Re: [RFC PATCH 3/3] i915: Expose PMU for Observation Architecture

[Chris Wilson <chris@chris-wilson.co.uk>]

On Wed, Oct 22, 2014 at 04:28:51PM +0100, Robert Bragg wrote:
> +	/* XXX: Not sure that this is really acceptable...
> +	 *
> +	 * i915_gem_context.c currently owns pinning/unpinning legacy
> +	 * context buffers and although that code has a
> +	 * get_context_alignment() func to handle a different
> +	 * constraint for gen6 we are assuming it's fixed for gen7
> +	 * here. Another option besides pinning here would be to
> +	 * instead hook into context switching and update the
> +	 * OACONTROL configuration on the fly.
> +	 */
> +	if (dev_priv->oa_pmu.specific_ctx) {
> +		struct intel_context *ctx = dev_priv->oa_pmu.specific_ctx;
> +		int ret;
> +
> +		ret = i915_gem_obj_ggtt_pin(ctx->legacy_hw_ctx.rcs_state,
> +					    4096, 0);

Right if you pin it here with a different alignment, when we try to pin
it with the required hw ctx alignment it will fail. Easiest way is to
record the ctx->legacy_hw_ctx.alignment and reuse that here.

> +		if (ret) {
> +			DRM_DEBUG_DRIVER("Couldn't pin %d\n", ret);
> +			ret = -EBUSY;

As an exercise, think of all the possible error values from pin() and
tell me why overriding that here is a bad, bad idea.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre