linux-kernel - Re: [RFC PATCH 00/17] DRM_USE_DYNAMIC

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Date:   Fri, 13 Jan 2023 19:48:03 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     jim.cromie@...il.com
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        amd-gfx@...ts.freedesktop.org, intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, jani.nikula@...el.com,
        ville.syrjala@...ux.intel.com, seanpaul@...omium.org,
        robdclark@...il.com, jbaron@...mai.com, gregkh@...uxfoundation.org,
        daniel.vetter@...ll.ch
Subject: Re: [RFC PATCH 00/17] DRM_USE_DYNAMIC_DEBUG regression

On Fri, Jan 13, 2023 at 11:29:57AM -0700, jim.cromie@...il.com wrote:
> On Wed, Jan 11, 2023 at 4:09 PM Daniel Vetter <daniel@...ll.ch> wrote:
> >
> > On Mon, Dec 05, 2022 at 05:34:07PM -0700, Jim Cromie wrote:
> > > Hi everyone,
> > >
> > > DRM_USE_DYNAMIC_DEBUG=y has a regression on rc-*
> > >
> > > Regression is due to a chicken-egg problem loading modules; on
> > > `modprobe i915`, drm is loaded 1st, and drm.debug is set.  When
> > > drm_debug_enabled() tested __drm_debug at runtime, that just worked.
> > >
> > > But with DRM_USE_DYNAMIC_DEBUG=y, the runtime test is replaced with a
> > > post-load enablement of drm_dbg/dyndbg callsites (static-keys), via
> > > dyndbg's callback on __drm_debug.  Since all drm-drivers need drm.ko,
> > > it is loaded 1st, then drm.debug=X is applied, then drivers load, but
> > > too late for drm_dbgs to be enabled.
> > >
> > > STATUS
> > >
> > > For all-loadable drm,i915,amdgpu configs, it almost works, but
> > > propagating drm.debug to dependent modules doesnt actually apply,
> > > though the motions are there.  This is not the problem I want to chase
> > > here.
> > >
> > > The more basic trouble is:
> > >
> > > For builtin drm + helpers, things are broken pretty early; at the
> > > beginning of dynamic_debug_init().  As the ddebug_sanity() commit-msg
> > > describes in some detail, the records added by _USE fail to reference
> > > the struct ddebug_class_map created and exported by _DEFINE, but get
> > > separate addresses to "other" data that segv's when used as the
> > > expected pointer. FWIW, the pointer val starts with "revi".
> >
> > So I honestly have no idea here, linker stuff is way beyond where I have
> > clue. So what's the way forward here?
> >
> 
> Ive fixed this aspect.
> Unsurprisingly, it wasnt the linker :-}

Awesome!

> > The DEFINE/USE split does like the right thing to do at least from the
> > "how it's used in drivers" pov. But if we're just running circles not
> > quite getting there I dunno :-/
> > -Daniel
> >
> 
> Sending new rev next.
> I think its getting close.

Thanks a lot for keeping on pushing this.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch