[RFC PATCH] cdc_ncm: support moving the NDP part of the frame to the end of NCM package

[Enrico Mioso <mrkiko.rs@gmail.com>]

The NCM specification as per
http://www.usb.org/developers/docs/devclass_docs/NCM10_012011.zip
does not define a fixed position for different frame parts.
The NCM header is effectively the head of a list of NDPs (NCM datagram
pointers), each of them pointing to a set of ethernet frames.
In general, the NDP can be anywhere after the header.
Some devices however are not quite respecting the specs - as they mandate
the NDP pointers to be after the payload, at the end of the NCM package.
This patch aims to support this scenario, introducing a module parameter
enabling this funcitonality.

Is this approach acceptable?

What does work:
- the module compiles, and seems to cause no crash
What doesn't:
- the device for now will ignore our frames
- I would need some guidance on flags to use with kzalloc.

thank you for the patience, and the review.

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>
---
 drivers/net/usb/cdc_ncm.c   | 30 ++++++++++++++++++++++++++++--
 include/linux/usb/cdc_ncm.h |  1 +
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 8067b8f..a6d0666b 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -60,6 +60,10 @@ static bool prefer_mbim;
 module_param(prefer_mbim, bool, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(prefer_mbim, "Prefer MBIM setting on dual NCM/MBIM functions");
 
+static bool move_ndp_to_end;
+module_param(move_ndp_to_end, bool, S_IRUGO | S_IWUSR);
+MODULE_PARM_DESC(move_ndp_to_end, "Move NDP block to the end of the NCM aggregate");
+
 static void cdc_ncm_txpath_bh(unsigned long param);
 static void cdc_ncm_tx_timeout_start(struct cdc_ncm_ctx *ctx);
 static enum hrtimer_restart cdc_ncm_tx_timer_cb(struct hrtimer *hr_timer);
@@ -684,6 +688,8 @@ static void cdc_ncm_free(struct cdc_ncm_ctx *ctx)
 		ctx->tx_curr_skb = NULL;
 	}
 
+	kfree(ctx->delayed_ndp);
+
 	kfree(ctx);
 }
 
@@ -994,6 +1000,9 @@ static struct usb_cdc_ncm_ndp16 *cdc_ncm_ndp(struct cdc_ncm_ctx *ctx, struct sk_
 		ndpoffset = le16_to_cpu(ndp16->wNextNdpIndex);
 	}
 
+	if ((move_ndp_to_end) && (ctx->delayed_ndp->dwSignature == sign))
+		return ndp16;
+
 	/* align new NDP */
 	cdc_ncm_align_tail(skb, ctx->tx_ndp_modulus, 0, ctx->tx_max);
 
@@ -1008,7 +1017,13 @@ static struct usb_cdc_ncm_ndp16 *cdc_ncm_ndp(struct cdc_ncm_ctx *ctx, struct sk_
 		nth16->wNdpIndex = cpu_to_le16(skb->len);
 
 	/* push a new empty NDP */
-	ndp16 = (struct usb_cdc_ncm_ndp16 *)memset(skb_put(skb, ctx->max_ndp_size), 0, ctx->max_ndp_size);
+	if (!move_ndp_to_end) {
+		ndp16 = (struct usb_cdc_ncm_ndp16 *)memset(skb_put(skb, ctx->max_ndp_size), 0, ctx->max_ndp_size);
+	} else {
+		ndp16 = kzalloc(sizeof(*ndp16), GFP_KERNEL);
+		ctx->delayed_ndp = ndp16;
+	}
+
 	ndp16->dwSignature = sign;
 	ndp16->wLength = cpu_to_le16(sizeof(struct usb_cdc_ncm_ndp16) + sizeof(struct usb_cdc_ncm_dpe16));
 	return ndp16;
@@ -1023,6 +1038,8 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
 	struct sk_buff *skb_out;
 	u16 n = 0, index, ndplen;
 	u8 ready2send = 0;
+	unsigned int skb_prev_len;
+	char *delayed_alloc_ptr;
 
 	/* if there is a remaining skb, it gets priority */
 	if (skb != NULL) {
@@ -1074,7 +1091,8 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
 		ndp16 = cdc_ncm_ndp(ctx, skb_out, sign, skb->len + ctx->tx_modulus + ctx->tx_remainder);
 
 		/* align beginning of next frame */
-		cdc_ncm_align_tail(skb_out,  ctx->tx_modulus, ctx->tx_remainder, ctx->tx_max);
+		if (!move_ndp_to_end)
+			cdc_ncm_align_tail(skb_out,  ctx->tx_modulus, ctx->tx_remainder, ctx->tx_max);
 
 		/* check if we had enough room left for both NDP and frame */
 		if (!ndp16 || skb_out->len + skb->len > ctx->tx_max) {
@@ -1111,6 +1129,14 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev, struct sk_buff *skb, __le32 sign)
 		dev_kfree_skb_any(skb);
 		skb = NULL;
 
+		if ((move_ndp_to_end) && (ctx->delayed_ndp)) {
+			skb_prev_len = skb_out->len;
+			delayed_alloc_ptr = memset(skb_put(skb_out, ctx->max_ndp_size), 0, ctx->max_ndp_size);
+			memcpy(ctx->delayed_ndp, delayed_alloc_ptr, sizeof(struct usb_cdc_ncm_ndp16));
+			kfree(ctx->delayed_ndp);
+			cdc_ncm_align_tail(skb_out,  ctx->tx_modulus, ctx->tx_remainder, ctx->tx_max);
+		}
+
 		/* send now if this NDP is full */
 		if (index >= CDC_NCM_DPT_DATAGRAMS_MAX) {
 			ready2send = 1;
diff --git a/include/linux/usb/cdc_ncm.h b/include/linux/usb/cdc_ncm.h
index 7c9b484..cc02a0d 100644
--- a/include/linux/usb/cdc_ncm.h
+++ b/include/linux/usb/cdc_ncm.h
@@ -93,6 +93,7 @@ struct cdc_ncm_ctx {
 	const struct usb_cdc_mbim_desc *mbim_desc;
 	const struct usb_cdc_mbim_extended_desc *mbim_extended_desc;
 	const struct usb_cdc_ether_desc *ether_desc;
+	struct usb_cdc_ncm_ndp16 *delayed_ndp;
 
 	struct usb_interface *control;
 	struct usb_interface *data;
-- 
2.4.2