On Wed, Feb 02, 2022 at 10:26:46AM +0000, Tvrtko Ursulin wrote:
On 01/02/2022 17:28, Lucas De Marchi wrote:
On Tue, Feb 01, 2022 at 07:09:14PM +0200, Jani Nikula wrote:
On Tue, 01 Feb 2022, Lucas De Marchi <lucas.demarchi@xxxxxxxxx> wrote:
On Tue, Feb 01, 2022 at 11:15:31AM +0000, Tvrtko Ursulin wrote:
From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Quicky and dirty hack based on some old ideas. Thought maybe
the approach might
interest the Arm port guys. But with IS_GEN_RANGE removed easy
gains are not so
big so meh.. Maybe some more easy wins with IS_DISPLAY_VER but
I haven't looked
into that side.
3884664 449681 6720 4341065 423d49 i915.ko.tip
3599989 429034 6688 4035711 3d947f i915.ko.noigp
By these numbers probably it's hard to justify. Another thing to
consider
is that it's very common to have on the same system both
integrated and discrete - doing this would remove at compile time any
chance of driving the integrated one.
I guess the point was, the arm systems won't have integrated, and it's
anyway going to be a separate build.
so probably the focus and argument here should not be about size
reduction. From patch 1 I see:
+config DRM_I915_INTEGRATED_GPU_SUPPORT
+ bool "Support integrated GPUs"
+ default y
+ depends on DRM_I915
+ help
+ Include support for integrated GPUs.
If it's something that depends on arch rather than providing an
option in menuconfig, then I think it could be some interesting
investigation. However, I can't see how it would help with removing
some code paths in the driver (e.g. the clflush() calls we were talking
about in another patch series) since the code elimination would all
happen at link time.
Clflush class of problems is yet another orthogonal set of problems.
Yes, idea was that the Kconfig option would be selected by Arm, or
deselected by x86, whatever. But there is also a case for letting it
be user visible.
In general, I thought at least, we should look into not
building/deploying binary code for irrelevant hardware on Arm builds.
If that is clear and agreeable then I think the approach how to get
there is really multi-pronged.
1)
What you are partly doing with "clflush" type series. Make Arm
relevant code paths actually compile on Arm.
to be clear, the goal is to be more architecture independent
and this is being tested with arm64.
2a)
What I sent in this series - it's simple/easy dead code elimination
from a single compilation unit.
2b)
*If* we resurrected GRAPHICS_VER check where "ver" is part of the
macro, eg. not doing "if (GRAPHICS_VER <=> N)" but "if
(GRAPHICS_VERN)", or "if IS_GRAPHICS_VER(N, FOREVER)", then the same
approach would be more effective.
Because if N or range is the macro parameter, we can make it dead code
based on Kconfig.
yes I remember that. But when it was done the IS_GRAPHICS_VER() was
already not much used. I doubt the handful places it was used in would
change much of the overall picture we had at that point.
This is what I demonstrated few years ago by being able to compile out
~3rd of a driver when selecting only execlists platforms, AFAIR.
And which is why I was a bit unhappy this was getting removed not so
long ago.
3)
Complex step is putting LTO into use to allow dead code elimination
between compilation units. Think:
file1.c:
RUNTIME_INFO->needs_blah = IS_PLATFORM && blah
file2.c
if (RUNTIME_INFO->needs_blah)
..dead code eliminated by LTO only..
Few years ago Andi Kleen had a proof of concept of KBuild LTO but I
don't know what is the status of that. If LTO can be enabled then work
from steps 2a&b automatically works much much better.
4)
If LTO is not there yet maybe see how to slightly refactor the code so
that we can completely drop compilation units from the Makefile. Drop
files like gt/intel_ring_submission.c, or similar if we have or can
create candidates.
I think this is something we have to do and to go after. I think it's
more future-proof to organize the code to have a better separation of
platforms, instead of relying on the linker to do the dead code
elimination based on user selection.
I'm fine with compiling out stuff on i915 right now because "this can
only and will ever (as far as we know) work on x86". I'm more reluctant
about adding a build option though. For the same reasons Jani mentioned
and that it suddenly may open the doors for a matrix of configuration
(e.g. build with/without display, or rc6, or dsi, or
name-the-feature-you-dont-want).
Lucas De Marchi
Regards,
Tvrtko
