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

[Daniel, Thomas]

> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On Behalf
> Of Wang, Zhi A
> Sent: Tuesday, February 10, 2015 1:27 PM
> To: Wang, Zhi A; Intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] About CTX_CONTEXT_CONTROL initialization in
> populate_lr_context() intel_lrc.c
> 
> Ping. If someone can confirm it. I can submit a patch, but I'm concerned
> about introducing new register bit needs some internal process....
> 
> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On Behalf
> Of Wang, Zhi A
> Sent: Monday, February 09, 2015 11:05 PM
> To: Intel-gfx at lists.freedesktop.org
> Subject: [Intel-gfx] About CTX_CONTEXT_CONTROL initialization in
> populate_lr_context() intel_lrc.c
> 
> 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?...

I don't think any of these events will be generated when execlists are enabled.  I think there is a plan to use semaphores again, and a similar codepath will be needed to deal with context pre-emption when we support that, in order to re-submit pre-empted contexts.

Thomas.