Add fast branch that skips serial cache entirely

libgphoto2_port/serial/unix.c

@@ -517,7 +517,7 @@ gp_port_serial_read (GPPort *dev, char *bytes, int size)
         FD_ZERO (&readfs);
         FD_SET (dev->pl->fd, &readfs);
 
-        while (readen < size) {
+        while (size > 0) {
 
 		/* Set timeout value within input loop */
                 timeout.tv_usec = (dev->timeout % 1000) * 1000;
@@ -564,25 +564,30 @@ gp_port_serial_read (GPPort *dev, char *bytes, int size)
 			/* Ok, we read 1 byte and it is 0xff */
 			/* FALLTHROUGH */
 		    }
+		} else if (dev->pl->cachep == dev->pl->cachee && size >= sizeof(dev->pl->cache)) {
+			/* We're trying to read a chunk larger than the cache and the cache is empty.
+			   In this case, skip the cache entirely and read as much as we can directly into the destination. */
+			now = read (dev->pl->fd, bytes, size);
+			if (now < 0)
+				return GP_ERROR_IO_READ;
 		} else {
 			if (dev->pl->cachep == dev->pl->cachee) {
-				/* Cache is empty, fill it up. */
+				/* We're reading only a few bytes and the cache is empty; fill it up. */
 				now = read (dev->pl->fd, dev->pl->cache, sizeof(dev->pl->cache));
-				if (now < 0) {
-					/* nothing more to read from the actual port */
+				if (now < 0)
 					return GP_ERROR_IO_READ;
-				}
 				/* Reset cache pointers */
 				dev->pl->cachep = dev->pl->cache;
 				dev->pl->cachee = dev->pl->cache + now;
 			}
 			/* read up to the required chunk size from cache */
-			now = MIN(size - readen, dev->pl->cachee - dev->pl->cachep);
+			now = MIN(size, dev->pl->cachee - dev->pl->cachep);
 			memcpy(bytes, dev->pl->cachep, now);
 			dev->pl->cachep += now;
 		}
 		bytes += now;
 		readen += now;
+		size -= readen;
         }
 
         return readen;