Re: [Dri-devel] question on lightweight vs heavyweight locking

[Keith W. <ke...@tu...>]

Philip Brown wrote:
> I've been reading
> http://dri.sourceforge.net/doc/hardware_locking_low_level.html
> 
> and the code, obviously... so I've seen references to the "lightweight"
> lock. ButI have yet to figure out why there is one.
> 
> The url document mentions that one supposedly exists, and that
> "the lightweight lock will not be designed to be shared between GLXContexts"
> and that
> "The lightweight lock will be stored in a user-space shared memory segment
>  that is available to all locking entities."
> 
> But it doesnt seem to spell out exactly what the lightweight lock
>  should be used for, or what the kernel's role in it should be.
> 
> Can I basically just ignore the "lightweight lock" at kernel level, and
> let the userspace code sort itself out?

I believe so, though in the linux drm, the kernel does make sure that when you 
grab the heavyweight lock, the lightweight lock reflects this.

> Also, as far as the heavyweight lock goes... It has been described as,
> "The hardware lock is obtained whenever the hardware is touched, and
>   holding the lock implies that no other entity in the system will touch the
>   hardware. "
>    [http://dri.sourceforge.net/doc/drm_low_level.html]
> 
> That being the case, I'm thinking of just
> having the heart of ioctl(DRM_IOCTL_LOCK) be a straight kernel mutex,
> since solaris supposedly handles mutex queueing fairly well, rather
> than implementating a whole separate queue mechanism.
> 
> Any comments on this?

I think that's reasonable.

Keith