[RFC] GPU reset notification interface

Ian Romanick
2012-07-17T22:17:38Z

I'm getting ready to implement the reset notification part of
GL_ARB_robustness in the i965 driver. There are a bunch of quirky bits
of the extension that are causing some grief in designing the kernel /
user interface. I think I've settled on an interface that should meet
all of the requirements, but I want to bounce it off people before I
start writing code.

Here's the list of requirements.

- Applications poll for reset status.

- Contexts that did not lose data or rendering should not receive a
reset notification. This isn't strictly a requirement of the spec, but
it seems like a good practice. Once an application receives a reset
notification for a context, it is supposed to destroy that context and
start over.

- If one context in an OpenGL share group receives a reset notification,
all contexts in that share group must receive a reset notification.

- All contexts in a single GL share group will have the same fd. This
isn't a requirement so much as a simplifying assumption. All contexts
in a share group have to be in the same address space, so I will assume
that means they're all controlled by one DRI driver instance with a
single fd.

- The reset notification given to the application should try to assign
guilt. There are three values possible: unknown guilt, you're not
guilty, or you are guilty.

- If there are multiple resets between polls, the application should get
the "most guilty" answer. In other words, if there are two resets and
the context was guilty for one and not the other, the application should
get the guilty notification.

- After the application polls the status, the status should revert to
"no reset occurred."

- If the application polls the status and the reset operation is still
in progress, it should continue to get the reset value until it is
"safe" to begin issuing GL commands again.

At some point I'd like to extend this to give slightly finer grained
mechanism so that a context could be told that everything after a
particular GL sync (fence) operation was lost. This could prevent some
applications from having to destroy and rebuild their context. This
isn't a requirement, but it's an idea that I've been mulling.

Each time a reset occurs, an internal count is incremented. This
associates a unique integer, starting with 1, with each reset event.
Each context affected by the reset will have the reset event ID stored
in one its three guilt levels. An ioctl will be provided that returns
the following data for all contexts associated with a particular fd.

In addition, it will return the index of any reset operation that is
still in progress.

I think this should be sufficient information for user space to meet all
of the requirements. I had a conversation with Paul and Ken about this.
Paul was pretty happy with the idea. Ken felt like too little policy
was in the kernel, and the resulting interface was too heavy (I'm
paraphrasing).

struct drm_context_reset_counts {
__u32 ctx_id;

/**
* Index of the most recent reset where this context was
* guilty. Zero if none.
*/
__u32 guilty;

/**
* Index of the most recent reset where this context was
* not guilty. Zero if none.
*/
__u32 not_guilty;

/**
* Index of the most recent reset where guilt was unknown.
* Zero if none.
*/
__u32 unknown_guilt;
};

struct drm_reset_counts {
/** Index of the in-progress reset. Zero if none. */
unsigned reset_index_in_progress;

/** Number of contexts. */
__u32 num_contexts;

struct drm_context_reset_counts contexts[0];
};