c - rdma connection manager driver pattern -


i'm using ofed 3.18r2 implementation of infiniband drivers application. in particular i'm using rdma connection manager wrapper functions. understand better what's going on under hood i'm used @ source code. doing came looks pattern cannot understand it. let's make example. rdma connection manager functions in cma.c. looking example @ rdma_listen call (this common every functions defined in library start "rdma_"): 

int rdma_listen(struct rdma_cm_id *id, int backlog) {     struct ucma_abi_listen cmd;     struct cma_id_private *id_priv;     int ret;      cma_init_cmd(&cmd, sizeof cmd, listen);     id_priv = container_of(id, struct cma_id_private, id);     cmd.id = id_priv->handle;     cmd.backlog = backlog;      ret = write(id->channel->fd, &cmd, sizeof cmd);     if (ret != sizeof cmd)         return (ret >= 0) ? err(enodata) : -1;      if (af_ib_support)         return ucma_query_addr(id);     else         return ucma_query_route(id); }

here can see pattern mentioned before:

ret = write(id->channel->fd, &cmd, sizeof cmd);

the first argument write call file descriptor associated /dev/infiniband/rdma_cm , cannot understand usage of cmd arguments. dig source find cmd struct comes abi definition of rdma cm function calls. don't understand if common pattern execute device driver calls , how works, real code associated cmd argument. please me?

using write() system call execute commands common method executing commands in rdma subsystem. used among others rdma_ucm module , ib_uverbs module. kernel code associate rdma_ucm can found in drivers/infiniband/core/ucma.c file. specifically, write() system call device implemented in ucma_write() function.

i don't think there lot of documentation on method of calling driver. user_verbs.txt document in kernel documentation states:

commands sent kernel via write()s on these device files. abi defined in drivers/infiniband/include/ib_user_verbs.h. structs commands require response kernel contain 64-bit field used pass pointer output buffer. status returned userspace return value of write() system call.

i think may small abuse of write() system call implements more similar ioctl().

edit: note added link upstream kernel module, ofed source structure similar. edit: added documentation pointers.


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