KSYNCMAP
p = kmalloc(sizeof(*p) + KSYNCMAP * sizeof(u32), M_DRM, GFP_KERNEL);
next = kzalloc(sizeof(*next) + KSYNCMAP * sizeof(next),
#define SHIFT ilog2(KSYNCMAP)
#define MASK (KSYNCMAP - 1)
BUILD_BUG_ON_NOT_POWER_OF_2(KSYNCMAP);
BUILD_BUG_ON(KSYNCMAP > BITS_PER_BYTE * sizeof((*root)->bitmap));
BUILD_BUG_ON(KSYNCMAP < I915_NUM_ENGINES);