Re: [PATCH] drm/i915: Do not lie about atomic wait granularity

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 01/02/16 13:30, Chris Wilson wrote:
> On Mon, Feb 01, 2016 at 01:17:35PM +0000, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Currently the wait_for_atomic_us only allows for a millisecond
>> granularity which is not nice towards callers requesting small
>> micro-second waits.
>>
>> Re-implement it so micro-second granularity is really supported
>> and not just in the name of the macro.
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> ---
>> Danger - this might break things which currently work by accident!
>> ---
>>   drivers/gpu/drm/i915/intel_drv.h | 21 ++++++++++++++++++---
>>   1 file changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
>> index f620023ed134..9e8a1202194c 100644
>> --- a/drivers/gpu/drm/i915/intel_drv.h
>> +++ b/drivers/gpu/drm/i915/intel_drv.h
>> @@ -63,10 +63,25 @@
>>   	ret__;								\
>>   })
>>
>> +#define _wait_for_atomic(COND, US) ({ \
>> +	unsigned long end__; \
>> +	int ret__ = 0; \
>> +	get_cpu(); \
>
> Hmm, by virtue of its name (and original intent), we are expected to be
> in an atomic context and could just do a BUG_ON(!in_atomic()) to catch
> misuse. Since the removal of the panic modeset, all callers outside of
> intel_uncore.c are definitely abusing this and we would be better to use
> a usleep[_range]() variant instead.

I considered a WARN_ON_ONCE and a BUILD_BUG_ON for very long waits but 
chickened out on both.

I'll respin with a WARN_ON_ONCE(!in_atomic)) to start with.

Regards,

Tvrtko
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