[Intel-gfx] [PATCH 1/5] drm/i915: Stop second guessing the caller for intel_uncore_wait_for_register()

[Chris Wilson]

On Tue, Apr 11, 2017 at 11:13:36AM +0100, Chris Wilson wrote:
> Allow the caller to use the fast_timeout_us to specify how long to wait
> within the atomic section, rather than transparently switching to a
> sleeping loop for larger values. This is required as some callsites may
> need a long wait and are in an atomic section.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Michal Wajdeczko <michal.wajdeczko at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_uncore.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index eb38392a2435..53c8457869f6 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -1601,7 +1601,7 @@ static int gen6_reset_engines(struct drm_i915_private *dev_priv,
>   *
>   * Otherwise, the wait will timeout after @slow_timeout_ms milliseconds.
>   * For atomic context @slow_timeout_ms must be zero and @fast_timeout_us
> - * must be not larger than 10 microseconds.
> + * must be not larger than 20,0000 microseconds.
>   *
>   * Note that this routine assumes the caller holds forcewake asserted, it is
>   * not suitable for very long waits. See intel_wait_for_register() if you
> @@ -1623,16 +1623,17 @@ int __intel_wait_for_register_fw(struct drm_i915_private *dev_priv,
>  	int ret;
>  
>  	/* Catch any overuse of this function */
> -	might_sleep_if(fast_timeout_us > 10 || slow_timeout_ms);
> +	might_sleep_if(slow_timeout_ms);

For whatever reason, replies are not in my mbox, so Tvrtko,Michal wrote:

> I agree with Michal here. Kerneldoc even says "must not be larger than 
> 20ms" so it would be better and completely fine in my opinion to:
>
>	if (GEM_WARN_ON(fast_timeout_us > 20000))
>		return -EINVAL;

Not EINVAL, ENODEV if we must. So GEM_BUG_ON(fast_timeout_us > 20000) as
documentation?

We need to keep the fast_timeout_us < X check for gcc.

> Hm but it would break the bisectability of the series and break the 
> sandybridge pcode.

No it doesn't, nobody is passing in such a large *fast_timeout_us*. What
have I missed?

> So patch 4/5 looks broken since it changes the timeout from 500ms to 
> 500us. I don't see how to fix that without splitting the _fw and atomic 
> concepts.

That's not being broken, thats part of the fix. 500ms timeout inside an
atomic section is insane. 500ms timeout outside of that is unwise and
deserves an error of its own. I am quite happy to add that error and
fight that battle later (as BAT is happy, EXT might not be, and users
never).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre