On Fri, Dec 06, 2024 at 03:34:52PM +0100, Maxime Ripard wrote:
> Hi,
> 
> On Fri, Dec 06, 2024 at 12:16:03PM +0200, Dmitry Baryshkov wrote:
> > The vc4_hdmi_connector_detect_ctx() via vc4_hdmi_handle_hotplug()
> > already reads EDID and propagates it to the drm_connector. Stop
> > rereading EDID as a part of the .get_modes() callback and just update
> > the list of modes. This matches the behaviour of the i915 driver.
> > 
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> > ---
> >  drivers/gpu/drm/vc4/vc4_hdmi.c | 21 ---------------------
> >  1 file changed, 21 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > index e5ab42f72f618b90f956482db6c9c8074c1e3bf1..3364ef90968dad3074800f02926300ffceb75c69 100644
> > --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > @@ -470,31 +470,10 @@ static int vc4_hdmi_connector_detect_ctx(struct drm_connector *connector,
> >  
> >  static int vc4_hdmi_connector_get_modes(struct drm_connector *connector)
> >  {
> > -	struct vc4_hdmi *vc4_hdmi = connector_to_vc4_hdmi(connector);
> >  	struct vc4_dev *vc4 = to_vc4_dev(connector->dev);
> > -	const struct drm_edid *drm_edid;
> >  	int ret = 0;
> >  
> > -	/*
> > -	 * NOTE: This function should really take vc4_hdmi->mutex, but doing so
> > -	 * results in reentrancy issues since cec_s_phys_addr() might call
> > -	 * .adap_enable, which leads to that funtion being called with our mutex
> > -	 * held.
> > -	 *
> > -	 * Concurrency isn't an issue at the moment since we don't share
> > -	 * any state with any of the other frameworks so we can ignore
> > -	 * the lock for now.
> > -	 */
> > -
> > -	drm_edid = drm_edid_read_ddc(connector, vc4_hdmi->ddc);
> > -	drm_edid_connector_update(connector, drm_edid);
> > -	cec_s_phys_addr(vc4_hdmi->cec_adap,
> > -			connector->display_info.source_physical_address, false);
> > -	if (!drm_edid)
> > -		return 0;
> > -
> >  	ret = drm_edid_connector_add_modes(connector);
> > -	drm_edid_free(drm_edid);
> 
> I don't think that works though, for mostly two reasons:
> 
>  1) We're not sure we'll even have an hotplug interrupt if the system
>     boots with the connector plugged in for example.
> 
>  2) afaik, the get_modes ioctl directly calls get_modes too.

I think both paths use .fill_modes, not get_modes. And fill_modes
explicitly calls .detect / .detect_ctx. It would be nice if somebody can
verify the change on the acual hw. But as I wrote, i915 driver does
exactly the same: the EDID is read / updated in .detect_ctx(), while
.get_modes() only performs drm_edid_connector_add_modes().
Okay, there is one significant difference which we should probably patch
out: the .force() is also implemented to update EDID in the forced case.
I should probably do the same for the drivers being handled in this
patchset.

-- 
With best wishes
Dmitry