Date:   Wed, 28 Jun 2017 12:16:17 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Guneshwor Singh <guneshwor.o.singh@...el.com>,
        Hardik T Shah <hardik.t.shah@...el.com>,
        Libin Yang <libin.yang@...el.com>,
        "Subhransu S. Prusty" <subhransu.s.prusty@...el.com>,
        Vinod Koul <vinod.koul@...el.com>,
        Julia Lawall <Julia.Lawall@...6.fr>,
        Jaroslav Kysela <perex@...ex.cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] BUG-REPORT: snd-hda: hacked-together EPROBE_DEFER support

On Mon, 26 Jun 2017 19:54:49 +0200,
Daniel Vetter wrote:
> 
> On Mon, Jun 26, 2017 at 7:47 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > On Mon, 26 Jun 2017 18:16:30 +0200,
> > Daniel Vetter wrote:
> >>
> >> On Wed, Jun 21, 2017 at 05:30:10PM +0200, Takashi Iwai wrote:
> >> > On Wed, 21 Jun 2017 17:23:57 +0200,
> >> > Chris Wilson wrote:
> >> > >
> >> > > Quoting Daniel Vetter (2017-06-21 16:08:54)
> >> > > > So back when the i915 power well support landed in
> >> > > >
> >> > > > commit 99a2008d0b32d72dfc2a54e7be1eb698dd2e3bd6
> >> > > > Author: Wang Xingchao <xingchao.wang@...ux.intel.com>
> >> > > > Date:   Thu May 30 22:07:10 2013 +0800
> >> > > >
> >> > > >     ALSA: hda - Add power-welll support for haswell HDA
> >> > > >
> >> > > > the logic to handle the cross-module depencies was hand-rolled using a
> >> > > > async work item, and that just doesn't work.
> >> > > >
> >> > > > The correct way to handle cross-module deps is either:
> >> > > > - request_module + failing when the other module isn't there
> >> > > >
> >> > > > OR
> >> > > >
> >> > > > - failing the module load with EPROBE_DEFER.
> >> > > >
> >> > > > You can't mix them, if you do then the entire load path just
> >> > > > busy-spins blowing through cpu cycles forever with no way to stop
> >> > > > this.
> >> > > >
> >> > > > snd-hda-intel does mix it, because the hda codec drivers are loaded
> >> > > > using request_module, but the i915 depency is handled using
> >> > > > PROBE_DEFER (or well, should be, but I haven't found any code at all).
> >> > > > This is a major pain when trying to debug i915 load failures.
> >> > > >
> >> > > > This patch here is a horrible hackish attempt at somewhat correctly
> >> > > > wriing EPROBE_DEFER through. Stuff that's missing:
> >> > > > - Check all the other places where load errors are conveniently
> >> > > >   dropped on the floor.
> >> > > > - Also fix up the firmware_cb path.
> >> > > > - Drop the debug noise I've left in to make it clear this isn't
> >> > > >   anything for merging.
> >> > >
> >> > > This tames "hdaudio hdaudioC0D0: Unable to bind the codec" which was
> >> > > continuously spewing previously, and now the system is usable again.
> >> >
> >> > Could you give a failing scenario?  I'm not opposing to the suggested
> >> > solution, we need to fix the mess in anyway, but I just would like to
> >> > know how to trigger the problem easily.
> >>
> >> Disable i915 loading e.g. with i915.modeset=0. Watch how snd-hda*
> >> collective blow through 100% of the cpu time spewing into dmesg (and make
> >> the system completely unuseable for kernel work because you can't find
> >> your own debug printk anymore).
> >
> > Ah, that's the case we discussed in the past.  We know that it's
> > problematic for component binding, but we're ignoring this scenario
> > because it's supposed to be no real use-case but only for some
> > temporary workarounds.
> >
> > We had some bigger-hammer patchset, but it didn't justify for the
> > further development of the reasoning above.
> >
> >> This is on a snb, where we don't even need the cross-module stuff ... But
> >> I think it goes sideways in other cases too, if you simply build but don't
> >> load i915. So every time an i915 breaks module load things become real
> >> painful.
> >
> > Even on SNB, we still need i915 for the HDMI/DP ELD notification.  The
> > hardware inquiry over HD-audio verb was so unstable, so we rather take
> > a path directly inquiring to the gfx driver.
> 
> Ah right, forgot about that.
> 
> >> Unfortunately the patch is a bit too big for our fixup branch in drm-tip,
> >> so plan B would be to stop building snd-hda (which will make the intel
> >> audio team unhappy, but mea culpa if they don't fix this mess).
> >
> > OK, let me think and take a look for older patchset, too.
> 
> Yeah would be great if we can somehow address this, preferrably using
> EPROBE_DEFER or something else that's standard. At least the component
> stuff really doesn't work without wiring EPROBE_DEFER through.

Now I took a closer look, and this appears rather like a brown paper
bag bug, not about the deferred probe or module dependency.
The fix patch is below.  Could you check whether it works?


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai@...e.de>
Subject: [PATCH] ALSA: hda - Fix endless loop of codec configure

azx_codec_configure() loops over the codecs found on the given
controller via a linked list.  The code used to work in the past, but
in the current version, this may lead to an endless loop when a codec
binding returns an error.

The culprit is that the snd_hda_codec_configure() unregisters the
device upon error, and this eventually deletes the given codec object
from the bus.  Since the list is initialized via list_del_init(), the
next object points to the same device itself.  This behavior change
was introduced at splitting the HD-audio code code, and forgotten to
adapt it here.

For fixing this bug, just use a *_safe() version of list iteration.

Fixes: d068ebc25e6e ("ALSA: hda - Move some codes up to hdac_bus struct")
Reported-by: Daniel Vetter <daniel.vetter@...ll.ch>
Cc: <stable@...r.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
---
 sound/pci/hda/hda_codec.h      | 2 ++
 sound/pci/hda/hda_controller.c | 8 ++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/sound/pci/hda/hda_codec.h b/sound/pci/hda/hda_codec.h
index d6fb2d5d01a7..60ce1cfc300f 100644
--- a/sound/pci/hda/hda_codec.h
+++ b/sound/pci/hda/hda_codec.h
@@ -295,6 +295,8 @@ struct hda_codec {
 
 #define list_for_each_codec(c, bus) \
 	list_for_each_entry(c, &(bus)->core.codec_list, core.list)
+#define list_for_each_codec_safe(c, n, bus)				\
+	list_for_each_entry_safe(c, n, &(bus)->core.codec_list, core.list)
 
 /* snd_hda_codec_read/write optional flags */
 #define HDA_RW_NO_RESPONSE_FALLBACK	(1 << 0)
diff --git a/sound/pci/hda/hda_controller.c b/sound/pci/hda/hda_controller.c
index 3715a5725613..1c60beb5b70a 100644
--- a/sound/pci/hda/hda_controller.c
+++ b/sound/pci/hda/hda_controller.c
@@ -1337,8 +1337,12 @@ EXPORT_SYMBOL_GPL(azx_probe_codecs);
 /* configure each codec instance */
 int azx_codec_configure(struct azx *chip)
 {
-	struct hda_codec *codec;
-	list_for_each_codec(codec, &chip->bus) {
+	struct hda_codec *codec, *next;
+
+	/* use _safe version here since snd_hda_codec_configure() deregisters
+	 * the device upon error and deletes itself from the bus list.
+	 */
+	list_for_each_codec_safe(codec, next, &chip->bus) {
 		snd_hda_codec_configure(codec);
 	}
 	return 0;
-- 
2.13.2

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