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[98.38.17.99]) by smtp.googlemail.com with ESMTPSA id 5614622812f47-4673434a35csm4795987b6e.20.2026.03.13.06.21.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Mar 2026 06:21:50 -0700 (PDT) From: Jim Cromie To: airlied@gmail.com, simona@ffwll.ch, jbaron@akamai.com, gregkh@linuxfoundation.org Cc: Jim Cromie , mripard@kernel.org, tzimmermann@suse.de, maarten.lankhorst@linux.intel.com, jani.nikula@intel.com, ville.syrjala@linux.intel.com, christian.koenig@amd.com, matthew.auld@intel.com, arunpravin.paneerselvam@amd.com, louis.chauvet@bootlin.com, skhan@linuxfoundation.org, pmladek@suse.com, ukaszb@chromium.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org, amd-gfx@lists.freedesktop.org Subject: [PATCH v11 00/65] Fix DRM_USE_DYNAMIC_DEBUG=y Date: Fri, 13 Mar 2026 07:19:25 -0600 Message-ID: <20260313132103.2529746-1-jim.cromie@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit resend - apologies, faf'g with --cc-cover & to: lists The drm subsystem has *lots* of debug statements, in 11 categories $> ack '\w*_dbg' drivers/gpu/drm/ | wc 5532 29318 553806 $> ack 'DRM_DEBUG\w*' drivers/gpu/drm/ | wc 2208 12856 212035 All of these are bit-tests on __drm_debug, exposed to users as /sys/module/drm/parameters/debug. Many of these are done often, vblank is done ~100/sec for some displays. Over the uptime of many boxes, this is a lot of cpu cycles, on bits that are almost always off. Dynamic-debug excels at replacing such tests with NOOPS. Classmaps was devised to bring that 0-off-cost to drm's categories. Classmaps-v1 went into the kernel in Sept 2022, in 2 chunks: b7b4eebdba7b..6ea3bf466ac6 # core dyndbg changes 0406faf25fb1..ee7d633f2dfb # drm adoption Sadly DRM-CI found a regression during init with drm.debug=; the static-keys underneath the drm-dbgs in drm.ko got enabled, but those in drivers & helpers did not. Root Problem: DECLARE_DYNDBG_CLASSMAP defined the classmap, but it repeated use in both core and drivers violated a K&R rule "define once, refer afterwards". This flaw resulted in a regression; with drm.debug=0xFF boot arg, drm-core got enabled, but drivers,etc did not. So in feb 2023, this resulted in: commit bb2ff6c27bc9 ("drm: Disable dynamic debug as broken") This patchset replaces it with DYNDBG_CLASSMAP_DEFINE (invoked once in drm-core) and DYNDBG_CLASSMAP_USE (invoked repeatedly, in drivers & helpers). _DEFINE exports the classmap it creates (in drm.ko), other modules _USE the classmap. The _USE adds a record ref'g the _DEFINEd (& exported) classmap, in a 2nd __dyndbg_class_users section. So now at modprobe, dyndbg scans the new section after the 1st __dyndbg_class_maps section, follows the linkage to the _DEFINEr module, finds the (optional) kernel-param controlling the classmap, examines its drm.debug=, and applies it to the module being initialized. Savings effect: No real effort was made to quantify the savings; bit-tests are almost unmeasurable individually, and [1] DRM_USE_DYNAMIC_DEBUG=y purposely depends upon JUMP_LABEL to avoid its use where it cannot help. That said, Ive booted one box with drm.debug=0x1ff on the boot-line, with a script run by systemd to turn it off once boot completes. It issued ~25k messages in that time. Status-v11: Now rebased on drm-misc-next, to allow grinding on DRM-CI. https://gitlab.freedesktop.org/jim.cromie/kernel-drm-next-dd/-/pipelines/1622778 DRM-CI revealed 2 bugs, fixes now included: 1. drm_buddy had 32bit truncation error, causing unexpected pass 2. drm_printer_debug_fn disregarded drm.debug, spewing msgs, causing timeout. Also tuned vblank* to avoid drm_debug_enabled() bit-test. Probably should defer this until [1]=y is default. It isnt yet. Up