Re: [PATCH 3/6] drm/i915: Compact memcmp in i915_vma_compare()

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

On 12/01/2017 11:33, Chris Wilson wrote:
> On Wed, Jan 11, 2017 at 09:51:03PM +0000, Chris Wilson wrote:
>> In preparation for the next patch to convert to using an anonymous union
>> and leaving the excess bytes in the union uninitialised, we first need
>> to make sure we do not compare using those uninitialised bytes. We also
>> want to preserve the compactness of the code, avoiding a second call to
>> memcmp or introducing a switch, so we take advantage of using the type
>> as an encoded size (as well as a unique identifier for each type of view).
>>
>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
>> ---
>>  drivers/gpu/drm/i915/i915_gem_gtt.h | 14 +++++++-------
>>  drivers/gpu/drm/i915/i915_vma.h     | 15 +++++++++------
>>  2 files changed, 16 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h b/drivers/gpu/drm/i915/i915_gem_gtt.h
>> index 3187a260e6e1..36d85599ffc9 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.h
>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
>> @@ -145,12 +145,6 @@ typedef uint64_t gen8_ppgtt_pml4e_t;
>>
>>  struct sg_table;
>>
>> -enum i915_ggtt_view_type {
>> -	I915_GGTT_VIEW_NORMAL = 0,
>> -	I915_GGTT_VIEW_ROTATED,
>> -	I915_GGTT_VIEW_PARTIAL,
>> -};
>> -
>>  struct intel_rotation_info {
>>  	struct intel_rotation_plane_info {
>>  		/* tiles */
>> @@ -184,10 +178,16 @@ static inline u64 intel_partial_get_page_offset(const struct intel_partial_info
>>  	return pi->offset_size >> INTEL_PARTIAL_SIZE_BITS;
>>  }
>>
>> +enum i915_ggtt_view_type {
>> +	I915_GGTT_VIEW_NORMAL = 0,
>> +	I915_GGTT_VIEW_ROTATED = sizeof(struct intel_rotation_info),
>> +	I915_GGTT_VIEW_PARTIAL = sizeof(struct intel_partial_info),
>
> One alternative to packing intel_partial_info into a u64 is to use
>
> I915_GGTT_VIEW_PARTIAL = 12, /* ideal sizeof(struct intel_partial_info) */
>
> How horrible is that?

Really horrible, nightmare to debug. :)

You are happy with 12 packed bytes I gather so no need for it?

Regards,

Tvrtko

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