[PATCH] gfp: further document GFP_DMA32

[Miles Chen <miles.chen@mediatek.com>]

kmalloc(..., GFP_DMA32) does not return DMA32 memory
because the DMA32 kmalloc cache array is not implemented.
(Reason: there is no such user in kernel).

Put a short comment about this so people can understand this by
reading the comment.

[1] https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031696.html

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
---
 include/linux/gfp.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index b976c4177299..c7bfaa9f3903 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -302,7 +302,9 @@ struct vm_area_struct;
  * lowest zone as a type of emergency reserve.
  *
  * %GFP_DMA32 is similar to %GFP_DMA except that the caller requires a 32-bit
- * address.
+ * address. Note that kmalloc(..., GFP_DMA32) does not return DMA32 memory
+ * because the DMA32 kmalloc cache array is not implemented.
+ * (Reason: there is no such user in kernel).
  *
  * %GFP_HIGHUSER is for userspace allocations that may be mapped to userspace,
  * do not need to be directly accessible by the kernel but that cannot
-- 
2.18.0