to this point, classmaps-v2 has been stuck, straddled on the fence between drm and dynamic-debug; it needs a little help towards one side or the other. ISTM that drm is the actual user, and the acid-test of its proper function; the dyndbg selftest recaps the 2-module scenario, but this cannot finally prove anything absent drm. Verification: DRM-CI Pipeline #1622778 - KUnit (arm32, arm64, x86_64): All 621 tests PASSED. Specifically confirms the 64-bit truncation fixes in drm_buddy. - i915-CML: 370 tests PASSED, 0 FAIL. Confirms the quelling of the UART storm in shared print callbacks. - Overall: No regressions found in dyndbg or buddy allocator logic. Future: Lukas Bartosik has been patiently waiting for classmaps to land, so he can add dyndbg-to-ftrace functionality. This new feature will allow steering any/all drm.debug messages to a private ftrace buffer, and also to mix in other pr_debug messages. Other ftrace streams could also be sent there (possibly w/o code changes, tbd). It could be a big step towards a featureful crash-buffer. Ive also cobbled together a maple-tree based replacement for the __dyndbg_descriptors linker section; it copies the section contents into 3 maple trees, then drops the section in __init. By eliminating the redundant columns (module, file, function) it reduces the storage needed by 40%. Early measurements show no net runtime costs, maple trees make good furniture. Given that progress, I also hacked up a maple-tree based dynamic prefix cache. Currently the dynamic-prefix is re-written for every prefixed callsite; this can get expensive. By careful keying (flags on top, descriptor address underneath), a single prefix string can span an entire range (smallest of enabled module, file, function). That last one doesn't help DRM, cuz it doesnt use dynamic-prefixes, but with this optimization in place, it could be adapted to do so. Lastly, I have an RFC jump-label patchset that dramatically cuts IPIs, currently 1 per pr_debug (to ~1/140), by sorting the (existing) queue, rather than flushing it when a non-increasing patch-addr is queued. I'd like to move forward with all these, but I anticipate a "fix classmaps 1st" response. Jim Cromie (65): dyndbg: fix NULL ptr on i386 due to section alignment dyndbg: factor ddebug_match_desc out from ddebug_change dyndbg: add stub macro for DECLARE_DYNDBG_CLASSMAP docs/dyndbg: update examples \012 to \n docs/dyndbg: explain flags parse 1st test-dyndbg: fixup CLASSMAP usage error dyndbg: reword "class unknown," to "class:_UNKNOWN_" dyndbg: make ddebug_class_param union members same size dyndbg: drop NUM_TYPE_ARRAY dyndbg: tweak pr_fmt to avoid expansion conflicts dyndbg: reduce verbose/debug clutter dyndbg: refactor param_set_dyndbg_classes and below dyndbg: tighten fn-sig of ddebug_apply_class_bitmap dyndbg: replace classmap list with a vector dyndbg: macrofy a 2-index for-loop pattern dyndbg,module: make proper substructs in _ddebug_info dyndbg: hoist classmap-filter-by-modname up to ddebug_add_module dyndbg: move mod_name down from struct ddebug_table to _ddebug_info dyndbg-API: remove DD_CLASS_TYPE_(DISJOINT|LEVEL)_NAMES and code selftests-dyndbg: add a dynamic_debug run_tests target dyndbg: change __dynamic_func_call_cls* macros into expressions dyndbg-API: replace DECLARE_DYNDBG_CLASSMAP dyndbg: detect class_id reservation conflicts dyndbg: check DYNAMIC_DEBUG_CLASSMAP_DEFINE args at compile-time dyndbg-test: change do_prints testpoint to accept a loopct dyndbg-API: promote DYNAMIC_DEBUG_CLASSMAP_PARAM to API dyndbg: treat comma as a token separator dyndbg: split multi-query strings with % selftests-dyndbg: add test_mod_submod dyndbg: resolve "protection" of class'd pr_debug dyndbg: add