drm/dsi: use stack buffer in mipi_dsi_dcs_write()

Currently the function heap allocates when we have any payload. Where in
many case the payload is 1 byte - ouch.

>From casual observation, vast majority of the payloads are smaller than
8 bytes - so use a stack array tx[8] to avoid the senseless kmalloc and
kfree dance.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200505160329.2976059-1-emil.l.velikov@gmail.com

drivers/gpu/drm/drm_mipi_dsi.c

@@ -748,26 +748,26 @@ ssize_t mipi_dsi_dcs_write(struct mipi_dsi_device *dsi, u8 cmd,
 {
 	ssize_t err;
 	size_t size;
+	u8 stack_tx[8];
 	u8 *tx;
 
-	if (len > 0) {
-		size = 1 + len;
-
+	size = 1 + len;
+	if (len > ARRAY_SIZE(stack_tx) - 1) {
 		tx = kmalloc(size, GFP_KERNEL);
 		if (!tx)
 			return -ENOMEM;
-
-		/* concatenate the DCS command byte and the payload */
-		tx[0] = cmd;
-		memcpy(&tx[1], data, len);
 	} else {
-		tx = &cmd;
-		size = 1;
+		tx = stack_tx;
 	}
 
+	/* concatenate the DCS command byte and the payload */
+	tx[0] = cmd;
+	if (data)
+		memcpy(&tx[1], data, len);
+
 	err = mipi_dsi_dcs_write_buffer(dsi, tx, size);
 
-	if (len > 0)
+	if (tx != stack_tx)
 		kfree(tx);
 
 	return err;