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Sat, 01 Aug 2020 09:06:14 -0700 (PDT) Date: Sat, 1 Aug 2020 16:06:03 +0000 From: Sidong Yang To: Daniel Vetter Cc: Melissa Wen , Haneen Mohammed , Rodrigo Siqueira , David Airlie , Rodrigo Siqueira , Trevor Woerner , Linux Kernel Mailing List , dri-devel , kernel-usp@googlegroups.com Subject: Re: [PATCH] drm/vkms: add missing drm_crtc_vblank_put to the get/put pair on flush Message-ID: <20200801160602.GA23836@realwakka> References: <20200725174514.esh4gqek6oddizvh@smtp.gmail.com> <20200730100901.daos4v6maqlcd2uj@smtp.gmail.com> <20200731090834.GR6419@phenom.ffwll.local> <20200731161314.GA21381@realwakka> <20200731164706.r62jjqu7nu3spyhx@smtp.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 31, 2020 at 08:33:25PM +0200, Daniel Vetter wrote: > On Fri, Jul 31, 2020 at 6:47 PM Melissa Wen wrote: > > > > On 07/31, Sidong Yang wrote: > > > On Fri, Jul 31, 2020 at 11:08:34AM +0200, daniel@ffwll.ch wrote: > > > > On Thu, Jul 30, 2020 at 07:09:25AM -0300, Melissa Wen wrote: > > > > > On 07/29, Daniel Vetter wrote: > > > > > > On Wed, Jul 29, 2020 at 9:09 PM Melissa Wen wrote: > > > > > > > > > > > > > > Melissa Wen > > > > > > > > > > > > > > On Sat, Jul 25, 2020 at 3:12 PM Daniel Vetter wrote: > > > > > > > > > > > > > > > > On Sat, Jul 25, 2020 at 7:45 PM Melissa Wen wrote: > > > > > > > > > > > > > > > > > > On 07/25, Daniel Vetter wrote: > > > > > > > > > > On Sat, Jul 25, 2020 at 5:12 AM Sidong Yang wrote: > > > > > > > > > > > > > > > > > > > > > > On Wed, Jul 22, 2020 at 05:17:05PM +0200, Daniel Vetter wrote: > > > > > > > > > > > > On Wed, Jul 22, 2020 at 4:06 PM Melissa Wen wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > On 07/22, daniel@ffwll.ch wrote: > > > > > > > > > > > > > > On Wed, Jul 22, 2020 at 08:04:11AM -0300, Melissa Wen wrote: > > > > > > > > > > > > > > > This patch adds a missing drm_crtc_vblank_put op to the pair > > > > > > > > > > > > > > > drm_crtc_vblank_get/put (inc/decrement counter to guarantee vblanks). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > It clears the execution of the following kms_cursor_crc subtests: > > > > > > > > > > > > > > > 1. pipe-A-cursor-[size,alpha-opaque, NxN-(on-screen, off-screen, sliding, > > > > > > > > > > > > > > > random, fast-moving])] - successful when running individually. > > > > > > > > > > > > > > > 2. pipe-A-cursor-dpms passes again > > > > > > > > > > > > > > > 3. pipe-A-cursor-suspend also passes > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The issue was initially tracked in the sequential execution of IGT > > > > > > > > > > > > > > > kms_cursor_crc subtest: when running the test sequence or one of its > > > > > > > > > > > > > > > subtests twice, the odd execs complete and the pairs get stuck in an > > > > > > > > > > > > > > > endless wait. In the IGT code, calling a wait_for_vblank before the start > > > > > > > > > > > > > > > of CRC capture prevented the busy-wait. But the problem persisted in the > > > > > > > > > > > > > > > pipe-A-cursor-dpms and -suspend subtests. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Checking the history, the pipe-A-cursor-dpms subtest was successful when, > > > > > > > > > > > > > > > in vkms_atomic_commit_tail, instead of using the flip_done op, it used > > > > > > > > > > > > > > > wait_for_vblanks. Another way to prevent blocking was wait_one_vblank when > > > > > > > > > > > > > > > enabling crtc. However, in both cases, pipe-A-cursor-suspend persisted > > > > > > > > > > > > > > > blocking in the 2nd start of CRC capture, which may indicate that > > > > > > > > > > > > > > > something got stuck in the step of CRC setup. Indeed, wait_one_vblank in > > > > > > > > > > > > > > > the crc setup was able to sync things and free all kms_cursor_crc > > > > > > > > > > > > > > > subtests. