[Intel-gfx] [PATCH 06/43] drm/i915: protect force_wake_(get|put) with the gt_lock

[Keith Packard]

On Wed, 14 Dec 2011 13:57:03 +0100, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:

> The problem this patch solves is that the forcewake accounting
> necessary for register reads is protected by dev->struct_mutex. But the
> hangcheck and error_capture code need to access registers without
> grabbing this mutex because we hold it while waiting for the gpu.
> So a new lock is required. Because currently the error_state capture
> is called from the error irq handler and the hangcheck code runs from
> a timer, it needs to be an irqsafe spinlock (note that the registers
> used by the irq handler (neglecting the error handling part) only uses
> registers that don't need the forcewake dance).

I think this description is wrong -- the only difference between using
atomic objects and using a spinlock is that with the spinlock the call
to ->force_wake_get is correctly serialized so that no register access
can occur without the chip being awoken. Without a spinlock, a second
thread can pass right through gen6_gt_force_wake_get and then go touch
registers while the first thread is busy waking the chip up.

-- 
keith.packard at intel.com
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