Re: [PATCH 2/2] usb: gadget: ncm: Add support to update wMaxSegmentSize via configfs

From: Maciej Żenczykowski
Date: Mon Oct 09 2023 - 20:38:13 EST

On Mon, Oct 9, 2023 at 5:20 PM Maciej Żenczykowski <maze@xxxxxxxxxx> wrote:
>
> On Mon, Oct 9, 2023 at 5:17 PM Maciej Żenczykowski <maze@xxxxxxxxxx> wrote:
> >
> > On Mon, Oct 9, 2023 at 7:20 AM Krishna Kurapati
> > <quic_kriskura@xxxxxxxxxxx> wrote:
> > >
> > > Currently the NCM driver restricts wMaxSegmentSize that indicates
> > > the datagram size coming from network layer to 1514. However the
> > > spec doesn't have any limitation. For P2P connections over NCM,
> > > increasing MTU helps increasing throughput.
> > >
> > > 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 15014. Set the
> > > default MTU size for the ncm interface during function bind before
> > > network interface is registered allowing MTU to be set in parity
> > > with wMaxSegmentSize.
> > >
> > > Signed-off-by: Krishna Kurapati <quic_kriskura@xxxxxxxxxxx>
> > > ---
> > > drivers/usb/gadget/function/f_ncm.c | 51 +++++++++++++++++++++++++++++
> > > drivers/usb/gadget/function/u_ncm.h | 2 ++
> > > 2 files changed, 53 insertions(+)
> > >
> > > diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
> > > index feccf4c8cc4f..eab297b22200 100644
> > > --- a/drivers/usb/gadget/function/f_ncm.c
> > > +++ b/drivers/usb/gadget/function/f_ncm.c
> > > @@ -103,6 +103,8 @@ static inline struct f_ncm *func_to_ncm(struct usb_function *f)
> > > /* Delay for the transmit to wait before sending an unfilled NTB frame. */
> > > #define TX_TIMEOUT_NSECS 300000
> > >
> > > +#define MAX_DATAGRAM_SIZE 15014
> > > +
> > > #define FORMATS_SUPPORTED (USB_CDC_NCM_NTB16_SUPPORTED | \
> > > USB_CDC_NCM_NTB32_SUPPORTED)
> > >
> > > @@ -1408,6 +1410,7 @@ static int ncm_bind(struct usb_configuration *c, struct usb_function *f)
> > > ncm_opts = container_of(f->fi, struct f_ncm_opts, func_inst);
> > >
> > > if (cdev->use_os_string) {
> > > + ncm_opts->net->mtu = (ncm_opts->max_segment_size - ETH_HLEN);
> > > f->os_desc_table = kzalloc(sizeof(*f->os_desc_table),
> > > GFP_KERNEL);
> > > if (!f->os_desc_table)
> > > @@ -1469,6 +1472,8 @@ static int ncm_bind(struct usb_configuration *c, struct usb_function *f)
> > >
> > > status = -ENODEV;
> > >
> > > + ecm_desc.wMaxSegmentSize = ncm_opts->max_segment_size;
> >
> > I think you need byte swap here.
> >
> > > +
> > > /* allocate instance-specific endpoints */
> > > ep = usb_ep_autoconfig(cdev->gadget, &fs_ncm_in_desc);
> > > if (!ep)
> > > @@ -1569,11 +1574,56 @@ 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,
> > > };
> > >
> > > @@ -1616,6 +1666,7 @@ 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);
> >
> > and not here. ie. max_segment_size should be native endian
> >
> > > 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..d3403cf13f17 100644
> > > --- a/drivers/usb/gadget/function/u_ncm.h
> > > +++ b/drivers/usb/gadget/function/u_ncm.h
> > > @@ -31,6 +31,8 @@ struct f_ncm_opts {
> > > */
> > > struct mutex lock;
> > > int refcnt;
> > > +
> > > + u32 max_segment_size;
> > > };
> > >
> > > #endif /* U_NCM_H */
> > > --
> > > 2.42.0
> > >
> >
> > That said, I don't really follow what this is doing...
>
> Also
>
> static struct usb_cdc_ether_desc ecm_desc = {
> ...
> .wMaxSegmentSize = cpu_to_le16(ETH_FRAME_LEN),
>
> ^ I think this should be deleted now, right? since it's always overwritten?
> And if it isn't always overwritten, that would be a bug I think, cause
> what happens if you bring up 2 ncm devices and only change the setting
> on the 1st?

One last thing...

static int ncm_unwrap_ntb(struct gether *port,
...
unsigned frame_max = le16_to_cpu(ecm_desc.wMaxSegmentSize);

^ is this a problem now if we have >1 gadget?
how does it work then?