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

[Javier Martinez Canillas <javierm@redhat.com>]

Hello Sam,

On 11/4/21 18:57, Jani Nikula wrote:
> On Thu, 04 Nov 2021, Sam Ravnborg <sam@ravnborg.org> wrote:
>> Hi Javier,
>>
>> On Thu, Nov 04, 2021 at 05:07:06PM +0100, Javier Martinez Canillas wrote:
>>> Some DRM drivers check the vgacon_text_force() function return value as an
>>> indication on whether they should be allowed to be enabled or not.
>>>
>>> This function returns true if the nomodeset kernel command line parameter
>>> was set. But there may be other conditions besides this to determine if a
>>> driver should be enabled.
>>>
>>> Let's add a drm_drv_enabled() helper function to encapsulate that logic so
>>> can be later extended if needed, without having to modify all the drivers.
>>>
>>> Also, while being there do some cleanup. The vgacon_text_force() function
>>> is guarded by CONFIG_VGA_CONSOLE and there's no need for callers to do it.
>>>
>>> Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
>>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
>>> ---
>>>
>>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>>> index 8214a0b1ab7f..3fb567d62881 100644
>>> --- a/drivers/gpu/drm/drm_drv.c
>>> +++ b/drivers/gpu/drm/drm_drv.c
>>> @@ -975,6 +975,26 @@ int drm_dev_set_unique(struct drm_device *dev, const char *name)
>>>  }
>>>  EXPORT_SYMBOL(drm_dev_set_unique);
>>>  
>>> +/**
>>> + * 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)
>>> +{
>>> +	if (vgacon_text_force()) {
>>> +		DRM_INFO("%s driver is disabled\n", driver->name);
>>
>> DRM_INFO is deprecated, please do not use it in new code.
>> Also other users had an error message and not just info - is info
>> enough?
>>

Thanks, I didn't know that. Right, they had an error but I do wonder
if that was correct though. After all isn't an error but an explicit
disable due "nomodeset" being set in the kernel command line.

[snip]

>>>  
>>> -	if (vgacon_text_force() && i915_modparams.modeset == -1)
>>> +	ret = drm_drv_enabled(&driver);
>>
>> You pass the local driver variable here - which looks wrong as this is
>> not the same as the driver variable declared in another file.
>

Yes, Jani mentioned it already. I got confused and thought that the driver
variable was also defined in the same compilation unit...

Maybe I could squash the following change?

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index b18a250e5d2e..b8f399b76363 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -89,7 +89,7 @@
 #include "intel_region_ttm.h"
 #include "vlv_suspend.h"
 
-static const struct drm_driver driver;
+const struct drm_driver driver;
 
 static int i915_get_bridge_dev(struct drm_i915_private *dev_priv)
 {
@@ -1777,7 +1777,7 @@ static const struct drm_ioctl_desc i915_ioctls[] = {
        DRM_IOCTL_DEF_DRV(I915_GEM_VM_DESTROY, i915_gem_vm_destroy_ioctl, DRM_RENDER_ALLOW),
 };
 
-static const struct drm_driver driver = {
+const struct drm_driver driver = {
        /* Don't use MTRRs here; the Xserver or userspace app should
         * deal with them for Intel hardware.
         */
diff --git a/drivers/gpu/drm/i915/i915_module.c b/drivers/gpu/drm/i915/i915_module.c
index c890c1ca20c4..88f770920324 100644
--- a/drivers/gpu/drm/i915/i915_module.c
+++ b/drivers/gpu/drm/i915/i915_module.c
@@ -16,7 +16,7 @@
 #include "i915_selftest.h"
 #include "i915_vma.h"
 
-static const struct drm_driver driver;
+extern const struct drm_driver driver;
 
 static int i915_check_nomodeset(void)
 {

That should work and I actually got a laptop with an i915 and tested the change
both when CONFIG_DRM_I915=m and CONFIG_DRM_I915=y is set.

Another option is to declare it in i915_drv.h and not make the definition static.
 
> Indeed.
> 
>> Maybe move the check to new function you can add to init_funcs,
>> and locate the new function in i915_drv - so it has access to driver?
> 
> We don't really want that, though. This check is pretty much as early as
> it can be, and there's a ton of useless initialization that would happen
> if we waited until drm_driver is available.
>

Agreed.
 
> From my POV, drm_drv_enabled() is a solution that creates a worse
> problem for us than it solves.
>

I don't have a strong opinion on this. I could just do patch #2 without adding
a level of indirection through drm_drv_enabled(). But Thomas and Daniel Vetter
suggested that we should do this change before.

That is, the drivers could just check if should be enabled by calling to the
drm_check_modeset() function directly if people agree that encapsulating that
logic in a drm_drv_enabled() is not needed.

> 
> BR,
> Jani.
> 

Best regards,
-- 
Javier Martinez Canillas
Linux Engineering
Red Hat