DYNAMIC_DEBUG_CLASSMAP_USE_(dd_class_name, offset) dyndbg: Harden classmap and callsite validation docs/dyndbg: add classmap info to howto drm: use correct ccflags-y spelling drm-dyndbg: adapt drm core to use dyndbg classmaps-v2 drm-dyndbg: adapt DRM to invoke DYNAMIC_DEBUG_CLASSMAP_PARAM drm-print: modernize an archaic comment drm-print: fix config-dependent unused variable drm-dyndbg: DRM_CLASSMAP_USE in amdgpu driver drm-dyndbg: DRM_CLASSMAP_USE in i915 driver drm-dyndbg: DRM_CLASSMAP_USE in drm_crtc_helper drm-dyndbg: DRM_CLASSMAP_USE in drm_dp_helper drm-dyndbg: DRM_CLASSMAP_USE in nouveau drm-dyndbg: add DRM_CLASSMAP_USE to Xe driver drm-dyndbg: add DRM_CLASSMAP_USE to virtio_gpu drm-dyndbg: add DRM_CLASSMAP_USE to simpledrm drm-dyndbg: add DRM_CLASSMAP_USE to bochs drm-dyndbg: add DRM_CLASSMAP_USE to etnaviv drm-dyndbg: add DRM_CLASSMAP_USE to gma500 driver drm-dyndbg: add DRM_CLASSMAP_USE to radeon drm-dyndbg: add DRM_CLASSMAP_USE to vmwgfx driver drm-dyndbg: add DRM_CLASSMAP_USE to vkms driver drm-dyndbg: add DRM_CLASSMAP_USE to udl driver drm-dyndbg: add DRM_CLASSMAP_USE to mgag200 driver drm-dyndbg: add DRM_CLASSMAP_USE to the gud driver drm-dyndbg: add DRM_CLASSMAP_USE to the qxl driver drm-dyndbg: add DRM_CLASSMAP_USE to the drm_gem_shmem_helper driver accel: add -DDYNAMIC_DEBUG_MODULE to subdir-ccflags accel/ivpu: implement IVPU_DBG_* as a dyndbg classmap drm: restore CONFIG_DRM_USE_DYNAMIC_DEBUG un-BROKEN accel/ethosu: enable drm.debug control accel/rocket: enable drm.debug control drm_buddy: fix 64-bit truncation in power-of-2 rounding drm_print: fix drm_printer dynamic debug bypass drm_vblank: use dyndbg's static-key to avoid flag-check .../admin-guide/dynamic-debug-howto.rst | 184 +++- MAINTAINERS | 3 +- drivers/accel/Makefile | 7 +- drivers/accel/ethosu/ethosu_drv.c | 3 + drivers/accel/ivpu/ivpu_drv.c | 27 +- drivers/accel/ivpu/ivpu_drv.h | 45 +- drivers/accel/rocket/rocket_gem.c | 2 + drivers/gpu/buddy.c | 4 +- drivers/gpu/drm/Kconfig.debug | 1 - drivers/gpu/drm/Makefile | 3 +- drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c | 12 +- drivers/gpu/drm/display/drm_dp_helper.c | 12 +- drivers/gpu/drm/drm_crtc_helper.c | 12 +- drivers/gpu/drm/drm_gem_shmem_helper.c | 1 + drivers/gpu/drm/drm_print.c | 38 +- drivers/gpu/drm/drm_vblank.c | 10 +- drivers/gpu/drm/etnaviv/etnaviv_drv.c | 2 + drivers/gpu/drm/gma500/psb_drv.c | 2 + drivers/gpu/drm/gud/gud_drv.c | 2 + drivers/gpu/drm/i915/i915_params.c | 12 +- drivers/gpu/drm/mgag200/mgag200_drv.c | 2 + drivers/gpu/drm/nouveau/nouveau_drm.c | 12 +- drivers/gpu/drm/qxl/qxl_drv.c | 2 + drivers/gpu/drm/radeon/radeon_drv.c | 2 + drivers/gpu/drm/sysfb/simpledrm.c | 2 + drivers/gpu/drm/tiny/bochs.c | 2 + drivers/gpu/drm/udl/udl_main.c | 2 + drivers/gpu/drm/virtio/virtgpu_drv.c | 2 + drivers/gpu/drm/vkms/vkms_drv.c | 2 + drivers/gpu/drm/vmwgfx/vmwgfx_drv.c | 2 + drivers/gpu/drm/xe/xe_drm_client.c | 2 + include/asm-generic/vmlinux.lds.h | 19 +- include/drm/drm_print.h | 22 +- include/linux/dynamic_debug.h | 304 +++++-- kernel/module/main.c | 15 +- lib/Kconfig.debug | 24 +- lib/Makefile | 5 + lib/dynamic_debug.c | 814 ++++++++++++------ lib/test_dynamic_debug.c | 198 +++-- lib/test_dynamic_debug_submod.c | 21 + tools/testing/selftests/Makefile | 1 + .../testing/selftests/dynamic_debug/Makefile | 9 + tools/testing/selftests/dynamic_debug/config | 7 + .../dynamic_debug/dyndbg_selftest.sh | 373 ++++++++ 44 files changed, 1665 insertions(+), 561 deletions(-) create mode 100644 lib/test_dynamic_debug_submod.c create mode 100644 tools/testing/selftests/dynamic_debug/Makefile create mode 100644 tools/testing/selftests/dynamic_debug/config create mode 100755 tools/testing/selftests/dynamic_debug/dyndbg_selftest.sh -- 2.53.0