Matching 'struct usb_config' with usbconfig

[Hans Petter Selasky]

On 2019-11-26 00:21, Farhan Khan wrote:
> Hi all,
> 
> I have a clarification question about the USB protocol.
> 
> I ran "lsusb -v" (same as usbconfig dump_info) and saw that a device's wMaxPacketSize was 0x0200 (512 bytes). However, the "struct usb_config" seems to set the bufsize field as 16384 (16 * 1024). What is the reason for this difference? Also, why would the number of usb_config Endpoints and the usb_config struct differ?

Hi,

The buffer size is usually larger to avoid buffer underruns, but should 
be a multiplum of the wMaxPacketSize. We have an own flag for that, 
proxy buffer.

usb_config is usually a sub-set of the endpoints the device has, and for 
sake of double buffering you may repeat the same entry.

--HPS

> 
> Full context, the device is a Realtek RTL8188EU. The USB configuration variable is rtwn_config_common, as defined in sys/dev/rtwn/usb/rtwn_usb_ep.c. The bufsize is set to RTWN_USB_TXBUFSZ, which is defined as:
> #define RTWN_USB_TXBUFSZ (16 * 1024)
> 
> Also, note that there are 3 Endpoints, but 5 elements in the rtwn_config_common structure. What explains this?
> 
> Thanks! And see configuration below:
> 
> -----BEGIN COMMAND BLOCK-----
> Bus 003 Device 004: ID 0bda:8179 Realtek Semiconductor Corp. RTL8188EUS 802.11n Wireless Network Adapter
> Device Descriptor:
>    bLength                18
>    bDescriptorType         1
>    bcdUSB               2.00
>    bDeviceClass            0 (Defined at Interface level)
>    bDeviceSubClass         0
>    bDeviceProtocol         0
>    bMaxPacketSize0        64
>    idVendor           0x0bda Realtek Semiconductor Corp.
>    idProduct          0x8179 RTL8188EUS 802.11n Wireless Network Adapter
>    bcdDevice            0.00
>    iManufacturer           1
>    iProduct                2
>    iSerial                 3
>    bNumConfigurations      1
>    Configuration Descriptor:
>      bLength                 9
>      bDescriptorType         2
>      wTotalLength           39
>      bNumInterfaces          1
>      bConfigurationValue     1
>      iConfiguration          0
>      bmAttributes         0xa0
>        (Bus Powered)
>        Remote Wakeup
>      MaxPower              500mA
>      Interface Descriptor:
>        bLength                 9
>        bDescriptorType         4
>        bInterfaceNumber        0
>        bAlternateSetting       0
>        bNumEndpoints           3
>        bInterfaceClass       255 Vendor Specific Class
>        bInterfaceSubClass    255 Vendor Specific Subclass
>        bInterfaceProtocol    255 Vendor Specific Protocol
>        iInterface              0
>        Endpoint Descriptor:
>          bLength                 7
>          bDescriptorType         5
>          bEndpointAddress     0x81  EP 1 IN
>          bmAttributes            2
>            Transfer Type            Bulk
>            Synch Type               None
>            Usage Type               Data
>          wMaxPacketSize     0x0200  1x 512 bytes
>          bInterval               0
>        Endpoint Descriptor:
>          bLength                 7
>          bDescriptorType         5
>          bEndpointAddress     0x02  EP 2 OUT
>          bmAttributes            2
>            Transfer Type            Bulk
>            Synch Type               None
>            Usage Type               Data
>          wMaxPacketSize     0x0200  1x 512 bytes
>          bInterval               0
>        Endpoint Descriptor:
>          bLength                 7
>          bDescriptorType         5
>          bEndpointAddress     0x03  EP 3 OUT
>          bmAttributes            2
>            Transfer Type            Bulk
>            Synch Type               None
>            Usage Type               Data
>          wMaxPacketSize     0x0200  1x 512 bytes
>          bInterval               0
> -----END COMMAND BLOCK-----
> 
> 
> 
> --
> Farhan Khan
> PGP Fingerprint: 1312 89CE 663E 1EB2 179C 1C83 C41D 2281 F8DA C0DE
> 
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