linux-kernel - Re: [PATCH 2/2] gpu/drm: ingenic: Add workaround for disabled drivers

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Date:   Thu, 05 Aug 2021 22:05:27 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Rob Herring <robh@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Sam Ravnborg <sam@...nborg.org>, list@...ndingux.net,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/2] gpu/drm: ingenic: Add workaround for disabled drivers

Hi Greg,

Le jeu., août 5 2021 at 21:35:34 +0200, Greg Kroah-Hartman 
<gregkh@...uxfoundation.org> a écrit :
> On Thu, Aug 05, 2021 at 09:21:09PM +0200, Paul Cercueil wrote:
>>  When the drivers of remote devices (e.g. HDMI chip) are disabled in 
>> the
>>  config, we want the ingenic-drm driver to be able to probe 
>> nonetheless
>>  with the other devices (e.g. internal LCD panel) that are enabled.
>> 
>>  Signed-off-by: Paul Cercueil <paul@...pouillou.net>
>>  ---
>>   drivers/gpu/drm/ingenic/ingenic-drm-drv.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>> 
>>  diff --git a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c 
>> b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
>>  index d261f7a03b18..5e1fdbb0ba6b 100644
>>  --- a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
>>  +++ b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
>>  @@ -1058,6 +1058,18 @@ static int ingenic_drm_bind(struct device 
>> *dev, bool has_components)
>>   	for (i = 0; ; i++) {
>>   		ret = drm_of_find_panel_or_bridge(dev->of_node, 0, i, &panel, 
>> &bridge);
>>   		if (ret) {
>>  +			/*
>>  +			 * Workaround for the case where the drivers for the
>>  +			 * remote devices are not enabled. When that happens,
>>  +			 * drm_of_find_panel_or_bridge() returns -EPROBE_DEFER
>>  +			 * endlessly, which prevents the ingenic-drm driver from
>>  +			 * working at all.
>>  +			 */
>>  +			if (ret == -EPROBE_DEFER) {
>>  +				ret = driver_deferred_probe_check_state(dev);
>>  +				if (ret == -ENODEV || ret == -ETIMEDOUT)
>>  +					continue;
>>  +			}
> 
> So you are mucking around with devices on other busses within this
> driver?  What could go wrong?  :(

I'm doing the same thing as everybody else. This is the DRM driver, and 
there is a driver for the external HDMI chip which gives us a DRM 
bridge that we can obtain from the device tree.

> Please use the existing driver core functionality for this type of
> thing, it is not unique, no need for this function to be called.

I'm not sure you understand what I'm doing here. This driver calls 
drm_of_find_panel_or_bridge(), without guarantee that the driver for 
the remote device (connected via DT graph) has been enabled in the 
kernel config. In that case it will always return -EPROBE_DEFER and the 
ingenic-drm driver will never probe.

This patch makes sure that the driver can probe if the HDMI driver has 
been disabled in the kernel config, nothing more.

Cheers,
-Paul