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

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Date:   Fri, 13 Jan 2023 11:29:57 -0700
From:   jim.cromie@...il.com
To:     Jim Cromie <jim.cromie@...il.com>, 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
Cc:     daniel.vetter@...ll.ch
Subject: Re: [RFC PATCH 00/17] DRM_USE_DYNAMIC_DEBUG regression

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 :-}

> 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.