Re: drm/i915: Watchdog timeout: DRM kernel interface to set the timeout

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2019-01-07 12:38:47)
> On 05/01/2019 02:39, Carlos Santa wrote:
> > +/* Return the timer count threshold in microseconds. */
> > +int i915_gem_context_get_watchdog(struct i915_gem_context *ctx,
> > +                               struct drm_i915_gem_context_param *args)
> > +     if (__copy_to_user(u64_to_user_ptr(args->value),
> > +                        &threshold_in_us,
> > +                        sizeof(threshold_in_us))) {
> 
> I think user pointer hasn't been verified for write access yet so 
> standard copy_to_user should be used.

The starting point here is that this bakes in OTHER_CLASS as uABI when
clearly it is not uABI. The array must be variable size and so the first
pass is to report the size to userspace (if they pass args->size = 0)
and then to copy up to the user requested size. Since it is a variable
array with uncertain indexing, the output should be along the lines of
[class, instance, watchdog]. But what about virtual engine? Good
question. (Remember set must follow the same pattern as get, so you can
always do a rmw cleanly.)

I guess we just have to accept class == -1 as meaning use instance as an
index into ctx->engines[]. Tvrtko? Virtual engine would be reported as
(-1, 0, watchdog), and settable similarly with the other engines being
addressable either by index or by class-instance.
-Chris
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