Re: [PATCH RFC] 2/2 huawei_cdc_ncm: introduce new TX ncm stack

Hi Oliver.
Thank you for your patience, and review. I apreciated it very much.

On Thu, 25 Jun 2015, Oliver Neukum wrote:

Date: Thu, 25 Jun 2015 11:49:29
From: Oliver Neukum <oneukum@suse.com>
To: Enrico Mioso <redacted>
Cc: linux-usb@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [PATCH RFC] 2/2 huawei_cdc_ncm: introduce new TX ncm stack

On Tue, 2015-06-23 at 00:32 +0200, Enrico Mioso wrote:

quoted

This patch introduces a new NCM tx engine, able to operate in standard-
and huawei-style mode.
In the first case, the NDP is disposed after the initial headers and
before any datagram.

What works:
- is able to communicate with compliant NCM devices:
	I tested this with a board running the Linux g_ncm gadget driver.

What doesn't work:
- After some packets I start gettint LOTS of EVENT_RX_MEMORY from usbnet,
	which fails to allocate an RX SKB in rx_submit(). Don't understand why,
	any suggestion would be very welcome.

The tx_fixup function given here, even if actually working, should be
considered as an example: the NCM manager is used here simulating the
cdc_ncm.c behaviour.

Signed-off-by: Enrico Mioso <redacted>
---
 drivers/net/usb/huawei_cdc_ncm.c | 187 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 185 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/huawei_cdc_ncm.c b/drivers/net/usb/huawei_cdc_ncm.c
index 735f7da..217802a 100644
--- a/drivers/net/usb/huawei_cdc_ncm.c
+++ b/drivers/net/usb/huawei_cdc_ncm.c

@@ -29,6 +29,35 @@
 #include <linux/usb/cdc-wdm.h>
 #include <linux/usb/cdc_ncm.h>

+/* NCM management operations: */
+
+/* NCM_INIT_FRAME: prepare for a new frame.
+ * NTH16 header is written to output SKB, NDP data is reset and last
+ * committed NDP pointer set to NULL.
+ * Now, data may be added to this NCM package.
+ */
+#define NCM_INIT_FRAME		1
+
+/* NCM_UPDATE_NDP: adds data to an NDP structure, hence updating it.
+ * Some checks are performed to be sure data fits in, respecting device and
+ * spec constrains.
+ * Normally the NDP is kept in memory and committed to the SKB only when
+ * requested. However, calling this "method" after NCM_COMMIT_NDP, causes it to
+ * work directly on the already committed SKB copy. this allows for flexibility
+ * in frame ordering.
+ */
+#define NCM_UPDATE_NDP		2
+
+/* Write NDP: commits NDP to output SKB.
+ * This method should be called only once per frame.
+ */
+#define NCM_COMMIT_NDP		3
+
+/* Finalizes NTH16 header: to be called when working in
+ * update-already-committed mode.
+ */
+#define NCM_FINALIZE_NTH	5
+
 /* Driver data */
 struct huawei_cdc_ncm_state {
 	struct cdc_ncm_ctx *ctx;

@@ -36,6 +65,16 @@ struct huawei_cdc_ncm_state {
 	struct usb_driver *subdriver;
 	struct usb_interface *control;
 	struct usb_interface *data;
+
+	/* Keeps track of where data starts and ends in SKBs. */
+	int data_start;
+	int data_len;
+
+	/* Last committed NDP for post-commit operations. */
+	struct usb_cdc_ncm_ndp16 *skb_ndp;
+
+	/* Non-committed NDP */
+	struct usb_cdc_ncm_ndp16 *ndp;
 };

 static int huawei_cdc_ncm_manage_power(struct usbnet *usbnet_dev, int on)

@@ -53,6 +92,149 @@ static int huawei_cdc_ncm_manage_power(struct usbnet *usbnet_dev, int on)
 	return 0;
 }

+/* huawei_ncm_mgmt: flexible TX NCM manager.
+ *
+ * Once a non-zero status value is rturned, current frame should be discarded
+ * and operations restarted from scratch.
+ */

Is there any advantage in keeping this in a single function?

I did this choice in the light of the fact I think the tx_fixup function will 
become more complex than it is now, when aggregating frames.
I answer here your other message to make it more convenient to read: my 
intention for the tx_fixup function would be to:
- aggregate frames
- send them out when:
 	- a timer expires
 	OR
 	- we have enough data in the aggregate, and cannot add more.

