X.Org To Pay For Better Nouveau GPU Reclocking

Written by Michael Larabel in Nouveau on 25 May 2012 at 05:42 AM EDT. 49 Comments
With X.Org / Mesa / Wayland not being part of this year's GSoC, the X.Org Foundation is independently paying for some projects to motivate students over to summer to make some open-source driver improvements.

As part of the rarely-used X.Org Endless Vacation of Code, a few proposals came in and it looks like the X.Org Foundation Board will go ahead and finance the work. The work mainly comes down to better re-clocking support in the Nouveau driver for reverse-engineered NVIDIA support.

The work comes down to NVIDIA monitoring improvements and a re-clocking scheme for newer (Fermi and Kepler) graphics cards. Below are the two proposals that will be worked on for the next few months and hopefully result in some nice upstream enhancements for the reverse-engineered NVIDIA driver.
NVidia cards have long had the possibility to reclock at least some of the engines of its GPUs. Up to the geforce 7 (included), reclocking used to happen at boot time and usually didn't involve memory reclocking at all.

It changed with geforce 8 (nv50) where almost all laptops got the capability to reclock both the VRAM and the main engines. This was introduced in order to lower power consumption when the GPU was mostly idle. The default boot clocks were usually in some intermediate state between the slowest and the fastest clocks. The reclocking process for these cards is mostly understood and Nouveau is not far from being safely reclock on the fly, even while gaming.

Geforce 200 (nva3) introduced load-based reclocking on all the cards. This started being a real problem because the default boot clocks are a third to a half of the maximum clocks. This explains a lot of the performance problem on those cards with Nouveau.
The reclocking process on these cards is yet to be perfected but it is hoped that it will soon reach the level of stability of nv50.

On Fermi (geforce 400), the performance problem got worse as default boot clocks are often set to about 10% the maximum clocks. With such low clocks, desktop usage suffers many performance problems. This makes reclocking on these card an urging task. Reclocking Fermi's engines is mostly understood while memory reclocking (GDDR5) is being investigated.

In order for memory reclocking to be done safely, it is needed to deny access to VRAM to all applications. This can be done radeon-style (by unmapping the BOs in TTM while locking BO mapping) or by putting the card off-bus. The latter being more efficient, it has been the method used on nv50.

When the card is off-bus, no MMIO access can be done and so, no fiddling with the PLL registers can be done. This is why reclocking on nv50 is carried on by an internal "scripting" engine called HWSQ (HardWare SeQuencer). This engine has the ability to put the card off-bus, wait and write to MMIO registers.

Unfortunately, HWSQ was removed when Fermi was released as it was superseded by a more general-purpose engine called PDAEMON. PDAEMON was introduced on Geforce 200 (nva3) and can be considered as a 200 MHz microcontroller with full access to the registers of the card and that is also capable of sending IRQs to the host. It is used by nvidia for both hardware monitoring and reclocking.

The ISA of this microcontroller is FµC (flexible microcode). This ISA is being used in most engines of the GPU on Fermi and almost all of them in Kepler. An open-source implementation of PDAEMON with basic command-submission (host ->pdaemon) and fan-management has been written[1]. This implementation works on PDAEMON from nva3 to nvd9 and should work out of the box on Kepler (never tested but the needed RE is done).

The goal of this project would be to propose a replacement for HWSQ by implementing some scripting capabilities in PDAEMON. This requires some improvements on PDAEMON such as PDAEMON -> Host communication and a very simple scheduler to manage multiple "applications" (thermal management, hardware monitoring and reclocking).

Theoretically, it would be possible to replace hwsq by straight FµC and ask PDAEMON to execute this. However, FµC isn't a simple ISA and embedding an assembler inside the Linux kernel doesn't seem like a good idea.
It is also a very stupid idea from a security point of view as this engine can do pretty much everything.
Furthermore, it could generate some corner effects that could hinder the GPU monitoring processes.
Lastly, reclocking is already hard on its own. Implementing it in FµC seems like an impossible mission, especially when considering all the chipset-specific variations.

On the contrary, a script could be guaranteed not to interfere with the critical mission of PDAEMON (thermal monitoring).

In order to be useful, the proposed scripting ISA must be capable of cleanly implementing both memory reclocking. Designing the ISA of this script and implementing it in PDAEMON will be the main challenge of this project.

The scripting engine will then be put to a test by implementing safe engine reclocking and safe memory reclocking if the reverse engineering has been completed.

This task is likely to require REing and various fixing to accommodate all the testers.

In parallel of this task, PDAEMON should be merged to Linux. This involves creating a new Nouveau submodule, porting the user-space command-submission program to the kernel and then, porting some hardware monitoring features to PDAEMON.

And the second Nouveau EVoC one...
Nvidia GPUs provide an engine to monitor "hardware signals" called PCOUNTER. Monitoring
these signals may lead to a better understanding of the internal structure of the GPU, better
power management or better performance.

PCOUNTER is already sufficiently understood to be set up to listen to arbitrary signals.
However, the current number of documented signals is pretty slim. We will be looking at further
expanding this and working on it in a manner which is most suitable for the user.

The way we currently reverse engineer signals is to write minimal tests for some features and
see how signals react to these tests. During reverse engineering, we will document the signals.
After reverse engineering, we will develop an interface in the kernel to make signals more
efficient. The final phase will include logging various components of the card and visually
representing them which will help the user tweak his settings for maximum utilization.

Once more useful signals have been reverse engineered, a userspace tool could be used to
monitor those signals to detect bottlenecks in a GL or CL program.
Right now Nouveau re-clocking can be a mess but yield nice performance improvements so any improvements in this area for the open-source GPU driver is most welcome. Right now in the mainline code there's also no re-clocking support for Kepler -- or even the correct reading of clock frequencies.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter, LinkedIn, or contacted via MichaelLarabel.com.

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