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

[Krishna Kurapati PSSNV]

On 10/10/2023 5:47 AM, Maciej Żenczykowski 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...

Hi Maciej,

Can you help clarify what you think here is wrong ?
Since increasing segment size give better throughputs in P2P connections 
that don't necessarily exchange internet data, we need to have the 
felxibility to change the value as per requirement of the application 
using the interface and hence this attempt at adding the configfs parameter.

Regards,
Krishna,