Cross-buffer access

EVL drivers can access the out-of-band endpoint of a cross-buffer, to exchange data with a user application which attends the in-band endpoint. The companion driver of the latmus application illustrates this, by transmitting the latency data it measures in kernel space to the display front-end which runs in user space, via a shared cross-buffer.

A driver can achieve this as follows:

  1. given a file descriptor to the cross-buffer element created by an EVL application, the driver obtains a stable reference to the struct evl_xbuf descriptor associated to that cross-buffer in kernel space, by a call to evl_get_xbuf(). This in-kernel descriptor is used for other operations on the cross-buffer.

  2. from that point, the driver may send/receive data to/from the application via the cross-buffer using the evl_write_xbuf() and evl_read_xbuf() respectively.

  3. when the cross-buffer is not needed anymore, the driver should release the reference it has obtained, by calling evl_put_xbuf().

Cross-buffer services

struct evl_xbuf *evl_get_xbuf(int efd, struct evl_file **efilpp)

Retrieve a cross-buffer from a file descriptor. Typically, the peer application would pass the file descriptor to the driver using a dedicated ioctl(2) system call in order to establish the communication. The file descriptor is resolved in the context of the caller, i.e. by a lookup into the file table of the current process.

A file descriptor which should point at a cross-buffer element available from user space in the context of the calling process.
A pointer to an EVL file descriptor, referring to the cross-buffer internally. A reference is held on this file, to guarantee it never goes stale until dropped by a converse call to [evl_put_xbuf()](#evl_put_xbuf).

evl_get_xbuf() returns the in-kernel struct evl_xbuf descriptor of the cross-buffer on success, otherwise NULL if efd is either an invalid descriptor, or does not correspond to a cross-buffer element.

void evl_put_xbuf(struct evl_file *efilp)

Release a cross-buffer previously obtained by a call to evl_get_xbuf(). The reference to the corresponding element is dropped.

A pointer to the EVL file descriptor received from [evl_get_xbuf()](#evl_get_xbuf) for the cross-buffer to be released.
ssize_t evl_read_xbuf(struct evl_xbuf *xbuf, void *buf, size_t count, int f_flags)

Attempt to read up to count bytes from the cross-buffer xbuf into the buffer starting at buf from the out-of-band execution stage. The data is received from a peer application in user space which sent it by calling the write(2) system call to the same file referred to by the descriptor passed to evl_get_xbuf().

If O_NONBLOCK is clear in f_flags, there are no short reads: the caller always gets a complete message of the requested length, blocking if necessary except if a sender is currently blocked on the other end of the channel, waiting for some space to be available into the ring buffer to complete a write operation. In this case, a short read is performed to prevent a deadlock, which means the caller may receive fewer data than requested in the evl_read_xbuf() call. This situation arises when the size picked for the ring buffer does not fit the transfer pattern, as illustrated below:

Alt text

A cross-buffer descriptor to read from.

  • buf

    A buffer to read the cross-buffer data into.

  • count

    The number of bytes to read into buf.

  • f_flags

    Operation flags. Can be either zero or O_NONBLOCK for non-blocking input.

    • The number of bytes copied to buf is returned on success, otherwise:

    • -EAGAIN if O_NONBLOCK is set in f_flags not enough data is available from the cross-buffer to satisfy the request.

    • -EINVAL if count is greater than the size of the ring buffer associated with the traffic direction. See evl_create_xbuf().

    • -ENOBUFS if there is no ring buffer space associated with the outbound traffic. See evl_create_xbuf().

    ssize_t evl_write_xbuf(struct evl_xbuf *xbuf, const void *buf, size_t count, int f_flags)

    Attempt to write up to count bytes to the cross-buffer xbuf from the buffer starting at buf from the out-of-band execution stage. If not enough space is available from the cross-buffer for completing the operation immediately, the caller blocks until enough space is freed by reader(s) unless O_NONBLOCK is set in f_flags. The peer application in user space can later read the output data by calling oob_read() for the same file referred to by the descriptor passed to evl_get_xbuf().

    A cross-buffer descriptor to write to.

  • buf

    A buffer containing the data to be written.

  • count

    The number of bytes to write starting from buf.

  • f_flags

    Operation flags. Can be either zero or O_NONBLOCK for non-blocking output.

    • The number of bytes written is returned on success, otherwise:

    • -EAGAIN if O_NONBLOCK is set in f_flags and the cross-buffer is out of memory space for receiving the requested amount of data.

    • -EINVAL if count is greater than the size of the ring buffer associated with the traffic direction. See evl_create_xbuf().

    • -ENOBUFS if there is no ring buffer space associated with the inbound traffic. See evl_create_xbuf().

    Last modified: Tue, 28 Jun 2022 15:34:31 +0200