Messages in this thread | | | Date | Mon, 18 Mar 2019 16:36:11 -0300 | From | Rodrigo Siqueira <> | Subject | Re: [PATCH] drm/vkms: Solve bug on kms_crc_cursor tests |
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On 03/15, Ville Syrjälä wrote: > On Fri, Mar 15, 2019 at 08:51:57AM -0300, Rodrigo Siqueira wrote: > > On 03/11, Ville Syrjälä wrote: > > > On Sun, Mar 10, 2019 at 05:35:05PM -0300, Rodrigo Siqueira wrote: > > > > On 03/01, Ville Syrjälä wrote: > > > > > On Fri, Mar 01, 2019 at 03:35:35PM -0300, Shayenne Moura wrote: > > > > > > Em sex, 1 de mar de 2019 às 12:26, Ville Syrjälä > > > > > > <ville.syrjala@linux.intel.com> escreveu: > > > > > > > > > > > > > > On Fri, Mar 01, 2019 at 11:55:11AM -0300, Shayenne Moura wrote: > > > > > > > > Em qui, 28 de fev de 2019 às 11:03, Ville Syrjälä > > > > > > > > <ville.syrjala@linux.intel.com> escreveu: > > > > > > > > > > > > > > > > > > On Thu, Feb 28, 2019 at 11:11:07AM +0100, Daniel Vetter wrote: > > > > > > > > > > On Mon, Feb 25, 2019 at 11:26:06AM -0300, Shayenne Moura wrote: > > > > > > > > > > > vkms_crc_work_handle needs the value of the actual frame to > > > > > > > > > > > schedule the workqueue that calls periodically the vblank > > > > > > > > > > > handler and the destroy state functions. However, the frame > > > > > > > > > > > value returned from vkms_vblank_simulate is updated and > > > > > > > > > > > diminished in vblank_get_timestamp because it is not in a > > > > > > > > > > > vblank interrupt, and return an inaccurate value. > > > > > > > > > > > > > > > > > > > > > > Solve this getting the actual vblank frame directly from the > > > > > > > > > > > vblank->count inside the `struct drm_crtc`, instead of using > > > > > > > > > > > the `drm_accurate_vblank_count` function. > > > > > > > > > > > > > > > > > > > > > > Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com> > > > > > > > > > > > > > > > > > > > > Sorry for the delay, I'm a bit swamped right now :-/ > > > > > > > > > > > > > > > > > > > > Debug work you're doing here is really impressive! But I have no idea > > > > > > > > > > what's going on. It doesn't look like it's just papering over a bug (like > > > > > > > > > > the in_vblank_irq check we've discussed on irc), but I also have no idea > > > > > > > > > > why it works. > > > > > > > > > > > > > > > > > > > > I'll pull in Ville, he understands this better than me. > > > > > > > > > > > > > > > > > > It's not entirely clear what we're trying to fix. From what I can see > > > > > > > > > the crc work seems to be in no way synchronized with page flips, so > > > > > > > > > I'm not sure how all this is really supposed to work. > > > > > > > > > > > > > > > > > > > > > > > > > Hi, Ville! > > > > > > > > > > > > > > > > Thank you for the review! :) > > > > > > > > > > > > > > > > I do not understand well what crc code is doing, but the issue that I found > > > > > > > > is related to the vblank timestamp and frame count. > > > > > > > > > > > > > > > > When vkms handles the crc_cursor it uses the start frame and end frame > > > > > > > > values to verify if it needs to call the function 'drm_crtc_add_crc_entry()' > > > > > > > > for each frame. > > > > > > > > > > > > > > > > However, when getting the frame count, the code is calling the function > > > > > > > > drm_update_vblank_count(dev, pipe, false) and, because of the 'false', > > > > > > > > subtracting the actual vblank timestamp (consequently, the frame count > > > > > > > > value), causing conflicts. > > > > > > > > > > > > > > The in_vblank_irq behavour looks sane to me. What are these conflicts? > > > > > > > > > > > > > > > > > > > The entire history was: > > > > > > - I sent the patch with bugfix for vblank extra frame. The patch changed > > > > > > our function vkms_get_vblank_timestamp() to look like this: > > > > > > > > > > > > bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe, > > > > > > int *max_error, ktime_t *vblank_time, > > > > > > bool in_vblank_irq) > > > > > > { > > > > > > struct vkms_device *vkmsdev = drm_device_to_vkms_device(dev); > > > > > > struct vkms_output *output = &vkmsdev->output; > > > > > > > > > > > > *vblank_time = output->vblank_hrtimer.node.expires; > > > > > > > > > > > > + if (!in_vblank_irq) > > > > > > + *vblank_time -= output->period_ns; > > > > > > > > > > > > return