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

[Daniel Vetter]

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 at 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 at chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Signed-off-by: Jani Nikula <jani.nikula at 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 at ffwll.ch>

> ---
>  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
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch