kernel: dma: Soften the dma_map_resources() interest

It is not clear who should remap the MMIO resources when using an
external DMA engine.

Signed-off-by: Miquel Raynal <[email protected]>

slides/kernel-driver-development-dma/kernel-driver-development-dma.tex

@@ -258,8 +258,8 @@ \subsection{Kernel APIs for DMA}
 
 \begin{frame}[fragile]
   \frametitle{\code{dma-mapping}: Setting up streaming I/O mappings}
-  Physical addresses with MMIO registers shall always be remapped
-  (otherwise it would not work when they are accessed through an IO-MMU)
+  Physical addresses with MMIO registers might need to be remapped in order to
+  be accessed through an IO-MMU:
   \vfill
 \begin{minted}[fontsize=\scriptsize]{c}
 #include <linux/dma-mapping.h>
@@ -312,7 +312,7 @@ \subsection{Kernel APIs for DMA}
   \item The CPU shall only access the buffer after unmapping!
   \item If however the same memory region has to be used for several DMA
     transfers, the same mapping can be kept in place. In this case the
-    data must be synchronized before CPU access:
+    data must be synchronized before access:
     \begin{itemize}
     \item The CPU needs to access the data:
 \begin{minted}[fontsize=\small]{c}