linux-kernel - Re: [PATCH v2 1/2] drm: Add a drm_drv_enabled() to check if drivers should be enabled

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Date:   Fri, 5 Nov 2021 10:48:38 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Cc:     David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        amd-gfx@...ts.freedesktop.org,
        VMware Graphics <linux-graphics-maintainer@...are.com>,
        Peter Robinson <pbrobinson@...il.com>,
        nouveau@...ts.freedesktop.org, Dave Airlie <airlied@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Michel Dänzer <michel@...nzer.net>,
        Hans de Goede <hdegoede@...hat.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        virtualization@...ts.linux-foundation.org,
        Pekka Paalanen <pekka.paalanen@...labora.com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        spice-devel@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        intel-gfx@...ts.freedesktop.org,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v2 1/2] drm: Add a drm_drv_enabled() to check if drivers
 should be enabled

Hello Thomas,

On 11/5/21 09:43, Thomas Zimmermann wrote:
> Hi
> 
> Am 04.11.21 um 21:09 schrieb Javier Martinez Canillas:
>> Hello Jani,
>>
>> On 11/4/21 20:57, Jani Nikula wrote:
>>> On Thu, 04 Nov 2021, Javier Martinez Canillas <javierm@...hat.com> wrote:
>>>> +/**
>>>> + * drm_drv_enabled - Checks if a DRM driver can be enabled
>>>> + * @driver: DRM driver to check
>>>> + *
>>>> + * Checks whether a DRM driver can be enabled or not. This may be the case
>>>> + * if the "nomodeset" kernel command line parameter is used.
>>>> + *
>>>> + * Return: 0 on success or a negative error code on failure.
>>>> + */
>>>> +int drm_drv_enabled(const struct drm_driver *driver)
> 
> Jani mentioned that i915 absolutely wants this to run from the 
> module_init function. Best is to drop the parameter.
>

Ok. I now wonder though how much value would add this function since
it will just be a wrapper around the nomodeset check.

We talked about adding a new DRIVER_GENERIC feature flag and check for
this, but as danvet mentioned that is not really needed. We just need
to avoid testing for nomodeset in the simpledrm driver.

Do you envision other condition that could be added later to disable a
DRM driver ? Or do you think that just from a code readability point of
view makes worth it ?

>>>> +{
>>>> +	if (vgacon_text_force()) {
>>>> +		DRM_INFO("%s driver is disabled\n", driver->name);
>>>> +		return -ENODEV;
>>>> +	}
> 
> If we run this from within a module_init function, we'd get plenty of 
> these warnings if drivers are compiled into the kernel. Maybe simply 
> remove the message. There's already a warning printed by the nomodeset 
> handler.
>

Indeed. I'll just drop it.

>>>> +
>>>> +	return 0;
>>>> +}
>>>> +EXPORT_SYMBOL(drm_drv_enabled);
>>>
>>> The name implies a bool return, but it's not.
>>>
>>> 	if (drm_drv_enabled(...)) {
>>> 		/* surprise, it's disabled! */
>>> 	}
>>>
>>
>> It used to return a bool in v2 but Thomas suggested an int instead to
>> have consistency on the errno code that was returned by the callers.
>>
>> I should probably name that function differently to avoid confusion.
> 
> Yes, please.
>

drm_driver_check() maybe ?
 
Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat