Re: [RFC PATCH] drm/vc4: Add a load tracker to prevent HVS underflow errors

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Mon, 29 Oct 2018 10:03:01 +0100
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Mon, Oct 29, 2018 at 09:41:36AM +0100, Boris Brezillon wrote:
> > On Mon, 29 Oct 2018 09:06:40 +0100
> > Daniel Vetter <daniel@ffwll.ch> wrote:
> >   
> > > On Fri, Oct 26, 2018 at 06:10:03PM +0300, Ville Syrjälä wrote:  
> > > > On Fri, Oct 26, 2018 at 04:52:33PM +0200, Boris Brezillon wrote:    
> > > > > On Fri, 26 Oct 2018 16:26:03 +0200
> > > > > Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > >     
> > > > > > On Fri, Oct 26, 2018 at 3:57 PM Boris Brezillon
> > > > > > <boris.brezillon@bootlin.com> wrote:    
> > > > > > >
> > > > > > > On Fri, 26 Oct 2018 15:30:26 +0200
> > > > > > > Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > > > >      
> > > > > > > > On Thu, Oct 25, 2018 at 11:41:14AM +0200, Boris Brezillon wrote:      
> > > > > > > > > On Thu, 25 Oct 2018 11:33:14 +0200
> > > > > > > > > Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > > > > > >      
> > > > > > > > > > On Thu, Oct 25, 2018 at 10:09:31AM +0200, Boris Brezillon wrote:      
> > > > > > > > > > > On Tue, 23 Oct 2018 15:44:43 +0200
> > > > > > > > > > > Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > > > > > > > >      
> > > > > > > > > > > > On Tue, Oct 23, 2018 at 09:55:08AM +0200, Boris Brezillon wrote:      
> > > > > > > > > > > > > Hi Daniel,
> > > > > > > > > > > > >
> > > > > > > > > > > > > On Tue, 16 Oct 2018 14:57:43 +0200
> > > > > > > > > > > > > Daniel Vetter <daniel@ffwll.ch> wrote:
> > > > > > > > > > > > >      
> > > > > > > > > > > > > > On Tue, Oct 16, 2018 at 11:40:45AM +0200, Boris Brezillon wrote:      
> > > > > > > > > > > > > > > The HVS block is supposed to fill the pixelvalve FIFOs fast enough to
> > > > > > > > > > > > > > > meet the requested framerate. The problem is, the HVS and memory bus
> > > > > > > > > > > > > > > bandwidths are limited, and if we don't take these limitations into
> > > > > > > > > > > > > > > account we might end up with HVS underflow errors.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > This patch is trying to model the per-plane HVS and memory bus bandwidth
> > > > > > > > > > > > > > > consumption and take a decision at atomic_check() time whether the
> > > > > > > > > > > > > > > estimated load will fit in the HVS and membus budget.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > Note that we take an extra margin on the memory bus consumption to let
> > > > > > > > > > > > > > > the system run smoothly when other blocks are doing heavy use of the
> > > > > > > > > > > > > > > memory bus. Same goes for the HVS limit, except the margin is smaller in
> > > > > > > > > > > > > > > this case, since the HVS is not used by external components.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
> > > > > > > > > > > > > > > ---
> > > > > > > > > > > > > > > This logic has been validated using a simple shell script and
> > > > > > > > > > > > > > > some instrumentation in the VC4 driver:
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > - capture underflow errors at the HVS level and expose a debugfs file
> > > > > > > > > > > > > > >   reporting those errors
> > > > > > > > > > > > > > > - add debugfs files to expose when atomic_check fails because of the
> > > > > > > > > > > > > > >   HVS or membus load limitation or when it fails for other reasons
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > The branch containing those modification is available here [1], and the
> > > > > > > > > > > > > > > script (which is internally using modetest) is here [2] (please note
> > > > > > > > > > > > > > > that I'm bad at writing shell scripts :-)).
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > Note that those modification tend to over-estimate the load, and thus
> > > > > > > > > > > > > > > reject setups that might have previously worked, so we might want to
> > > > > > > > > > > > > > > adjust the limits to avoid that.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > [1]https://github.com/bbrezillon/linux/tree/vc4/hvs-bandwidth-eval
> > > > > > > > > > > > > > > [2]https://github.com/bbrezillon/vc4-hvs-bandwidth-test      
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Any interest in using igt to test this stuff? We have at least a bunch of
> > > > > > > > > > > > > > tests already in there that try all kinds of plane setups. And we use
> > > > > > > > > > > > > > those to hunt for underruns on i915 hw.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > Wrt underrun reporting: On i915 we just dump them into dmesg at the error
> > > > > > > > > > > > > > level, using DRM_ERROR,      
> > > > > > > > > > > > >
> > > > > > > > > > > > > Are you masking the underrun interrupt after it's been reported? If we
> > > > > > > > > > > > > don't do that on VC4 we just end up flooding the kernel-log buffer until
> > > > > > > > > > > > > someone comes and update the config.      
> > > > > > > > > > > >
> > > > > > > > > > > > Yeah we do that too. Rule is that a full modeset will clear any underrun
> > > > > > > > > > > > masking (so tests need to make sure they start with a modeset, or it'll be
> > > > > > > > > > > > for nothing).
> > > > > > > > > > > >      
> > > > > > > > > > > > >      
> > > > > > > > > > > > > > plus a tracepoint. See e.g.
> > > > > > > > > > > > > > intel_pch_fifo_underrun_irq_handler(). If there's interest we could
> > > > > > > > > > > > > > perhaps extract this into something common, similar to what was done with
> > > > > > > > > > > > > > crc support already.
> > > > > > > > > > > > > >      
> > > > > > > > > > > > >
> > > > > > > > > > > > > I'm not a big fan of hardcoded trace points in general (because of the
> > > > > > > > > > > > > whole "it's part of the stable ABI" thing), and in this case, making the
> > > > > > > > > > > > > tracepoint generic sounds even more risky to me. Indeed, how can we know
> > > > > > > > > > > > > about all the HW specific bits one might want to expose. For instance,
> > > > > > > > > > > > > I see the intel underrun tracepoint exposes a struct with a frame and
> > > > > > > > > > > > > scanline field, and AFAICT, we don't have such information in the VC4
> > > > > > > > > > > > > case.
> > > > > > > > > > > > >
> > > > > > > > > > > > > Any opinion on that?      
> > > > > > > > > > > >
> > > > > > > > > > > > It's only abi if you're unlucky. If it's just for debugging and
> > > > > > > > > > > > validation, you can change it again. Tbh, no idea why we even have these
> > > > > > > > > > > > tracepoints, they're fairly useless imo. CI only relies upon the dmesg
> > > > > > > > > > > > output. Maybe run git blame and ask the original author, we can probably
> > > > > > > > > > > > update them to suit our needs.      
> > > > > > > > > > >
> > > > > > > > > > > Okay, I think I'll go for a generic debugfs entry that returns true
> > > > > > > > > > > when an underrun error happened since the last modeset, false otherwise.
> > > > > > > > > > >
> > > > > > > > > > > Next question is: should I attach the underrun status to the drm_device
> > > > > > > > > > > or have one per CRTC? In my case, I only care about the "has an
> > > > > > > > > > > underrun error occurred on any of the active CRTC" case, so I'd vote for
> > > > > > > > > > > a per-device underrun status.      
> > > > > > > > > >
> > > > > > > > > > Yeah probably good enough. For our CI all we care about is the warn/error
> > > > > > > > > > level dmesg output. Anything at that level is considered a CI failure.      
> > > > > > > > >
> > > > > > > > > So igt is grepping dmesg to detect when an underrun happens?      
> > > > > > > >
> > > > > > > > No, but the CI runner is also observing dmesg. Anything in there at
> > > > > > > > warning or higher level is considered a failure.      
> > > > > > >
> > > > > > > Eric, do you do the same when you launch the IGT testsuite?
> > > > > > >      
> > > > > > > >      
> > > > > > > > > > What do you need the debugfs file for?      
> > > > > > > > >
> > > > > > > > > I just thought having a debugfs file to expose the underrun information
> > > > > > > > > would be cleaner than having to grep in dmesg to detect such failures.      
> > > > > > > >
> > > > > > > > The issue is that you want to detect underruns everywhere, not just in the
> > > > > > > > specific tests you're checking for it. Anything that does a modeset could
> > > > > > > > cause an underrun (at least we've managed to do so pretty much everywhere
> > > > > > > > on i915 hw, if you misprogram is sufficiently).      
> > > > > > >
> > > > > > > In my specific case, I want to have the IGT test check the underrun
> > > > > > > value while the test is being executed so that I know which exact
> > > > > > > configuration triggers the underrun situation. At least that's how I
> > > > > > > did to adjust/debug the HVS load tracking code. Maybe it's not really a
> > > > > > > problem when all we do is tracking regressions.      
> > > > > > 
> > > > > > Ok, that makes sense, and explains why you want the overall underrun
> > > > > > counter exposed in debugfs.    
> > > > > 
> > > > > Just one tiny detail which is not exactly related to this discussion
> > > > > but I thought I'd mention it here: underrun is actually not a counter,
> > > > > it's just a boolean. I used an atomic_t type to avoid having to add a
> > > > > spinlock to protect the variable (the variable is modified from
> > > > > an interrupt context and read from a non-atomic context). So, the
> > > > > acceptable values for underrun are currently 0 or 1. I can make it a
> > > > > counter if required.    
> > > > 
> > > > One idea I had a while back for i915 would be to count the number of
> > > > frames that had an underrun. So basically something like this:
> > > > 
> > > > underrun_irq() {
> > > > 	underrun_frames=1
> > > > 	disable_underrun_irq();
> > > > 	vblank_get();
> > > > }
> > > > vblank_irq() {
> > > > 	if (underrun) {
> > > > 		underrun_frames++;
> > > > 	} else if (underrun_frames) {
> > > > 		vblank_put();
> > > > 		enable_underrun_irq();
> > > > 		DEBUG("%d frames had an underrun\n", underrun_frames);
> > > > 		underrun_frames=0;
> > > > 	}
> > > > }
> > > > 
> > > > This avoids drowning in underrun interrupts while still
> > > > reporting at least how many frames had problems.
> > > > 
> > > > But I haven't had time to implement that yet.    
> > 
> > May I ask why you need to know how many frames had underrun issues?  
> 
> Much quicker testing when you're experimenting around with e.g. watermark
> settings. Full modeset for resetting can easily take a few seconds. I
> think we even had patches that restored the interrupt after 1ms already,
> to have more accurate sampling to judge whether a given change made things
> worse or better. We might have somewhat strange hw :-)
> 
> > > The other upshot of a counter is that there's no problem with resetting.
> > > Userspace simply grabs the counter before and after the test and compares.
> > > If you only have 0/1 you need some way to reset, or otherwise automated
> > > running in a CI farm isn't possible.  
> > 
> > The reset was done during atomic commit in my proposal, so no action
> > was required on the user side apart from applying a new config.
> > Anyway, I'll change that for a real counter.  
> 
> We want to measure underruns while doing full modesets too (specifically,
> when shutting down the pipeline). At least for our hw this has
> historically helped greatly in catching programming sequence issues.

Looks like you have particular needs for intel HW which I don't know
about, maybe I shouldn't be the one driving this rework...
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