+/*
+ * Try to write quickly with an atomic kmap. Return true on success.
+ *
+ * If this fails (which includes a partial write), we'll redo the whole
+ * thing with the slow version.
+ *
+ * This is a workaround for the low performance of iounmap (approximate
+ * 10% cpu cost on normal 3D workloads). kmap_atomic on HIGHMEM kernels
+ * happens to let us map card memory without taking IPIs. When the vmap
+ * rework lands we should be able to dump this hack.
+ */
+static inline int fast_user_write(unsigned long pfn, char __user *user_data,
+ int l, int o)
+{
+#ifdef CONFIG_HIGHMEM
+ unsigned long unwritten;
+ char *vaddr_atomic;
+
+ vaddr_atomic = kmap_atomic_pfn(pfn, KM_USER0);
+#if WATCH_PWRITE
+ DRM_INFO("pwrite i %d o %d l %d pfn %ld vaddr %p\n",
+ i, o, l, pfn, vaddr_atomic);
+#endif
+ unwritten = __copy_from_user_inatomic_nocache(vaddr_atomic + o, user_data, l);
+ kunmap_atomic(vaddr_atomic, KM_USER0);
+ return !unwritten;
+#else
+ return 0;
+#endif
+}
+