[Intel-gfx] About CTX_CONTEXT_CONTROL initialization in populate_lr_context() intel_lrc.c

[Dave Gordon]

On 09/02/15 15:05, Wang, Zhi A wrote:
> Hi Gurus:
> Forgive my junior HW knowledge, I just found that in execlist
> context initialization function populate_lr_context(), this line:
>   
>           reg_state[CTX_CONTEXT_CONTROL+1] =
>                           _MASKED_BIT_ENABLE((1<<3) | MI_RESTORE_INHIBIT);
>   
> It seems the "Inhibit Synchronous Context Switch" bit is not set
> here, so when HW is trying to wait some events, e.g. semaphore, according to
> Bspec, per my basic understanding, it will switch out current context
> with some reason bit set. Here comes my question, I think if this
> situation happen, should i915 remember this context and try to
> re-schedule it in a proper time, e.g. before submitting a new context
> until the context_completed bit in CSB is set? I don't find a register
> that will remember the context switched out by waiting event, so I think
> it should be i915 to handle this situation or just set "Inhibit
> Synchronous Context Switch" bit here?...

But that's exactly what it does already ... it sets bit 3, which is the
"Inhibit Synchronous Context Switch" bit you refer to.

What's wrong here is that it should use a #define for this bit, and for
the "Restore Inhibit" bit -- for which it's actually using a value
defined in a completely different context (no pun intended), namely the
bits in a MI_SET_CONTEXT *instruction*. It just happens to work because
they have the same numerical value. So, what we need is:

<intel_lrc.h>

#define RING_CONTEXT_CONTROL(ring)   	   ((ring)->mmio_base+0x244)
#define	  CTX_CTRL_INHIBIT_SYN_CTX_SWITCH	(1 << 3)
#define	  CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT	(1 << 0)

and then change the line you pointed out to use the correct symbolic names.

.Dave.

> Thanks,
> Zhi.
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