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Tracing and comparing a clean run with a blocked one: > > > > > > > > > > > > > > > - in a clean one, vkms_crtc_atomic_flush enables vblanks; > > > > > > > > > > > > > > > - when blocked, only in next op, vkms_crtc_atomic_enable, the vblanks > > > > > > > > > > > > > > > started. Moreover, a series of vkms_vblank_simulate flow out until > > > > > > > > > > > > > > > disabling vblanks. > > > > > > > > > > > > > > > Also watching the steps of vkms_crtc_atomic_flush, when the very first > > > > > > > > > > > > > > > drm_crtc_vblank_get returned an error, the subtest crashed. On the other > > > > > > > > > > > > > > > hand, when vblank_get succeeded, the subtest completed. Finally, checking > > > > > > > > > > > > > > > the flush steps: it increases counter to hold a vblank reference (get), > > > > > > > > > > > > > > > but there isn't a op to decreased it and release vblanks (put). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Cc: Daniel Vetter > > > > > > > > > > > > > > > Cc: Rodrigo Siqueira > > > > > > > > > > > > > > > Cc: Haneen Mohammed > > > > > > > > > > > > > > > Signed-off-by: Melissa Wen > > > > > > > > > > > > > > > --- > > > > > > > > > > > > > > > drivers/gpu/drm/vkms/vkms_crtc.c | 1 + > > > > > > > > > > > > > > > 1 file changed, 1 insertion(+) > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c > > > > > > > > > > > > > > > index ac85e17428f8..a99d6b4a92dd 100644 > > > > > > > > > > > > > > > --- a/drivers/gpu/drm/vkms/vkms_crtc.c > > > > > > > > > > > > > > > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c > > > > > > > > > > > > > > > @@ -246,6 +246,7 @@ static void vkms_crtc_atomic_flush(struct drm_crtc *crtc, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > spin_unlock(&crtc->dev->event_lock); > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > + drm_crtc_vblank_put(crtc); > > > > > > > > > > > > > > > > > > > > > > > > > > > > Uh so I reviewed this a bit more carefully now, and I dont think this is > > > > > > > > > > > > > > the correct bugfix. From the kerneldoc of drm_crtc_arm_vblank_event(): > > > > > > > > > > > > > > > > > > > > > > > > > > > > * Caller must hold a vblank reference for the event @e acquired by a > > > > > > > > > > > > > > * drm_crtc_vblank_get(), which will be dropped when the next vblank arrives. > > > > > > > > > > > > > > > > > > > > > > > > > > > > So when we call drm_crtc_arm_vblank_event then the vblank_put gets called > > > > > > > > > > > > > > for us. And that's the only case where we successfully acquired a vblank > > > > > > > > > > > > > > interrupt reference since on failure of drm_crtc_vblank_get (0 indicates > > > > > > > > > > > > > > success for that function, failure negative error number) we directly send > > > > > > > > > > > > > > out the event. > > > > > > > > > > > > > > > > > > > > > > > > > > > > So something else fishy is going on, and now I'm totally confused why this > > > > > > > > > > > > > > even happens. > > > > > > > > > > > > > > > > > > > > > > > > > > > > We also have a pile of WARN_ON checks in drm_crtc_vblank_put to make sure > > > > > > > > > > > > > > we don't underflow the refcount, so it's also not that I think (except if > > > > > > > > > > > > > > this patch creates more WARNING backtraces). > > > > > > > > > > > > > > > > > > > > > > > > > > > > But clearly it changes behaviour somehow ... can you try to figure out > > > > > > > > > > > > > > what changes? Maybe print out the vblank->refcount at various points in > > > > > > > > > > > > > > the driver, and maybe also trace when exactly the fake vkms vblank hrtimer > > > > > > > > > > > > > > is enabled/disabled ... > > > > > > > > > > > > > > > > > > > > > > > > > > :( > > > > > > > > > > > > > > > > > > > > > > > > > > I