[RFC][PATCH] [POWERPC] Allow caching of kmap_atomic page

[Kumar Gala <galak@kernel.crashing.org>]

Skip updating the kmap_pte and flushing the TLB if the pte we
are about to write is the same as the one we wrote last time we
called kmap_atomic for this km_type.

Also expose the flags to allow a caller to specify their own
flags for things like non-cacheable IO memory.

---

This is the starts of some work Ben and I were discussion to allow us to
use kmap_atomic to access a page size region of PCI CFG space when its
provided as direct MMIO.

We also intend to provide a means to preload the TLB for SW managed TLB
machines.

- k

 include/asm-powerpc/highmem.h |   16 +++++++++++++---
 1 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/asm-powerpc/highmem.h b/include/asm-powerpc/highmem.h
index f7b21ee..a50bb00 100644
--- a/include/asm-powerpc/highmem.h
+++ b/include/asm-powerpc/highmem.h
@@ -73,10 +73,12 @@ static inline void kunmap(struct page *page)
  * be used in IRQ contexts, so in some (very limited) cases we need
  * it.
  */
-static inline void *kmap_atomic(struct page *page, enum km_type type)
+static inline void *__kmap_atomic(struct page *page,
+				  enum km_type type, unsigned long flags)
 {
 	unsigned int idx;
 	unsigned long vaddr;
+	pte_t pte;

 	/* even !CONFIG_PREEMPT needs this, for in_atomic in do_page_fault */
 	pagefault_disable();
@@ -88,12 +90,20 @@ static inline void *kmap_atomic(struct page *page, enum km_type type)
 #ifdef HIGHMEM_DEBUG
 	BUG_ON(!pte_none(*(kmap_pte+idx)));
 #endif
-	set_pte_at(&init_mm, vaddr, kmap_pte+idx, mk_pte(page, kmap_prot));
-	flush_tlb_page(NULL, vaddr);
+	pte = mk_pte(page, flags);
+	if (!pte_same(kmap_pte[idx], pte)) {
+		set_pte_at(&init_mm, vaddr, kmap_pte+idx, pte);
+		flush_tlb_page(NULL, vaddr);
+	}

 	return (void*) vaddr;
 }

+static inline void *kmap_atomic(struct page *page, enum km_type type)
+{
+	return __kmap_atomic(page, type, kmap_prot);
+}
+
 static inline void kunmap_atomic(void *kvaddr, enum km_type type)
 {
 #ifdef HIGHMEM_DEBUG
-- 
1.5.3.4