true; > > > > > > } > > > > > > > > > > > > - This patch solve the issue that I was looking for (extra vblank > > > > > > frames on kms_flip). > > > > > > > > > > > > - However, kms_cursor_crc tests, which passed before my patch, started to fail. > > > > > > > > > > > > - Debugging them, I found that the problem was inside of function > > > > > > `vkms_vblank_simulate()` > > > > > > when it was handling the crc_enabled (inside if (state && output->crc_enabled)) > > > > > > and inside the function vkms_crc_work_handle() too. > > > > > > > > > > > > - Following the steps: > > > > > > 1. Inside vkms_vblank_simulate() we call drm_crtc_accurate_vblank_count() > > > > > > 2. In its turn, drm_crtc_accurate_vblank_count() calls the function > > > > > > drm_update_vblank_count(dev, pipe, false). /* This false is default */ > > > > > > 3. Finally, the “false” used in drm_update_vblank_count(), will be > > > > > > passed to vkms_get_vblank_timestamp() and the condition “if > > > > > > (!in_vblank_irq)” will be executed multiple times (we don’t want it). > > > > > > > > > > > > - Inside vkms_crc, the issue is that the returned frame value change for > > > > > > every call of drm_crtc_accurate_vblank_count() because > > > > > > in_vblank_irq == false. > > > > > > > > Hi Ville, > > > > > > > > > OK. So why is it changing? AFAICS it should not change unless the > > > > > timer was moved forward in between the calls. > > > > > > > > Yes Ville, you’re right. We have to update it only when the function > > > > vkms_vblank_simulate() is invoked (timer move forward), and FWIU we do > > > > it when we call drm_crtc_handle_vblank(). However, the current > > > > implementation of vkms, has a call to drm_crtc_accurate_vblank_count() > > > > inside the vkms_vblank_simulate() which is a problem because it also > > > > update the vblank value. FWIU, this patch fixes this issue by taking the > > > > count value in the data struct instead of call > > > > drm_crtc_accurate_vblank_count() which will avoid the extra update. > > > > > > But why does that extra update change the vblank count? > > > > Hi, > > > > I think that I’m failing to explain the issue and the solution in this > > patch, sorry for that... and I apologize in advance for the lengthy > > email. > > > > In this sense, allow me to elaborate the whole history in ‘chronological > > sections’: > > > > 1. About the ‘fake’ Vblank inside VKMS > > > > Since VKMS is a virtual driver, we do not have real Vblank, and we > > simulate it by using hrtimers. I registered the function > > vkms_vblank_simulate() as a callback inside hrtimer; in this function, > > we do some stuff that can be summarized in the sequence below: > > > > I) drm_crtc_handle_vblank() > > II) Check if crc is enabled or not > > III) Schedule the next vblank with hrtimer_forward_now() > > > > 2. Override the default behaviour for get_vblank_timestamp() > > > > As I told before, everytime that vkms_vblank_simulate() is invoked the > > function drm_crtc_handle_vblank() will be requested. For us, the > > important thing about drm_crtc_handle_vblank() is the sequence: > > > > a) drm_crtc_handle_vblank() > > b) drm_handle_vblank() > > c) drm_update_vblank_count() > > d) drm_get_last_vbltimestamp() > > e) get_vblank_timestamp() > > > > Since VKMS is a virtual driver, we cannot rely on the default > > implementation of get_vblank_timestamp(). In this context, we > > implemented our wrapper named vkms_get_vblank_timestamp(), and we have > > to return the expires time from hrtimer to the userspace. In my first > > implementation of this function, I had something like: > > > > bool vkms_get_vblank_timestamp(struct drm_device *dev, unsigned int pipe, > > int *max_error, ktime_t *vblank_time, > > bool in_vblank_irq) > > { > > [..] > > *vblank_time = output->vblank_hrtimer.node.expires; > > return true; > > } > > > > Around 80% of the kms_flip tests were passing with this code. However, > > we had some tests that break due to an extra vblank. > > > > 3. The extra vblank bug > > > > Shayenne and Daniel figured out that the extra vblank came from the > > function drm_crtc_arm_vblank_event() invoked inside the > > vkms_crtc_atomic_flush(). If we take a look inside > > drm_crtc_arm_vblank_event(), we can find: > > > > void