diff --git a/lib/iomap.c b/lib/iomap.c
index 864f2ec..33651f8 100644
--- a/lib/iomap.c
+++ b/lib/iomap.c
@@ -247,9 +247,14 @@ EXPORT_SYMBOL(ioport_unmap);
  * @maxlen: length of the memory to map
  *
  * Using this function you will get a __iomem address to your device BAR.
- * You can access it using ioread*() and iowrite*(). These functions hide
- * the details if this is a MMIO or PIO address space and will just do what
- * you expect from them in the correct way.
+ * If you use pci_iomap() it is recommended you use ioread*() and iowrite*(). If
+ * your device is MMIO-only and as long as future architectures don't break the
+ * assumption that pcio_iomap()-returned cookie can be used by read*()/write*()
+ * then you can use read*()/write*() instead. The ioread*() and iowrite*() 
+ * functions hide the details if this is a MMIO or PIO address space and will
+ * just do what you expect from them in the correct way. Drivers for MMIO-only 
+ * devices would use this only to save themselves from having to call 
+ * &pci_resource_start().
  *
  * @maxlen specifies the maximum length to map. If you want to get access to
  * the complete BAR without checking for its length first, pass %0 here.