[PATCH v2 0/9] drm/print: add and use drm_debug_enabled()

[Eric Engestrom]

On Tuesday, 2019-10-01 14:03:55 +0300, Jani Nikula wrote:
> On Thu, 26 Sep 2019, Eric Engestrom <eric at engestrom.ch> wrote:
> > On Tuesday, 2019-09-24 15:58:56 +0300, Jani Nikula wrote:
> >> Hi all, v2 of [1], a little refactoring around drm_debug access to
> >> abstract it better. There shouldn't be any functional changes.
> >> 
> >> I'd appreciate acks for merging the lot via drm-misc. If there are any
> >> objections to that, we'll need to postpone the last patch until
> >> everything has been merged and converted in drm-next.
> >> 
> >> BR,
> >> Jani.
> >> 
> >> Cc: Eric Engestrom <eric.engestrom at intel.com>
> >> Cc: Alex Deucher <alexander.deucher at amd.com>
> >> Cc: Christian König <christian.koenig at amd.com>
> >> Cc: David (ChunMing) Zhou <David1.Zhou at amd.com>
> >> Cc: amd-gfx at lists.freedesktop.org
> >> Cc: Ben Skeggs <bskeggs at redhat.com>
> >> Cc: nouveau at lists.freedesktop.org
> >> Cc: Rob Clark <robdclark at gmail.com>
> >> Cc: Sean Paul <sean at poorly.run>
> >> Cc: linux-arm-msm at vger.kernel.org
> >> Cc: freedreno at lists.freedesktop.org
> >> Cc: Francisco Jerez <currojerez at riseup.net>
> >> Cc: Lucas Stach <l.stach at pengutronix.de>
> >> Cc: Russell King <linux+etnaviv at armlinux.org.uk>
> >> Cc: Christian Gmeiner <christian.gmeiner at gmail.com>
> >> Cc: etnaviv at lists.freedesktop.org
> >> 
> >> 
> >> [1] http://mid.mail-archive.com/cover.1568375189.git.jani.nikula@intel.com
> >> 
> >> Jani Nikula (9):
> >>   drm/print: move drm_debug variable to drm_print.[ch]
> >>   drm/print: add drm_debug_enabled()
> >>   drm/i915: use drm_debug_enabled() to check for debug categories
> >>   drm/print: rename drm_debug to __drm_debug to discourage use
> >
> > The above four patches are:
> > Reviewed-by: Eric Engestrom <eric at engestrom.ch>
> >
> > Did you check to make sure the `unlikely()` is propagated correctly
> > outside the `drm_debug_enabled()` call?
> 
> I did now.
> 
> Having drm_debug_enabled() as a macro vs. as an inline function does not
> seem to make a difference, so I think the inline is clearly preferrable.

Agreed :)

> 
> However, for example
> 
> 	unlikely(foo && drm_debug & DRM_UT_DP)
> 
> does produce code different from
> 
> 	(foo && drm_debug_enabled(DRM_UT_DP))
> 
> indicating that the unlikely() within drm_debug_enabled() does not
> propagate to the whole condition. It's possible to retain the same
> assembly output with
> 
> 	(unlikely(foo) && drm_debug_enabled(DRM_UT_DP))
> 
> but it's unclear to me whether this is really worth it, either
> readability or performance wise.
> 
> Thoughts?

That kind of code only happens 2 times, both in
drivers/gpu/drm/drm_dp_mst_topology.c (in patch 2/9), right?

I think your suggestion is the right thing to do here:

-   if (unlikely(ret && drm_debug & DRM_UT_DP)) {
+   if (unlikely(ret) && drm_debug_enabled(DRM_UT_DP)) {

It doesn't really cost much in readability (especially compared to what
it was before), and whether it's important performance wise I couldn't
tell, but I think it's best to keep the code optimised as it was before
unless there's a reason to drop it.

Lyude might know more since she wrote 2f015ec6eab69301fdcf5, if you want
to ping her?

> 
> BR,
> Jani.
> 
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center