Re: [PATCH] drm/i915: Avoid snooping with userptr where not supported

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

On 01/03/16 17:14, Chris Wilson wrote:
> On Tue, Mar 01, 2016 at 04:29:35PM +0000, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>>     commit e5756c10d841ddb448293c849392f3d6b809561f
>>     Author: Imre Deak <imre.deak@intel.com>
>>     Date:   Fri Aug 14 18:43:30 2015 +0300
>>
>>         drm/i915/bxt: don't allow cached GEM mappings on A stepping
>>
>> Added an exception of disallowing snooping for Broxton A
>> stepping hardware but userptr was still enabling it regardless.
>>
>> Move the check to HAS_SNOOP now that it is used from multiple
>> call sites and use it.
>>
>> Idea is that userspace which relies on userptr snooping, for
>> example for fine grained buffered SVM, can query the caching
>> mode on a created userptr object to determine whether coherency
>> is supported or not.
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> Cc: Imre Deak <imre.deak@intel.com>
>> ---
>>   drivers/gpu/drm/i915/i915_drv.h         | 1 +
>>   drivers/gpu/drm/i915/i915_gem.c         | 2 +-
>>   drivers/gpu/drm/i915/i915_gem_userptr.c | 2 +-
>>   3 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
>> index 671295523317..73f0db17b990 100644
>> --- a/drivers/gpu/drm/i915/i915_drv.h
>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>> @@ -2629,6 +2629,7 @@ struct drm_i915_cmd_table {
>>   #define HAS_LLC(dev)		(INTEL_INFO(dev)->has_llc)
>>   #define HAS_WT(dev)		((IS_HASWELL(dev) || IS_BROADWELL(dev)) && \
>>   				 __I915__(dev)->ellc_size)
>> +#define HAS_SNOOP(dev)		(!IS_BXT_REVID(dev, 0, BXT_REVID_A1))
>>   #define I915_NEED_GFX_HWS(dev)	(INTEL_INFO(dev)->need_gfx_hws)
>>
>>   #define HAS_HW_CONTEXTS(dev)	(INTEL_INFO(dev)->gen >= 6)
>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>> index 3d31d3ac589e..1c6e8fb9d392 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.c
>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>> @@ -3949,7 +3949,7 @@ int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
>>   		 * cacheline, whereas normally such cachelines would get
>>   		 * invalidated.
>>   		 */
>> -		if (IS_BXT_REVID(dev, 0, BXT_REVID_A1))
>> +		if (!HAS_SNOOP(dev))
>
> if (!HAS_LLC && !HAS_SNOOP)
>
> and HAS_SNOOP should be true for everything that is not llc, with the
> exception above. So make a dev_info->has_snoop feature bit (or
> uses_snoop?) and then clear the copy inside the struct for the special
> case.

Sure I can do that, but isn't it the same as this patch? Unless 
something with LLC and no snooping appears?

>>   			return -ENODEV;
>>
>>   		level = I915_CACHE_LLC;
>> diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
>> index 4b09c840d493..48aaff019b42 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
>> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
>> @@ -782,7 +782,7 @@ i915_gem_userptr_ioctl(struct drm_device *dev, void *data, struct drm_file *file
>>
>>   	drm_gem_private_object_init(dev, &obj->base, args->user_size);
>>   	i915_gem_object_init(obj, &i915_gem_userptr_ops);
>> -	obj->cache_level = I915_CACHE_LLC;
>> +	obj->cache_level = HAS_SNOOP(dev) ? I915_CACHE_LLC : I915_CACHE_NONE;
>
> This should be
> if (!HAS_SNOOP && !HAS_LLC)
> 	return -ENODEV;
>
> as we can not support a coherent pointer shared between memory and the GPU
> in this case.

Hm interesting, why not? I mean, what is the difference compared to 
normal BOs which will be uncached on !LLC?

Regards,

Tvrtko
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