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# Devtab Entries

## Name

Devtab Entries -- Data endpoint data structure

## Synopsis

    /dev/usb0c
    /dev/usb1r
    /dev/usb2w

## Devtab Entries

USB device drivers provide two ways of transferring data between host and peripheral. The first involves USB-specific functionality such as `usbs_start_rx_buffer`. This provides non-blocking I/O: a transfer is started, and some time later the device driver will call a supplied completion function. The second uses the conventional I/O model: there are entries in the device table corresponding to the various endpoints. Standard calls such as `open` can then be used to get a suitable handle. Actual I/O happens via blocking `read` and `write` calls. In practice the blocking operations are simply implemented using the underlying non-blocking functionality.

Each endpoint will have its own devtab entry. The exact names are controlled by the device driver package, but typically the root will be `/dev/usb`. This is followed by one or more decimal digits giving the endpoint number, followed by `c` for a control endpoint, `r` for a receive endpoint (host to peripheral), and `w` for a transmit endpoint (peripheral to host). If the target hardware involves more than one USB device then different roots should be used, for example `/dev/usb0c` and `/dev/usb1_0c`. This may require explicit manipulation of device driver configuration options by the application developer.

At present the devtab entry for a control endpoint does not support any I/O operations.

### `write` operations

`cyg_io_write` and similar functions in higher-level packages can be used to perform a transfer from peripheral to host. Successive write operations will not be coalesced. For example, when doing a 1000 byte write to an endpoint that uses the bulk transfer protocol this will involve 15 full-size 64-byte packets and a terminating 40-byte packet. USB device drivers are not expected to do any locking, and if higher-level code performs multiple concurrent write operations on a single endpoint then the resulting behaviour is undefined.

A USB `write` operation will never transfer less data than specified. It is the responsibility of higher-level code to ensure that the amount of data being transferred is acceptable to the host-side code. Usually this will be defined by a higher-level protocol. If an attempt is made to transfer more data than the host expects then the resulting behaviour is undefined.

There are two likely error conditions. `EPIPE` indicates that the connection between host and target has been broken. `EAGAIN` indicates that the endpoint has been stalled, either at the request of the host or by other activity inside the peripheral.

### `read` operations

`cyg_io_read` and similar functions in higher-level packages can be used to perform a transfer from host to peripheral. This should be a complete transfer: higher-level protocols should define an upper bound on the amount of data being transferred, and the `read` operation should involve at least this amount of data. The return value will indicate the actual transfer size, which may be less than requested.

Some device drivers may support partial reads, but USB device drivers are not expected to perform any buffering because that involves both memory and code overheads. One technique that may work for bulk transfers is to exploit the fact that such transfers happen in 64-byte packets. It is possible to `read` an initial 64 bytes, corresponding to the first packet in the transfer. These 64 bytes can then be examined to determine the total transfer size, and the remaining data can be transferred in another `read` operation. This technique is not guaranteed to work with all USB hardware. Also, if the delay between accepting the first packet and the remainder of the transfer is excessive then this could cause timeout problems for the host-side software. For these reasons the use of partial reads should be avoided.

There are two likely error conditions. `EPIPE` indicates that the connection between host and target has been broken. `EAGAIN` indicates that the endpoint has been stalled, either at the request of the host or by other activity inside the peripheral.

USB device drivers are not expected to do any locking. If higher-level code performs multiple concurrent read operations on a single endpoint then the resulting behaviour is undefined.

### `select` operations

Typical USB device drivers will not provide any support for `select`. Consider bulk transfers from the host to the peripheral. At the USB device driver level there is no way of knowing in advance how large a transfer will be, so it is not feasible for the device driver to buffer the entire transfer. It may be possible to buffer part of the transfer, for example the first 64-byte packet, and copy this into application space at the start of a `read`, but this adds code and memory overheads. Worse, it means that there is an unknown but potentially long delay between a peripheral accepting the first packet of a transfer and the remaining packets, which could confuse or upset the host-side software.

With some USB hardware it may be possible for the device driver to detect OUT tokens from the host without actually accepting the data, and this would indicate that a `read` is likely to succeed. However, it would not be reliable since the host-side I/O operation could time out. A similar mechanism could be used to implement `select` for outgoing data, but again this would not be reliable.

Some device drivers may provide partial support for `select` anyway, possibly under the control of a configuration option. The device driver's documentation should be consulted for further information. It is also worth noting that the USB-specific non-blocking API can often be used as an alternative to `select`.

### `get_config` and `set_config` operations

There are no `set_config` or `get_config` (also known as `ioctl`) operations defined for USB devices. Some device drivers may provide hardware-specific facilities this way.

> Note: Currently the USB-specific functions related to halted endpoints cannot be accessed readily via devtab entries. This functionality should probably be made available via `set_config` and `get_config`. It may also prove useful to provide a `get_config` operation that maps from the devtab entries to the underlying endpoint data structures.

### Presence

The devtab entries are optional. If the USB device is accessed primarily by class-specific code such as the USB-ethernet package and that package uses the USB-specific API directly, the devtab entries are redundant. Even if application code does need to access the USB device, the non-blocking API may be more convenient than the blocking I/O provided via the devtab entries. In these cases the devtab entries serve no useful purpose, but they still impose a memory overhead. It is possible to suppress the presence of these entries by disabling the configuration option `CYGGLO_IO_USB_SLAVE_PROVIDE_DEVTAB_ENTRIES`.

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