can check these, but I also have other suspicions. When I place the > > > > > > > > > > > > > drm_crct_vblank_put out of the if (at the end of flush), it not only solve > > > > > > > > > > > > > the issue of blocking on kms_cursor_crc, but also the WARN_ON on kms_flip > > > > > > > > > > > > > doesn't appear anymore (a total cleanup). Just after: > > > > > > > > > > > > > > > > > > > > > > > > > > vkms_output->composer_state = to_vkms_crtc_state(crtc->state); > > > > > > > > > > > > > > > > > > > > > > > > > > looks like there is something stuck around here. > > > > > > > > > > > > > > > > > > > > > > > > Hm do you have the full WARNING for this? Maybe this gives me an idea > > > > > > > > > > > > what's going wrong. > > > > > > > > > > > > > > > > > > > > > > > > > Besides, there is a lock at atomic_begin: > > > > > > > > > > > > > > > > > > > > > > > > > > /* This lock is held across the atomic commit to block vblank timer > > > > > > > > > > > > > * from scheduling vkms_composer_worker until the composer is updated > > > > > > > > > > > > > */ > > > > > > > > > > > > > spin_lock_irq(&vkms_output->lock); > > > > > > > > > > > > > > > > > > > > > > > > > > that seems to be released on atomic_flush and make me suspect something > > > > > > > > > > > > > missing on the composer update. > > > > > > > > > > > > > > > > > > > > > > > > atomic_begin/atomic_flush are symmetric functions an always called > > > > > > > > > > > > around all the plane updates. So having the spin_lock in _begin and > > > > > > > > > > > > the spin_unlock in _flush should be symmetric and correct. > > > > > > > > > > > > > > > > > > > > > > > > If you want to make sure, recompile with CONFIG_PROVE_LOCKING, which > > > > > > > > > > > > should immmediately give you a huge splat in dmesg if there's anything > > > > > > > > > > > > unbalanced with locking. > > > > > > > > > > > > > > > > > > > > > > > > > I'll check all these things and come back with news (hope) :) > > > > > > > > > > > > > > > > > > > > > > > > Have fun chasing stuff :-) > > > > > > > > > > > > > > > > > > > > > > > > Cheers, Daniel > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > > > > > > > > > Melissa > > > > > > > > > > > > > > > > > > > > > > > > > > > > I'm totally confused about what's going on here now. > > > > > > > > > > > > > > -Daniel > > > > > > > > > > > > > > > > > > > > > > Hi Daniel, Melissa. > > > > > > > > > > > I found something about this problem. > > > > > > > > > > > I traced vblank->refcount that it's important in the problem. > > > > > > > > > > > In normal case, first test run calls commit_tail() and enable vblank in > > > > > > > > > > > atomic_flush(). in drm_vblank_get(), it enable vblank when refcount was zero. > > > > > > > > > > > > > > > > > > > > > > in first test run, it disable crtc for cleanup test. drm_crtc_vblank_off() was > > > > > > > > > > > called by atomic_disable. in this function vblank's refcount was increased for > > > > > > > > > > > prevent subsequent drm_vblank_get() from re-enabling the vblank interrupt. > > > > > > > > > > > and refcount goes one not zero for next test run. > > > > > > > > > > > > > > > > > > > > > > and next test run, drm_vblank_get() was called but it didn't enable vblank > > > > > > > > > > > because refcount was already one. drm_crtc_vblank_on() was called in next. but > > > > > > > > > > > it didn't enable vblank but just increase refcount only. > > > > > > > > > > > > > > > > > > > > > > I think this is why this problem happen. don't know how to fix this correctly. > > > > > > > > > > > should we force to enable vblank after enabling crtc? > > > > > > > > > > > > > > > > > > > > Hm, between drm_crtc_vblank_off and drm_crtc_vblank_on > > > > > > > > > > drm_crtc_vblank_get