[Intel-gfx] [PATCH] drm/i915: Initialize all contexts

[Chris Wilson]

On Fri, Feb 27, 2015 at 01:40:18PM +0000, Michel Thierry wrote:
> From: Ben Widawsky <benjamin.widawsky at intel.com>
> 
> The problem is we're going to switch to a new context, which could be
> the default context. The plan was to use restore inhibit, which would be
> fine, except if we are using dynamic page tables (which we will). If we
> use dynamic page tables and we don't load new page tables, the previous
> page tables might go away, and future operations will fault.
> 
> CTXA runs.
> switch to default, restore inhibit
> CTXA dies and has its address space taken away.
> Run CTXB, tries to save using the context A's address space - this
> fails.
> 
> The general solution is to make sure every context has it's own state,
> and its own address space. For cases when we must restore inhibit, first
> thing we do is load a valid address space. I thought this would be
> enough, but apparently there are references within the context itself
> which will refer to the old address space - therefore, we also must
> reinitialize.
> 
> It was tricky to track this down as we don't have much insight into what
> happens in a context save.
> 
> This is required for the next patch which enables dynamic page tables.

This sneaks in a major change that is not mentioned at all above.
Namely:

> @@ -744,7 +747,7 @@ static int do_switch(struct intel_engine_cs *ring,
>  		i915_gem_context_unreference(from);
>  	}
>  
> -	uninitialized = !to->legacy_hw_ctx.initialized && from == NULL;
> +	uninitialized = !to->legacy_hw_ctx.initialized;
>  	to->legacy_hw_ctx.initialized = true;

That has nothing to do with VM spaces, but with w/a that need to be
applied to the context image.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre