Re: [PATCH] drm/irq: remove check on dev->dev_private

[Jani Nikula <jani.nikula@intel.com>]

On Tue, 11 Feb 2020, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Tue, Feb 11, 2020 at 04:47:53PM +0200, Jani Nikula wrote:
>> There is no real reason to require drivers to set and use
>> dev->dev_private. Indeed, the current recommendation, as documented in
>> drm_device.h, is to embed struct drm_device in the per-device struct
>> instead of using dev_private.
>> 
>> Remove the requirement for dev_private to have been set to indicate
>> driver initialization.
>
> Yeah this is nonsense. Also, drm_irq_install is purely optional
> semi-midlayer (it's not really a midlayer for the legacy drivers, but
> whatever, who cares about those).
>
> Now there might be some hilarious races this papers over, but:
>
> - Proper drivers should only call drm_dev_register once everything is set
>   up, including this stuff here. No race possible with anything else
>   really.
>
> - Slightly more wobbly drivers, including the legacy ones, all use
>   drm_global_mutex. This was the former BKL, which means that it was
>   impossible for soeone to go through the load/unload/reload (between
>   lastclose and firstopen) paths and also run the ioctl. But the ioctl had
>   to be made unlocked because blocking there killed X:
>
> 	commit 8f4ff2b06afcd6f151868474a432c603057eaf56
> 	Author: Ilija Hadzic <ihadzic@research.bell-labs.com>
> 	Date:   Mon Oct 31 17:46:18 2011 -0400
>
> 	    drm: do not sleep on vblank while holding a mutex
>
>   The even more legacy DRM_CONTROL ioctl stayed fully locked. But the file
>   open/close paths are still fully locked, and that's the only place
>   legacy drivers should call drm_irq_install/uninstall, so should all
>   still be fully ordered and protected and happy.
>
> Feel free to quote or not quote the above in the commit message.
>
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Daniel Vetter <daniel@ffwll.ch>
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>> 
>> ---
>> 
>> Any ideas for something else drm_irq_install() could/should check to
>> ensure "Driver must have been initialized"?
>> 
>> There are only a few instances of dev_private uses in i915, also to be
>> removed, and we could stop initializing dev_private altogether. We could
>> in fact do that without this patch too, as we don't use
>> drm_irq_install(). But it would be cleaner to not have any checks for
>> driver private stuff outside of drivers.
>
> I hope my review above answers your question here. Patch, as-is:
>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Many thanks, pushed to drm-misc-next with the details addded to commit
message.

BR,
Jani.


>
>> ---
>>  drivers/gpu/drm/drm_irq.c | 4 ----
>>  1 file changed, 4 deletions(-)
>> 
>> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
>> index 03bce566a8c3..588be45abd7a 100644
>> --- a/drivers/gpu/drm/drm_irq.c
>> +++ b/drivers/gpu/drm/drm_irq.c
>> @@ -111,10 +111,6 @@ int drm_irq_install(struct drm_device *dev, int irq)
>>  	if (irq == 0)
>>  		return -EINVAL;
>>  
>> -	/* Driver must have been initialized */
>> -	if (!dev->dev_private)
>> -		return -EINVAL;
>> -
>>  	if (dev->irq_enabled)
>>  		return -EBUSY;
>>  	dev->irq_enabled = true;
>> -- 
>> 2.20.1
>> 

-- 
Jani Nikula, Intel Open Source Graphics Center
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