should fail (and leave the refcount unchanged). > > > > > > > > > > It's convoluted logic, but the check for vblank->enabled should catch > > > > > > > > > > that and return -EINVAL for this case. Does that not happen? > > > > > > > > > > > > > > > > > > > > It would indeed explain the bug (I think, I've been wrong way too many > > > > > > > > > > times with this). > > > > > > > > > > -Daniel > > > > > > > > > > > > > > > > > > > Hi Daniel and Sidong, > > > > > > > > > > > > > > > > > > I don't know if it will be confusing, but I will try to explain in a > > > > > > > > > little more detail (and newbie way) what I saw in this behavior of the > > > > > > > > > refcount (similar to what Sidong evaluated). > > > > > > > > > > > > > > > > > > 1. Starting with the loading of vkms is: > > > > > > > > > In vkms_init: > > > > > > > > > After drm_vblank_init (refcount=0), it calls: > > > > > > > > > vkms_modeset_init > > > > > > > > > --> vkms_output_init > > > > > > > > > ----> drm_mode_config_reset > > > > > > > > > -------> vkms_atomic_crtc_reset > > > > > > > > > (even more inside)--> drm_crtc_vblank_reset that bumps the refcount to > > > > > > > > > prevent vblank_get to enable vblank (refcount=1) > > > > > > > > > > > > > > > > > > 2. So, when we start a subtest, vblank is still disabled and in > > > > > > > > > commit_tail, commit_planes triggers a atomic_begin/flush->vblank_get that > > > > > > > > > return -EINVAL because !vblank->enabled (refcount ends 1) and send_vblank; > > > > > > > > > however the test fails before atomic_enable decrements refcount to 0 and > > > > > > > > > reset timestamp. > > > > > > > > > ** This warning also appears in this very first running: > > > > > > > > > WARNING: CPU: 0 PID: 708 at drivers/gpu/drm/vkms/vkms_crtc.c:91 vkms_get_vblank_timestamp+0x41/0x50 [vkms] > > > > > > > > > > > > > > > > Hm yeah I guess that's something we should paper over a bit, but maybe > > > > > > > > the bugfix will take care of that. > > > > > > > > > > > > > > > > > In the end, this sequence modeset_disable -> atomic_begin -> > > > > > > > > > atomic_flush: refcount going from 0 to 1 and than drm_vblank_enable > > > > > > > > > prepares to everything going well in the next subtest (because > > > > > > > > > atomic_disable is not called). > > > > > > > > > > > > > > > > > > 3. It could be nice, but in the next subtest (with success), as refcount + > > > > > > > > > vblank_enabled ok, after doind its job, it calls > > > > > > > > > atomic_disable->vblank_off and here refcount ends 1 and vblank disabled > > > > > > > > > (the problem returns). > > > > > > > > > So, we have a kind of good turn and bad turn. > > > > > > > > > > > > > > > > > > I tried different things, but the only relatively stable result was > > > > > > > > > putting the sequence modeset_disable + modeset_enables + commit_planes in > > > > > > > > > the commit_tail. That didn't convince me and then I keep trying things. > > > > > > > > > > > > > > > > This actually sounds like a good idea, I had the same one. Doing it > > > > > > > > this way should also resolve the WARNING you've pointed out I think? > > > > > > > > > > > > > > Hi Daniel, > > > > > > > > > > > > > > My uncertainty in this idea was related to a subtest, the cursor-suspend. > > > > > > > Although the reordering solves most of the blocking in kms-cursor-crc, the > > > > > > > suspend subtest fails because when vkms suspends, it disables vblank, > > > > > > > and when it resumes, vblank is not enabled in time. In this subtest, there is > > > > > > > a pipe-crc-start and adding a igt_wait_for_vblank solves... but again, > > > > > > > I know it is not the real fix. > > > > > > > > > > > > > > Would be the case to develop a specific feature of suspend/resume in vkms? > > > > > > > I