[RFC 0/7+1] Add in-kernel vblank delaying mechanism

[Paulo Zanoni]

From: Paulo Zanoni <paulo.r.zanoni at intel.com>

Hi

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TL;DR summary:
I wrote patches. Help me choose the best implementation and interface.
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So the i915.ko driver could use some mechanism to run functions after a given
number of vblanks. Implementations for this mechanism were already proposed in
the past (by Chris Wilson and others), but they were i915-specific instead of a
generic drm.ko implementation. We even had patches that make use of this new
mechanism.

Since this is very similar to the vblank IOCTL we currently have, but with the
caller being a Kernel module instead of user space, I decided to write an
implementation that tries to reuse the current IOCTL infrastructure.

In the first patch we add all the necessary code to allow the modules to call
the vblank ioctl from Kernel space: they provide a callback function that is
going to be called instead of the traditional send_vblank_event(), which means
the Kernel callers don't have to deal with the file descriptors and DRM events.

In the second patch we add a simple extension of the feature, to allow the
drivers to have their callbacks called in a non-atomic context.

In all the other subsequent patches we rework the underlying code so that
the common aspects between the user space vblank IOCTL and the Kernel interface
are all stored in a single structure (struct drm_vblank_wait_item), and then
anything that is specific to the different users is stored in a structure that
contains struct drm_vblank_wait_item. This way, most of the drm.ko code only
needs to deal with struct drm_vblank_wait_item, not really knowing how it is
wrapped. The advantage of this rework is that it reduces the number of memory
allocations needed in some cases (from 2 to 1) and it also reduces the amount of
memory used on each allocation.

But while this all sounds nice in the above description, I had the feeling that
this adds a lot of complexity and makes the code not-so-beautiful. If we ever
need more extensions/options to this feature, we're going to have to untangle
the code even more from the IOCTL part. We also have a small "abstraction break"
in change introduced in patch 6. And there's the risk that some of the reworks
may have added a regression somewhere.

Based on that, I decided to also provide an alternative implementation. In this
implementation we add a new dev->vblank_kernel_list instead of reusing
dev->vblank_event_list, and add new code to handle that this. This new code is
completely based on the code that handles dev->vblank_kernel_list, so there's
some duplication here. On the other hand, since the in-Kernel infrastructure is
not tied to the IOCTL structure, we can very easily grow and adapt the Kernel
code without having to fight against the IOCTL code. And the risk of regressions
is greatly reduced.

The second difference of this alternative implementation is the signature of the
drm_wait_vblank_kernel() function. While it could be exactly the same as the one
used in the other series, I decided to make it different so we can also choose
which one to use. In the 7-patch series implementation, all the user needs to do
is to allocate the structure, and call the function, properly setting all its
arguments. Then the function is responsible for parsing the arguments and
populating the structure based on that. On the alternative implementation, the
user has to fill the structure with the appropriate arguments, and then call
drm_wait_vblank_kernel() passing just the allocated structure as an argument.

If you notice, you will see that these patches add a new debugfs file to
i915.ko. This file is just a very simple example user of the new interface,
which I used while developing. If you have difficulty understanding the new
interface, please also look at the i915/i915_debugfs.c diff. Of course, I plan
to exclude this debugfs interface from the final to-be-merged patches.

So, in summary, we have a few things we need to discuss:

 1. Which implementation to use?

   a. Just the 2 first patches of the 7-patch series?
      Pros:
        - the Kernel users basically call the vblank IOCTL
        - the code changes are minimal
     Cons:
        - The amount of memory allocations and memory space consumed is not
          optimal

   b. The full 7-patch series?
      Pros:
        - The same backend is used for both the Kernel and IOCTL.
      Cons:
        - The code gets more complex.
        - Extending the Kernel interface can get complicated due to the fact
          that it shares code with the IOCTL interface.
        - I didn't really like this solution.

   c. The alternative implementation I wrote?
      Pros:
        - It's independent from the IOCTL code, making further patches easier.
      Cons:
        - There is some duplicated code.

   d. Something totally different from these alternatives?

 2. How should the driver interface look like?

   a. All the possibilities are passed through the function call, so the drm.ko
      code needs to set the struct members itself.
   b. The caller already sets the struct members instead of passing them as
      parameters to the function.
   c. Something different?

All these patches are based on top of Daniel Vetter's drm-intel-nightly tree.
It's all just an RFC, so don't expect something very well tested.

Flamewars and bikeshedding are welcome!

Thanks,
Paulo

Paulo Zanoni (7):

  - First implementation:
  drm: allow the drivers to call the vblank IOCTL internally
  drm: allow drm_wait_vblank_kernel() callback from workqueues
  drm: introduce struct drm_vblank_wait_item
  drm: add wanted_seq to drm_vblank_wait_item
  drm: change the drm vblank callback item type
  drm: make vblank_event_list handle drm_vblank_wait_item types
  drm: make the callers of drm_wait_vblank() allocate the memory

  - Alternative implementation:
  drm: add a mechanism for drivers to schedule vblank callbacks

Stats for only the first implementation:

 drivers/gpu/drm/drm_fops.c          |  15 ++-
 drivers/gpu/drm/drm_ioctl.c         |   2 +-
 drivers/gpu/drm/drm_irq.c           | 178 ++++++++++++++++++++++++++----------
 drivers/gpu/drm/i915/i915_debugfs.c |  90 ++++++++++++++++++
 include/drm/drmP.h                  |  35 ++++++-
 5 files changed, 264 insertions(+), 56 deletions(-)

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