This is something done in cdc_ncm.c for example.
But here I have a question: by reading the comment in file 
drivers/net/usb/rndis_host.c at line 572, there seem to be different opinions 
in this matter.
What to do ?

quoted

+int
+huawei_ncm_mgmt(struct usbnet *dev,
+		struct huawei_cdc_ncm_state *drvstate, struct sk_buff *skb_out, int mode) {
+	struct usb_cdc_ncm_nth16 *nth16 = (struct usb_cdc_ncm_nth16 *)skb_out->data;
+	struct cdc_ncm_ctx *ctx = drvstate->ctx;
+	struct usb_cdc_ncm_ndp16 *ndp16 = NULL;
+	int ret = -EINVAL;
+	u16 ndplen, index;
+
+	switch (mode) {
+	case NCM_INIT_FRAME:
+
+		/* Write a new NTH16 header */
+		nth16 = (struct usb_cdc_ncm_nth16 *)memset(skb_put(skb_out, sizeof(struct usb_cdc_ncm_nth16)), 0, sizeof(struct usb_cdc_ncm_nth16));
+		if (!nth16) {
+			ret = -EINVAL;
+			goto error;
+		}
+
+		/* NTH16 signature and header length are known a-priori. */
+		nth16->dwSignature = cpu_to_le32(USB_CDC_NCM_NTH16_SIGN);
+		nth16->wHeaderLength = cpu_to_le16(sizeof(struct usb_cdc_ncm_nth16));
+
+		/* TX sequence numbering */
+		nth16->wSequence = cpu_to_le16(ctx->tx_seq++);
+
+		/* Forget about previous SKB NDP */
+		drvstate->skb_ndp = NULL;

This is probably better done after you know you cannot fail.

Sure. Thank you.

quoted

+
+		/* Allocate a new NDP */
+		ndp16 = kzalloc(ctx->max_ndp_size, GFP_NOIO);

Where is this freed?

The intention wqas to free it in the NCM_COMMIT_NDP case.
Infact after allocating the pointer, I make a copy of it in the driver state 
(drvstate) variable, and get back to it later.
Is this wrong?

quoted

+		if (!ndp16)
+			return ret;
+
+		/* Prepare a new NDP to add data on subsequent calls. */
+		drvstate->ndp = memset(ndp16, 0, ctx->max_ndp_size);

Either kzalloc() or memset(). Using both never makes sense.

True. Sorry - I knew that the "z" in kzalloc stood for ZERO and also looked at 
the code, but I forgot it at some point.

quoted

+
+		/* Initial NDP length */
+		ndp16->wLength = cpu_to_le16(sizeof(struct usb_cdc_ncm_ndp16) + sizeof(struct usb_cdc_ncm_dpe16));
+
+		/* Frame signature: to be reworked in general. */
+			ndp16->dwSignature = cpu_to_le32(USB_CDC_NCM_NDP16_NOCRC_SIGN);
+
+			ret = 0;
+			break;
+
+	case NCM_UPDATE_NDP:
+
+		if (drvstate->skb_ndp) {
+			ndp16 = drvstate->skb_ndp;
+		} else {
+			ndp16 = drvstate->ndp;
+
+			/* Do we have enough space for the data? */
+			if (skb_out->len + ctx->max_ndp_size > ctx->tx_max)
+				goto error;
+		}
+
+		/* Calculate frame number within this NDP */
+		ndplen = le16_to_cpu(ndp16->wLength);
+		index = (ndplen - sizeof(struct usb_cdc_ncm_ndp16)) / sizeof(struct usb_cdc_ncm_dpe16) - 1;
+
+		if (index >= CDC_NCM_DPT_DATAGRAMS_MAX)
+			goto error;
+
+		/* tx_max shouldn't be exceeded after committing. */
+		if (ndplen + sizeof(struct usb_cdc_ncm_dpe16) > ctx->tx_max)
+				goto error;
+
+		/* Adding a DPT entry. */
+		ndp16->dpe16[index].wDatagramLength = cpu_to_le16(drvstate->data_len);
+		ndp16->dpe16[index].wDatagramIndex = cpu_to_le16(drvstate->data_start);
+		ndp16->wLength = cpu_to_le16(ndplen + sizeof(struct usb_cdc_ncm_dpe16));
+
+		ret = 0;
+		break;
+
+	case NCM_COMMIT_NDP:
+
+		if (drvstate->skb_ndp)
+			goto error;	/* Call this only once please. */
+
+		ndp16 = drvstate->ndp;
+
+		nth16->wNdpIndex = cpu_to_le16(skb_out->len);
+
+		/* "commit" NDP */
+		drvstate->skb_ndp = memcpy(skb_put(skb_out, ctx->max_ndp_size), ndp16, ctx->max_ndp_size);
+
+		kfree(ndp16);
+		ndp16 = NULL;
+		drvstate->ndp = NULL;
+
+	case NCM_FINALIZE_NTH:
+
+		/* Finalize NTH16 header, setting it's block length */
+		nth16->wBlockLength = cpu_to_le16(skb_out->len);
+
+		ret = 0;
+		break;
+	default:
+		break;
+	}
+
+error:

The purpose of a label is kind of defeated by having a conditional after
it.

The purpose here was to be sure to deallocate everything could needed to, even 
reaching the label from multiple places.
I'll look at it again now.

quoted

+	if (ret)
+		kfree(drvstate->ndp);
+	return ret;
+}

	Regards
		Oliver

Thank you very much Oliver.