mean, something to enable vblank when resume. I am trying to figure out how > > > > > > > to develop it, but still without success. > > > > > > > > > > > > Hm since it's all software I expected that the hrtimer will simply > > > > > > continue to run as if nothing happened. For real hw we'd need to use > > > > > > drm_mode_config_helper_suspend/resume, but for vkms I dont think > > > > > > that's required. Is the vblank hrtimer not working after resume? Or is > > > > > > it simply reporting a garbage timestamp and that's why the testcase > > > > > > fails? > > > > > > > > > > The testcase fails for the same timeout in waiting the first crc > > > > > (already applying the change in the commit_tail sequence): > > > > > > > > > > (kms_cursor_crc:732) igt_aux-DEBUG: Test requirement passed: (power_dir = open("/sys/power", O_RDONLY)) >= 0 > > > > > (kms_cursor_crc:732) igt_aux-DEBUG: Test requirement passed: get_supported_suspend_states(power_dir) & (1 << state) > > > > > (kms_cursor_crc:732) igt_aux-DEBUG: Test requirement passed: test == SUSPEND_TEST_NONE || faccessat(power_dir, "pm_test", R_OK | W_OK, 0) == 0 > > > > > (kms_cursor_crc:732) igt_aux-DEBUG: Test requirement passed: !(state == SUSPEND_STATE_DISK && !intel_get_total_swap_mb()) > > > > > (kms_cursor_crc:732) igt_aux-DEBUG: Test requirement passed: ret == 0 > > > > > (kms_cursor_crc:732) igt_core-INFO: [cmd] rtcwake: wakeup from "mem" using /dev/rtc0 at Thu Jul 30 09:23:59 2020 > > > > > (kms_cursor_crc:732) igt_debugfs-DEBUG: Opening debugfs directory '/sys/kernel/debug/dri/0' > > > > > (kms_cursor_crc:732) igt_core-INFO: Timed out: Opening crc fd, and poll for first CRC. > > > > > > > > > > What I could check was, when suspend, vblanks are disabled (calling > > > > > vkms_disable_vblank), and when resume, the testcase fails and only after > > > > > the failure vblanks are enabled (vkms_enable_vblank) and > > > > > hrtimer_init/starts. > > > > > > > > Hm, what is disabling the vblank there? Can you grab a full backtrace for > > > > that? I have no idea why that's even happening ... > > > > > > > > > If I "force" enabling vblanks via testcase (adding a > > > > > igt_wait_for_vblank before igt_pipe_crc_start), things work fine. > > > > > This is why I thought about anticipating the restarting of activities by > > > > > placing a vblank "wakeup" in a resume function. Or perhaps prevent vblank > > > > > from being disabled when suspended, since that last attempt to place a > > > > > vblank_put at the end of the flush made this test case (suspend) work, > > > > > because it prevented the vblank from being disabled. > > > > > > > > > > This failure in suspend subtest is so closer to the previous ones, that I > > > > > was unsure if my attempt in fix by reordering commit_tail would be enough. > > > > > But maybe they are different situations that deserve different treats. > > > > > Do you think restarting a vblank in resume can make sense for vkms? > > > > > > > > tbh I'm just really confused what's going on :-/ > > > > > > Hi Daniel, Mellisa. > > > I made up the situation Melissa said, and thought about what's going on now. > > > What pipe-A-cursor-suspend different from size-change is that it has some > > > suspend command just before disabling cursor in test. usually vblank is enabled > > > by drm_vblank_get() and it's handled by vblank interrupt (vkms_vblank_simulate > > > in vkms). by calling drm_crtc_handle_vblank(), drm_vblank_put() is called and it > > > disable vblank with disable timer. the timer will disable vblank after 5 seconds > > > (drm_vblank_offdelay) later in default. that time is enough to execute crc > > > command in simple size-change test. but in suspending situation, if suspend and > > > wakup command takes more than 5 seconds, vblank will