Re: [RFC 1/2] drm/i915: Engine discovery uAPI

From: Kenneth Graunke <kenneth@whitecape.org>
To: mesa-dev@lists.freedesktop.org
Cc: Oscar Mateo <oscar.mateo@intel.com>,
	Ben Widawsky <ben@bwidawsk.net>,
	Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
	Tvrtko Ursulin <tvrtko.ursulin@intel.com>,
	Intel-gfx@lists.freedesktop.org, "Rogozhkin,
	Dmitry V" <dmitry.v.rogozhkin@intel.com>,
	Tvrtko Ursulin <tursulin@ursulin.net>,
	Jon Bloomfield <jon.bloomfield@intel.com>,
	Daniel Vetter <daniel.vetter@intel.com>,
	intel-vaapi-media@lists.01.org, "Gong,
	Zhipeng" <zhipeng.gong@intel.com>
Subject: Re: [RFC 1/2] drm/i915: Engine discovery uAPI
Date: Tue, 18 Apr 2017 22:22:18 -0700	[thread overview]
Message-ID: <6570224.bZGvjJtkdM@eiger> (raw)
In-Reply-To: <20170418165615.27666-2-tvrtko.ursulin@linux.intel.com>

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On Tuesday, April 18, 2017 9:56:14 AM PDT Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> Engine discovery uAPI allows userspace to probe for engine
> configuration and features without needing to maintain the
> internal PCI id based database.

I don't understand why I would want to query the existence of engines
from the kernel.  As a userspace driver developer, I have to know how to
program the specific generation of hardware I'm on.  I better know what
engines that GPU has, or else there's no way I can competently program it.

In Mesa, we recently imported libdrm and deleted all the engine checks
(a460e1eb51406e5ca54abda42112bfb8523ff046).  All generations have an
RCS, Gen6+ has a separate BLT, and we don't need to use the others.
It's completely statically determinable with a simple check.  Runtime
checks make sense for optional things...but not "is there a 3D engine?".

Plus, even if you add this to the kernel, we still support back to 3.6
(and ChromeOS needs us to continue supporting 3.8), so we won't be able
to magically use the new uABI - we'd need to support both.  Which, if
the point is to delete code...we'd actually have /more/ code for a few
years.  Or, we could not use it...but then nobody would be testing it,
and if a bug creeps in...that pushes it back more years still.

Sorry :(

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