Re: [RFC] usb: gadget: ncm: Add support to configure wMaSegmentSize
From: Krishna Kurapati PSSNV
Date: Mon Feb 13 2023 - 20:11:49 EST
On 2/14/2023 3:17 AM, Maciej Żenczykowski wrote:
This isn't a review per say - just some loose comments.
On Sun, Feb 12, 2023 at 9:57 AM Krishna Kurapati
<quic_kriskura@xxxxxxxxxxx> wrote:
Currently the NCM driver restricts wMasxSegmentSize that indicates
there's a number of spelling mistakes, here, and in the commit title
the datagram size coming from network layer to 1514. However the
spec doesn't have any limitation.
Add support to configure this value before configfs symlink is
created. Also since the NTB Out/In buffer sizes are fixed at 16384
bytes, limit the segment size to an upper cap of 8192 bytes so that
at least 2 packets can be aggregated.
Hi Maciej, Thanks for the comments:
I've experimented with increasing mtu to boost performance before
(have some half-baked patches/scripts somewhere).
And while it did improve point-to-point performance, it wasn't
actually useful for any real world use cases,
as internet mtu is simply never above 1500.
My use case is actually a P2P connection and data is not strictly
internet data. So I was trying to change the MTU and max segment size
both at the same time to increase throughput.
Note that you cannot simply receive, aggregate (lro/gro) and forward
aggregated packets without splitting them back up.
A change like this to be useful would require negotiating some sort of
gso capabilities between the two devices
(and thus extending the NCM standard). I've been meaning to do
this... but time...
I have very little knowledge of GSO capabilities. Will get back and read
up on them.
Is it something to do with splitting/aggregation of packets at Phy layer
(segments of NTB) before giving them back to network stack ?
Signed-off-by: Krishna Kurapati <quic_kriskura@xxxxxxxxxxx>
---
drivers/usb/gadget/function/f_ncm.c | 55 +++++++++++++++++++++++++++++
drivers/usb/gadget/function/u_ncm.h | 1 +
2 files changed, 56 insertions(+)
diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
index 424bb3b666db..1969e276017f 100644
--- a/drivers/usb/gadget/function/f_ncm.c
+++ b/drivers/usb/gadget/function/f_ncm.c
@@ -118,6 +118,12 @@ static inline unsigned ncm_bitrate(struct usb_gadget *g)
/* Delay for the transmit to wait before sending an unfilled NTB frame. */
#define TX_TIMEOUT_NSECS 300000
+/*
+ * Currently the max NTB Buffer size is set to 16384. For atleast 2 packets
+ * to be aggregated, the size of datagram must at max be 8192.
+ */
+#define MAX_DATAGRAM_SIZE 8192
From what I recall, there's a fair bit of overhead, and 8192 x2
doesn't actually fit in 16384...
That said... is it reasonable to require 2 to fit? why? what's wrong
with 15000?
Thanks for this, I missed the header being present here. I was under the
impression that we might need to aggregate at least 2 packets. So made
it (fixed_out_len/2). But seems like we can even put it to 15000 as
well. Will remove this check in configfs store func.
+
#define FORMATS_SUPPORTED (USB_CDC_NCM_NTB16_SUPPORTED | \
USB_CDC_NCM_NTB32_SUPPORTED)
@@ -1440,6 +1446,7 @@ static int ncm_bind(struct usb_configuration *c, struct usb_function *f)
*/
if (!ncm_opts->bound) {
mutex_lock(&ncm_opts->lock);
+ ncm_opts->net->mtu = (ncm_opts->max_segment_size - 14);
this should use a constant, ETH_HLEN probably
Sure, will make this a constant.
gether_set_gadget(ncm_opts->net, cdev->gadget);
status = gether_register_netdev(ncm_opts->net);
mutex_unlock(&ncm_opts->lock);
@@ -1484,6 +1491,8 @@ static int ncm_bind(struct usb_configuration *c, struct usb_function *f)
status = -ENODEV;
+ ecm_desc.wMaxSegmentSize = ncm_opts->max_segment_size;
+
Curious... wasn't this set previously?
The one set above was MTU size before register net device. This here is
to update the required segment size to ecm_descriptor before requesting
ep's and providing descs to composite.
Regards,
Krishna,
/* allocate instance-specific endpoints */
ep = usb_ep_autoconfig(cdev->gadget, &fs_ncm_in_desc);
if (!ep)
@@ -1586,11 +1595,56 @@ USB_ETHERNET_CONFIGFS_ITEM_ATTR_QMULT(ncm);
/* f_ncm_opts_ifname */
USB_ETHERNET_CONFIGFS_ITEM_ATTR_IFNAME(ncm);
+static ssize_t ncm_opts_max_segment_size_show(struct config_item *item,
+ char *page)
+{
+ struct f_ncm_opts *opts = to_f_ncm_opts(item);
+ u32 segment_size;
+
+ mutex_lock(&opts->lock);
+ segment_size = opts->max_segment_size;
+ mutex_unlock(&opts->lock);
+
+ return sprintf(page, "%u\n", segment_size);
+}
+
+static ssize_t ncm_opts_max_segment_size_store(struct config_item *item,
+ const char *page, size_t len)
+{
+ struct f_ncm_opts *opts = to_f_ncm_opts(item);
+ int ret;
+ u32 segment_size;
+
+ mutex_lock(&opts->lock);
+ if (opts->refcnt) {
+ ret = -EBUSY;
+ goto out;
+ }
+
+ ret = kstrtou32(page, 0, &segment_size);
+ if (ret)
+ goto out;
+
+ if (segment_size > MAX_DATAGRAM_SIZE) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ opts->max_segment_size = segment_size;
+ ret = len;
+out:
+ mutex_unlock(&opts->lock);
+ return ret;
+}
+
+CONFIGFS_ATTR(ncm_opts_, max_segment_size);
+
static struct configfs_attribute *ncm_attrs[] = {
&ncm_opts_attr_dev_addr,
&ncm_opts_attr_host_addr,
&ncm_opts_attr_qmult,
&ncm_opts_attr_ifname,
+ &ncm_opts_attr_max_segment_size,
NULL,
};
@@ -1633,6 +1687,7 @@ static struct usb_function_instance *ncm_alloc_inst(void)
kfree(opts);
return ERR_CAST(net);
}
+ opts->max_segment_size = cpu_to_le16(ETH_FRAME_LEN);
INIT_LIST_HEAD(&opts->ncm_os_desc.ext_prop);
descs[0] = &opts->ncm_os_desc;
diff --git a/drivers/usb/gadget/function/u_ncm.h b/drivers/usb/gadget/function/u_ncm.h
index 5408854d8407..fab99d997476 100644
--- a/drivers/usb/gadget/function/u_ncm.h
+++ b/drivers/usb/gadget/function/u_ncm.h
@@ -31,6 +31,7 @@ struct f_ncm_opts {
*/
struct mutex lock;
int refcnt;
+ u32 max_segment_size;
};
#endif /* U_NCM_H */
--
2.39.0