[iobuf] Relax alignment requirement for small I/O buffers

iPXE currently aligns all I/O buffers on a 2kB boundary.  This is
overkill for transmitted packets, which are typically much smaller
than 2kB.

Align I/O buffers on their own size.  This reduces the alignment
requirement for small buffers, while preserving the guarantee that I/O
buffers will never cross boundaries that might cause problems for some
DMA engines.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/iobuf.c

 FILE_LICENCE ( GPL2_OR_LATER );
 
 #include <stdint.h>
+#include <strings.h>
 #include <errno.h>
 #include <ipxe/malloc.h>
 #include <ipxe/iobuf.h>
  */
 struct io_buffer * alloc_iob ( size_t len ) {
 	struct io_buffer *iobuf = NULL;
+	size_t align;
 	void *data;
 
 	/* Pad to minimum length */
 	if ( len < IOB_ZLEN )
 		len = IOB_ZLEN;
 
-	/* Align buffer length */
-	len = ( len + __alignof__( *iobuf ) - 1 ) &
-		~( __alignof__( *iobuf ) - 1 );
-	
+	/* Align buffer length to ensure that struct io_buffer is aligned */
+	len = ( len + __alignof__ ( *iobuf ) - 1 ) &
+		~( __alignof__ ( *iobuf ) - 1 );
+
+	/* Align buffer on its own size to avoid potential problems
+	 * with boundary-crossing DMA.
+	 */
+	align = ( 1 << fls ( len - 1 ) );
+
 	/* Allocate memory for buffer plus descriptor */
-	data = malloc_dma ( len + sizeof ( *iobuf ), IOB_ALIGN );
+	data = malloc_dma ( len + sizeof ( *iobuf ), align );
 	if ( ! data )
 		return NULL;
 
 	return iobuf;
 }
 
+
 /**
  * Free I/O buffer
  *

src/include/ipxe/iobuf.h

 #include <assert.h>
 #include <ipxe/list.h>
 
-/**
- * I/O buffer alignment
- *
- * I/O buffers allocated via alloc_iob() are guaranteed to be
- * physically aligned to this boundary.  Some cards cannot DMA across
- * a 4kB boundary.  With a standard Ethernet MTU, aligning to a 2kB
- * boundary is sufficient to guarantee no 4kB boundary crossings.  For
- * a jumbo Ethernet MTU, a packet may be larger than 4kB anyway.
- */
-#define IOB_ALIGN 2048
-
 /**
  * Minimum I/O buffer length
  *