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

Dave Airlie
2014-11-26T23:25:15Z

>> ---
>> TL;DR summary:
>> I wrote patches. Help me choose the best implementation and interface.
>> ---
>>
>> 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!
>
> As we've discussed on irc a bit I still think cleaning up this rathole is
> beneficial. Getting the event handling and vblank callbacks right just in
> drivers seems to be really hard, so better to clean this up and unify
> things.
>
> But like you've said patches are a lot of churn, so do you have a git
> branch somewhere for the lazy to look at?
>
> Thanks, Daniel
>

I also think vmwgfx might already have something like this, maybe take a look
and see if this is the same use case and can cover that as well.

Dave.
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