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Date:   Thu, 12 Jan 2023 00:09:41 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Jim Cromie <jim.cromie@gmail.com>
Cc:     linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        amd-gfx@lists.freedesktop.org, intel-gvt-dev@lists.freedesktop.org,
        intel-gfx@lists.freedesktop.org, jani.nikula@intel.com,
        ville.syrjala@linux.intel.com, daniel.vetter@ffwll.ch,
        seanpaul@chromium.org, robdclark@gmail.com, jbaron@akamai.com,
        gregkh@linuxfoundation.org
Subject: Re: [RFC PATCH 00/17] DRM_USE_DYNAMIC_DEBUG regression
Message-ID: <Y79Btep8JnPKvuAp@phenom.ffwll.local>
Mail-Followup-To: Jim Cromie <jim.cromie@gmail.com>,
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References: <20221206003424.592078-1-jim.cromie@gmail.com>
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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?

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

> 
> OVERVIEW
> 
> DECLARE_DYNDBG_CLASSMAP is broken: it is one-size-fits-all-poorly.
> It muddles the distinction between a (single) definition, and multiple
> references.  Something exported should suffice.
> 
> The core of this patchset splits it into:
> 
> DYNDBG_CLASSMAP_DEFINE	used once per subsystem to define each classmap
> DYNDBG_CLASSMAP_USE	declare dependence on a DEFINEd classmap
> 
> This makes the weird coordinated-changes-by-identical-classmaps
> "feature" unnecessary; the DEFINE can export the var, and USE refers
> to the exported var.
> 
> So this patchset adds another section: __dyndbg_class_refs.
> 
> It is like __dyndbg_classes; it is scanned under ddebug_add_module(),
> and attached to each module's ddebug_table.  Once attached, it can be
> used like classes to validate and apply class FOO >control queries.
> 
> It also maps the class user -> definer explicitly, so that when the
> module is loaded, the section scan can find the kernel-param that is
> wired to dyndbg's kparam-callback, and apply its state-var, forex:
> __drm_debug to the just loaded helper/driver module.
> 
> Theres plenty to address Im sure.
> 
> Jim Cromie (17):
>   test-dyndbg: fixup CLASSMAP usage error
>   test-dyndbg: show that DEBUG enables prdbgs at compiletime
>   dyndbg: fix readback value on LEVEL_NAMES interfaces
>   dyndbg: replace classmap list with a vector
>   dyndbg: make ddebug_apply_class_bitmap more selective
>   dyndbg: dynamic_debug_init - use pointer inequality, not strcmp
>   dyndbg: drop NUM_TYPE_ARRAY
>   dyndbg: reduce verbose/debug clutter
>   dyndbg-API: replace DECLARE_DYNDBG_CLASSMAP with
>     DYNDBG_CLASSMAP(_DEFINE|_USE)
>   dyndbg-API: specialize DYNDBG_CLASSMAP_(DEFINE|USE)
>   dyndbg-API: DYNDBG_CLASSMAP_USE drop extra args
>   dyndbg-API: DYNDBG_CLASSMAP_DEFINE() improvements
>   drm_print: fix stale macro-name in comment
>   dyndbg: unwrap __ddebug_add_module inner function NOTYET
>   dyndbg: ddebug_sanity()
>   dyndbg: mess-w-dep-class
>   dyndbg: miss-on HACK
> 
>  drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c |  14 +-
>  drivers/gpu/drm/display/drm_dp_helper.c |  14 +-
>  drivers/gpu/drm/drm_crtc_helper.c       |  14 +-
>  drivers/gpu/drm/drm_print.c             |  22 +--
>  drivers/gpu/drm/i915/i915_params.c      |  14 +-
>  drivers/gpu/drm/nouveau/nouveau_drm.c   |  14 +-
>  include/asm-generic/vmlinux.lds.h       |   3 +
>  include/drm/drm_print.h                 |   6 +-
>  include/linux/dynamic_debug.h           |  57 ++++--
>  include/linux/map.h                     |  54 ++++++
>  kernel/module/main.c                    |   2 +
>  lib/dynamic_debug.c                     | 240 +++++++++++++++++++-----
>  lib/test_dynamic_debug.c                |  47 ++---
>  13 files changed, 344 insertions(+), 157 deletions(-)
>  create mode 100644 include/linux/map.h
> 
> -- 
> 2.38.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch