Say I have an USB device, a camera for instance, and I would like to load the image sequence captured by the camera to the host, using libusb API.
It is not clear to me for the following points:
How is the IN Endpoint on the device populated? Is it always the full image data of one frame (and optionally plus some status data)?
libusb_bulk_transfer() has a parameter length to specify how long is the data the host wants to read IN, and another parameter transferred indicating how much data actually had been transferred. The question is: should I always request the same amount of data that the IN Endpoint would send? If so, then what would be the case where transferred was smaller than length?
How is it determined how much data would be sent by the In Endpoint upon each transfer request?
Related
With a control PC, I am addressing a R&S ESPI Receiver device to perform a frequency scan and return the measurement results back via BAT-EMC control software and a NI GPIB-USB controller in between. My target is to track the binary measurement data (Definite Length Block Data according to IEEE 488.2) sent to the control PC to understand how the device is deciding on the byte size of each binary block sent.
The trace shows that binary blocks are sent with no consistent pattern or rule!
E.g, running the same scan with the same frequency range and step twice may result in a different distribution of the measurement values' bytes on binary blocks (and possibly different total number of blocks sent), although the amount of data delivered is the same.
Any help to figure out how the device and control software are communicating the measurement data?
PS: The NI trace at the level of GPIB controller is not showing that the control software is specifying a byte size when querying for the next block, neither is the instrument sending this piece of info when it is issuing a service request so that it is queried for more available data by the software (according to the trace).
Make sure that you are giving enough time for the instrument to respond. Possibly you are sending commands from the PC which would assert the ATN line and interrupt the response. You should be able to configure the instrument to send one result. Configure the instrument as a listener and talker and set the instrument to send only one response per trigger. Then send the group execute trigger (GET) and read the results off the bus. When it’s done measure how long it took for that packet to get sent. If you are sending triggers before the full response you will be terminating the output stream. I suspect this because the streams are randomly different.
I’m just starting to learn GPIB so please write back what happened.
Digging around with/for HID reports, I ran into a strange problem within a USB HID device. I'm implementing an HID class device and have based my program on the HID USB program supplied by Keil. Some codes have been changed in this project and it seems working fine with 32 bytes input and 32 bytes output reports. Somehow, after thousands times data transferring, the Endpoint 1 out would hang and become a bad pipe. Then I searched the google for some tips, a topic remind me that we should write a data length zero packet after sending a length of packet match what you defined in the report description. But it's not working for me. Then I write a data length zero to the control pipe after I receive a out packet and magically, it works! It would never hang after million times transferring!
Here is my question: Why does it works after writing a data length zero to a control pipe. The data transferring in the out pipe should have no relationship with the data in the control pipe. It confuses me!
If you transfer any data that is less than the expected payload size, you must send a Zero Length Packet to indicate that data has transferred.
But it depends heavily on the implementation on the host controller, and not all devices follow the specification to the point and may stall.
Source:
When do USB Hosts require a zero-length IN packet at the end of a Control Read Transfer?
I am writing code for a USB device. Suppose the USB host starts a control read transfer to read some data from the device, and the amount of data requested (wLength in the Setup Packet) is a multiple of the Endpoint 0 max packet size. Then after the host has received all the data (in the form of several IN transactions with maximum-sized data packets), will it initiate another IN transaction to see if there is more data even though there can't be more?
Here's an example sequence of events that I am wondering about:
USB enumeration process: max packet size on endpoint 0 is reported to be 64.
SETUP-DATA-ACK transaction starts a control read transfer, wLength = 128.
IN-DATA-ACK transaction delivers first 64 bytes of data to host.
IN-DATA-ACK transaction delivers last 64 bytes of data to host.
IN-DATA-ACK with zero-length DATA packet? Does this transaction ever happen?
OUT-DATA-ACK transaction completes Status Phase of the transfer; transfer is over.
I tested this on my computer (Windows Vista, if it matters) and the answer was no: the host was smart enough to know that no more data can be received from the device, even though all the packets sent by the device were full (maximum size allowed on Endpoint 0). I'm wondering if there are any hosts that are not smart enough, and will try to perform another IN transaction and expect to receive a zero-length data packet.
I think I read the relevant parts of the USB 2.0 and USB 3.0 specifications from usb.org but I did not find this issue addressed. I would appreciate it if someone can point me to the right section in either of those documents.
I know that a zero-length packet can be necessary if the device chooses to send less data than the host requested in wLength.
I know that I could make my code flexible enough to handle either case, but I'm hoping I don't have to.
Thanks to anyone who can answer this question!
Read carefully USB specification:
The Data stage of a control transfer from an endpoint to the host is complete when the endpoint does one of
the following:
Has transferred exactly the amount of data specified during the Setup stage
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
So, in your case, when wLength == transfer size, answer is NO, you don't need ZLP.
In case wLength > transfer size, and (transfer size % ep0 size) == 0 answer is YES, you need ZLP.
In general, USB uses a less-than-max-length packet to demarcate an end-of-transfer. So in the case of a transfer which is an integer multiple of max-packet-length, a ZLP is used for demarcation.
You see this in bulk pipes a lot. For example, if you have a 4096 byte transfer, that will be broken down into an integer number of max-length packets plus one zero-length-packet. If the SW driver has a big enough receive buffer set up, higher-level SW receives the entire transfer at once, when the ZLP occurs.
Control transfers are a special case because they have the wLength field, so ZLP isn't strictly necessary.
But I'd strongly suggest SW be flexible to both, as you may see variations with different USB host silicon or low-level HCD drivers.
I would like to expand on MBR's answer. The USB specification 2.0, in section 5.5.3, says:
The Data stage of a control transfer from an endpoint to the host is
complete when the endpoint does one of the following:
Has transferred exactly the amount of data specified during the Setup stage
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
When a Data stage is complete, the Host Controller advances to the
Status stage instead of continuing on with another data transaction.
If the Host Controller does not advance to the Status stage when the
Data stage is complete, the endpoint halts the pipe as was outlined in
Section 5.3.2. If a larger-than-expected data payload is received from
the endpoint, the IRP for the control transfer will be
aborted/retired.
I added emphasis to one of the sentences in that quote because it seems to specifically say what the device should do: it should "halt" the pipe if the host tries to continue the data phase after it was done, and it is done if all the requested data has been transmitted (i.e. the number of bytes transferred is greater than or equal to wLength). I think halting refers to sending a STALL packet.
In other words, the device does not need a zero-length packet in this situation and in fact the USB specification says it should not provide one.
You don't have to. (*)
The whole point of wLength is to tell the host the maximum number of bytes it should attempt to read (but it might read less !)
(*) I have seen devices that crash when IN/OUT requests were made at incorrect time during control transfers (when debugging our host solution). So any host doing what you are worried about, would of killed those devices and is hopefully not in the market.
I am trying to write an app that exchanges data with other iPhones running the app through the Game Kit framework. The iPhones discover each other and connect fine, but the problems happens when I send the data. I know the iPhones are connected properly because when I serialize an NSString and send it through the connection it comes out on the other end fine. But when I try to archive a larger object (using NSKeyedArchiver) I get the error message "AGPSessionBroadcast failed (801c0001)".
I am assuming this is because the data I am sending is too large (my files are about 500k in size, Apple seems to recommend a max of 95k). I have tried splitting up the data into several transfers, but I can never get it to unarchive properly at the other end. I'm wondering if anyone else has come up against this problem, and how you solved it.
I had the same problem w/ files around 300K. The trouble is the sender needs to know when the receiver has emptied the pipe before sending the next chunk.
I ended up with a simple state engine that ran on both sides. The sender transmits a header with how many total bytes will be sent and the packet size, then waits for acknowledgement from the other side. Once it gets the handshake it proceeds to send fixed size packets each stamped with a sequence number.
The receiver gets each one, reads it and appends it to a buffer, then writes back to the pipe that it got packet with the sequence #. Sender reads the packet #, slices out another buffer's worth, and so on and so forth. Each side keeps track of the state they're in (idle, sending header, receiving header, sending data, receiving data, error, done etc.) The two sides have to keep track of when to read/write the last fragment since it's likely to be smaller than the full buffer size.
This works fine (albeit a bit slow) and it can scale to any size. I started with 5K packet sizes but it ran pretty slow. Pushed it to 10K but it started causing problems so I backed off and held it at 8096. It works fine for both binary and text data.
Bear in mind that the GameKit isn't a general file-transfer API; it's more meant for updates of where the player is, what the current location or other objects are etc. So sending 300k for a game doesn't seem that sensible, though I can understand hijacking the API for general sharing mechanisms.
The problem is that it isn't a TCP connection; it's more a UDP (datagram) connection. In these cases, the data isn't a stream (which gets packeted by TCP) but rather a giant chunk of data. (Technically, UDP can be fragmented into multiple IP packets - but lose one of those, and the entire UDP is lost, as opposed to TCP, which will re-try).
The MTU for most wired networks is ~1.5k; for bluetooth, it's around ~0.5k. So any UDP packet that you sent (a) may get lost, (b) may be split into multiple MTU-sized IP packets, and (c) if one of those packets is lost, then you will automatically lose the entire set.
Your best strategy is to emulate TCP - it sends out packets with a sequence number. The receiving end can then request dupe transmissions of packets which went missing afterwards. If you're using the equivalent of an NSKeyedArchiver, then one suggestion is to iterate through the keys and write those out as individual keys (assuming each keyed value isn't that big on its own). You'll need to have some kind of ACK for each packet that gets sent back, and a total ACK when you're done, so the sender knows it's OK to drop the data from memory.
This Question is extension of the following
OpenFlow Rule Metadata
I would like to have this clarified, on my question about Metadata
Let us say, I have an Open Flow rules, as below
Cookie=0x8000001, duration=228925.445s, table=17, n_packets=350, n_bytes=32424, priority=10,metadata=0xc000f30000000000/0xffffff0000000000 actions=goto_table:19
I wanted to understand the following
Do we have certain rule/ Algorithm , to determine these Metadata from a Packet.
because the Packet in OVS is actually switched based on Matching Metadata, Is that correct ?? ( At least according to the above flow rule )
And the Packet itself does not carry the Metadata, then how exactly
the packet hitting a flow matched against the Metadata.
So, If I understood it correctly the Packets those are traversed between the flow-tables, are within the OVS application itself or Handled #OVS Application level, until it had determined Egress Port
So in that Case, the MetaData are handled #OVS-Application level, until the Packets is send via Egress Port.
Is this correct??
Finally which Module in ODL is responsible for determine the Metadata, and I would like to understand from the code how exactly it was done.
The OpenFlow metadata field starts with a value of zero for every packets. Tables can then writes to this field and you can match on it in subsequent tables. It is only used to carry information from one table to the next, as explained in the OpenFlow specifications:
Metadata: a maskable register that is used to carry information from one table to the next.
first of all you can try Ryu instead, its code is more easy to read and understand.
Then, I think metadata/instructions/actions.... these things are belong to the processing of OVS forwarding, but these things needs to attach to something and that is the packet that OVS received. About the question "Do we have certain rule/ Algorithm , to determine these Metadata from a Packet. " I think the value of the Metadata is determind by the controller, which means that it depends on 'how do you design your own network instance using some(e.g. RYU) controller application'.