Re: [PATCH v2 08/12] nova-core: sequencer: Add register opcodes

[Lyude Paul <lyude@redhat.com>]

On Mon, 2025-11-10 at 10:16 -0500, Joel Fernandes wrote:
> On 11/5/2025 6:19 PM, Timur Tabi wrote:
> > On Wed, 2025-11-05 at 13:55 -0800, John Hubbard wrote:
> > > > #define nvdev_trace(d,f,a...) nvdev_printk((d), TRACE,   info, f, ##a)
> > > > #define nvdev_spam(d,f,a...)  nvdev_printk((d),  SPAM,    dbg, f, ##a)
> > > 
> > > ...and those are unusable, unfortunately. I've tried.
> > 
> > This works great for me:
> > 
> > modprobe nouveau dyndbg="+p" modeset=1 debug="gsp=spam" config=NvGspRm=1
> > 
> > I get all sequencer messages when I boot with these options.
> > 
> > > ftrace/bpftrace, maybe those are the real way to "trace"...or something
> > > other than this.
> > 
> > You could say the same thing about most dev_dbg() statements.
> > 
> > I agree that dev_dbg for sequencer commands is excessive, and that implementing new debug levels
> > just to get sequencer prints is also excessive.  But Nouveau implement nvkm_trace for a reason.  And
> > we all know that because of ? in Rust, NovaCore does a terrible job at telling us where an error
> > actually occurred.  So there is a lot of room for improvement.
> 
> IMO, the best way to do this is the tracing subsystem. It is the lowest overhead
> runtime kernel logging system that I know off, lockless, independent of the
> serial console etc, next to no runtime overhead when off, etc.
> 

I agree. FWIW, it's worth noting that honestly avoiding logging is the way to
go for anything spammy. I've seen quite a number of heisenbugs that only
appear when trace logging isn't turned on in nouveau or vice-versa (igt tests
that fail because logging causes things to time out…).

> I recommend we use the tracing subsystem for "trace" and even "spam" level
> logging levels for Nova. The brave souls can always ask the tracing subsystem to
> also spam to kernel logs if they so wish.
> 
> ++ Tracing Czar Steven Rostedt as well. Steve, Nova is a new modern Nvidia GPU
> driver.
> 
> I guess we have to decide how to do this - what kind of tracepoints do we need
> for Nova. One use case that just came up is RPC message buffer dumps for
> debugging communication with the firmware.
> 
> thanks,
> 
>  - Joel

-- 
Cheers,
 Lyude Paul (she/her)
 Senior Software Engineer at Red Hat

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