Hi,
I have some questions about the HID USB driver in Linux. I have
a Linux From Scratch LFS 6.2 system, with many additions
according to BLFS 6.2. I am using the 2.6.16.27 kernel.
I am communicating with a Cypress PSoc device, part number
CY8C24894, over usb. The device is configured as an HID with
bInterval of 1ms. I am communicating over interrupt in and
out endpoints. Here is the relevant portion from a lsusb
listing which shows how the interrupt endpoints are set up:
---------------------------------------------------------------
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x81 EP 1 IN
bmAttributes 3
Transfer Type Interrupt
Synch Type None
Usage Type Data
wMaxPacketSize 0x0040 1x 64 bytes
bInterval 1
Endpoint Descriptor:
bLength 7
bDescriptorType 5
bEndpointAddress 0x02 EP 2 OUT
bmAttributes 3
Transfer Type Interrupt
Synch Type None
Usage Type Data
wMaxPacketSize 0x0040 1x 64 bytes
bInterval 1
Device Status: 0x0000
(Bus Powered)
-------------------------------------------------------
As you can see the PSoc device is configured with one
interrupt in endpoint and one interrupt out endpoint,
with maximum packet size of 64 bytes and 1ms polling
interval.
My program, which is written in C and uses libusb
calls, is continually sending out usb interrupt
messages with 64 bytes each, and after each message
is sent the program requests a reply from the
PSoc device, which consists of a packet of 62
bytes. I am seeing about an 8ms round trip for
this process, i.e. from the time one 64-byte packet
is sent, the 62-byte reply is received, and the
next 64-byte packet is sent out.
I have enabled debugfs and I've used usbmon to
capture a typical cycle of the usb transactions.
Here is a representative set, where I have added
numbers at the beginning of each line for
reference:
1. dfb324c0 1787068080 S Io:002:02 -115 64 = 043e0100 0c000e0e 00000000
00000000 00000000 00000000 00000000 00000000
2. dfb324c0 1787069920 C Io:002:02 0 64 >
3. dfb324c0 1787069957 S Ii:002:01 -115 62 <
4. dfb324c0 1787072920 C Ii:002:01 0 62 = 01000c00 0e0e0000 00000000
00000000 00000000 00000000 00000000 00000000
5. dfb324c0 1787076043 S Io:002:02 -115 64 = 043e0100 0c001c0e 00000000
00000000 00000000 00000000 00000000 00000000
I don't completely understand this report, so any help with
interpretation will be appreciated. I think it is clear that
line 1 is where the PC sends an interrupt out message to
the PSoc device consisting of 64 bytes. This is accomplished
in the program by a libusb usb_interrupt_write() call.
Immediately after that call, the program starts looking
for a 62-byte reply from the PSoc, via a usb_interrupt_read()
call. I am assuming that in line 2 the PC is sending out
a message that it is looking for this reply. This occurs
1.84ms after the 64-byte out transaction. I'm not sure
what is happening in line 3, I am guessing that the PSoc
device is notifying the PC that it has 62 bytes ready to
send via an in transaction, or maybe it is telling the
PC that the reply is not yet ready. Then on line 4, 3ms after it
was requested in line 2, the 62-byte in transaction is
received from the PSoc device. Finally in line 5, 3.12ms
after the in transaction is received, the next 64-byte
out transaction in the series is sent.
My basic question regards some of the delays in this
process. In my program, there should be very little
delay between the time a 62-byte in transaction is
received, and the time when the next 64-byte out
transaction is sent. But it appears that it takes over
3ms before the next out transaction is sent. I wouldn't
expect this delay to be more than 1ms. Does anyone
have an idea why it takes so long to send the next
message?
My understanding is that with a 1ms polling interval,
all usb requested actions should be initiated in at
most 1ms. Does the HID driver strictly enforce this
timing?
Also, my program immediately starts looking for
a 62-byte reply after it sends a 64-byte out
message, however it appears that the usb is
waiting 1.84ms before requesting this reply.
Again, any insight as to why this takes so long?
The 3ms delay between when the 62-byte in transaction
is requested in line 2, and when it is actually
received in line 4, is understandable because this
is essentially the time it takes the PSoc device
to process the data it has received and to formulate
its reply message, plus some extra delay to coincide
with the next usb frame sync, I would imagine.
Are the libusb calls good to use for this
application, or should I be using another interface
to the usb system? Any help with these timing
issues or suggestions about how I could speed up
the usb communications will be very much appreciated.
Thanks,
Richard
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