drm_crtc_arm_vblank_event([..]) > > { > > [..] > > e->sequence = drm_crtc_accurate_vblank_count(crtc) + 1; > > [..] > > } > > > > This function increment the sequence by one (there is a detailed > > explanation about it in the documentation of this function). > > > > 4. The bug fix > > > > After inspecting the code, we identified the following sequence when the > > function drm_crtc_arm_vblank_event() is invoked: > > > > a) drm_crtc_arm_vblank_event() > > b) drm_crtc_accurate_vblank_count() > > c) drm_update_vblank_count(dev, pipe, false) > > > > Noticed that drm_update_vblank_count() is invoked again, but now with > > ‘false’ in the in_vblank_irq because it does not have to report anything > > for userspace or something like that. With this in mind, remember that > > drm_update_vblank_count() will call get_vblank_timestamp(); with my > > first implementation of vkms_get_vblank_timestamp() (see Section 2 for > > recap) we will update the vblank_time with the wrong information. After > > understanding all of these details and the meaning of the parameter > > in_vblank_irq, the following fix was added: > > > > bool vkms_get_vblank_timestamp([..]) { > > [..] > > if (!in_vblank_irq) > > *vblank_time -= output->period_ns; > > [..] > > } > > > > With this validation, we can solve the problem of the extra Vblank added > > by drm_crtc_arm_vblank_event(). Afterward, all the kms_flip test started > > to pass. > > > > 5. The new bug on CRC operation > > > > As a side effect of the above change, the CRC tests start to failing. > > The question was, why we break the CRC test with that fix? > > > > We start hunting the problem, and the first thing to look is the > > function vkms_vblank_simulate(). Inside this function, we have this > > code: > > > > vkms_vblank_simulate([..]) { > > [..] > > if (state && output->crc_enabled) { > > u64 frame = drm_crtc_accurate_vblank_count(crtc); > > [..] > > } > > > > The output->crc_enabled is a module parameter that enables CRC > > computation which is required for CRC tests, but not for kms_flip. > > Additionally, notice that we save the value of the current frame because > > it is necessary to use it on vkms_crc_work_handle(). The only thing that > > we need to know about vkms_crc_work_handle() is the fact we call > > drm_crtc_add_crc_entry() which requires the frame number, and we also > > use it to check if the frame was updated or not. > > > > As I said in Section 2, drm_crtc_accurate_vblank_count() calls > > drm_update_vblank_count() with false parameter in in_vblank_irq. Due to > > the last change made at vkms_get_vblank_timestamp(), we broke the CRC > > code because now we invoke get_vblank_timestamp() multiple times. The > > consequence of this change is the excessive decrement of the timestamp > > since the following condition execute multiple time: > > > > bool vkms_get_vblank_timestamp([..]) { > > if (!in_vblank_irq) > > *vblank_time -= output->period_ns; > > The full code is > *vblank_time = output->vblank_hrtimer.node.expires; > if (!in_vblank_irq) > *vblank_time -= output->period_ns; > > So no matter how many times you call it you should always get the same > answer (unless the timer was moved forwared in between the calls).
Touché! :)
You're right! Thanks for your patience, I finally understood the problem and now I will dig into it.
Thanks
> > > > 6. The bug fix (this patch) > > > > Since we only want the frame number, as I described in Section 5, the > > fix is quite simple: avoid call drm_crtc_accurate_vblank_count() and > > just get the frame value from the data structure. In this patch, > > Shayenne removed the function drm_crtc_accurate_vblank_count() which > > avoid the extra decrement in the vkms_get_vblank_timestamp(). With this > > patch, everything started to work well. > > > > So... In a few words, it is a VKMS problem, not a vblank issue. > > > > I’m not sure if the solution here is the best one, but I believe that > > the idea behind it is correct. > > > > Best Regards > > > > -- > > Rodrigo Siqueira > > https://siqueira.tech > > Graduate Student > > Department of Computer Science > > University of São Paulo > > > > -- > Ville Syrjälä > Intel
-- Rodrigo Siqueira https://siqueira.tech Graduate Student Department of Computer Science University of São Paulo [unhandled content-type:application/pgp-signature] | |