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

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Date:	Thu, 23 Oct 2014 08:47:39 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Robert Bragg <robert@...bynine.org>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Rob Clark <robdclark@...il.com>,
	Samuel Pitoiset <samuel.pitoiset@...il.com>,
	Ben Skeggs <bskeggs@...hat.com>
Subject: Re: [RFC PATCH 3/3] i915: Expose PMU for Observation Architecture

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
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