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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