disabled and also crc > > > command is failed from polling crc file. the test are passed if I make my > > > environment wake up earlier. > > Oh, that makes tons of sense and explains (I think at least) all the > bugs seen around vblank. Nice debugging! > > btw you can also set offdelay=0, then it should never disable. > > > Oh, nice! I tested what you say extending the offdelay, and the test goes well. > > But what would be the right way to fix? > > Well vkms crc code needs a working vblank, so when we enable crc we > need to call drm_crtc_vblank_get, and when we disable crc, we need to > call drm_crtc_vblank_put. That should ensure that vblanks are always > happening, and crcs are captured no matter how much time (e.g. > suspend/resume) passes meanwhile. On real hw usually the crc is always > generated as long as the crtc is on, and don't need the vblank > interrupt enabled. But vkms is all fake, so works a bit different. I think it's good. I tested some code that insert drm_crtc_vblank_get/put in vkms_set_crc_source and it make successful with igt test. with this modified code, when user code opens crc, vkms_set_crc_source is called with enabling crc. On the contrary, when closing crc, set_crc_source is called with disabling src_name and composer_enabled goes false. -Sidong > > Cheers, Daniel > > > > > Melissa > > > > > In the same way, if there is the code delaying crc command in igt test, it make > > > same problem even if it's in simple size-change. > > > > > > -Sidong > > > > > > > -Daniel > > > > > > > > > > > > > > Melissa > > > > > > > > > > > > Not sure how to wire it up for fake drivers like vkms, but maybe doing > > > > > > the suspend/resume like for real drivers helps. I think ideally we'd > > > > > > try to attach a platform driver to our platform device we create (but > > > > > > not sure how to do that). > > > > > > -Daniel > > > > > > > > > > > > > > > > > > > > Melissa > > > > > > > > > > > > > > > > But I'm still wondering why after step 3 we don't get -EINVAL from > > > > > > > > vblank_get() - after vblank_off() vblank->enabled should be false > > > > > > > > again, getting us back to the same state as after 1. Is that not > > > > > > > > happening? > > > > > > > > -Daniel > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > -Sidong > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > crtc->state->event = NULL; > > > > > > > > > > > > > > > } > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > > > > > > 2.27.0 > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > > > > > Daniel Vetter > > > > > > > > > > > > > > Software Engineer, Intel Corporation > > > > > > > > > > > > > > http://blog.ffwll.ch > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > > > Daniel Vetter > > > > > > > > > > > > Software Engineer, Intel Corporation > > > > > > > > > > > > http://blog.ffwll.ch > > > > > > > > > > > > _______________________________________________ > > > > > > > > > > > > dri-devel mailing list > > > > > > > > > > > > dri-devel@lists.freedesktop.org > > > > > > > > > > > > https://lists.freedesktop.org/mailman/listinfo/dri-devel > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > Daniel Vetter > > > > > > > > > > Software Engineer, Intel Corporation > > > > > > > > > > http://blog.ffwll.ch > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > Daniel Vetter > > > > > > > > Software Engineer, Intel Corporation > > > > > > > > http://blog.ffwll.ch > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > Daniel Vetter > > > > > > Software Engineer, Intel Corporation > > > > > > http://blog.ffwll.ch > > > > > > > > -- > > > > Daniel Vetter > > > > Software Engineer, Intel Corporation > > > > http://blog.ffwll.ch > > > > -- > Daniel Vetter > Software Engineer, Intel Corporation > http://blog.ffwll.ch