Intel(R) Management Engine (ME) Client bus API

Rationale

The MEI character device is useful for dedicated applications to send and receive data to the many FW appliance found in Intel’s ME from the user space. However, for some of the ME functionalities it makes sense to leverage existing software stack and expose them through existing kernel subsystems.

In order to plug seamlessly into the kernel device driver model we add kernel virtual bus abstraction on top of the MEI driver. This allows implementing Linux kernel drivers for the various MEI features as a stand alone entities found in their respective subsystem. Existing device drivers can even potentially be re-used by adding an MEI CL bus layer to the existing code.

MEI CL bus API

A driver implementation for an MEI Client is very similar to any other existing bus based device drivers. The driver registers itself as an MEI CL bus driver through the struct mei_cl_driver structure defined in include/linux/mei_cl_bus.c

struct mei_cl_driver {
        struct device_driver driver;
        const char *name;

        const struct mei_cl_device_id *id_table;

        int (*probe)(struct mei_cl_device *dev, const struct mei_cl_id *id);
        int (*remove)(struct mei_cl_device *dev);
};

The mei_cl_device_id structure defined in include/linux/mod_devicetable.h allows a driver to bind itself against a device name.

struct mei_cl_device_id {
        char name[MEI_CL_NAME_SIZE];
        uuid_le uuid;
        __u8    version;
        kernel_ulong_t driver_info;
};

To actually register a driver on the ME Client bus one must call the mei_cl_add_driver() API. This is typically called at module initialization time.

Once the driver is registered and bound to the device, a driver will typically try to do some I/O on this bus and this should be done through the mei_cl_send() and mei_cl_recv() functions. More detailed information is in API: section.

In order for a driver to be notified about pending traffic or event, the driver should register a callback via mei_cl_devev_register_rx_cb() and mei_cldev_register_notify_cb() function respectively.

API:

ssize_t mei_cldev_send_vtag(struct mei_cl_device *cldev, const u8 *buf, size_t length, u8 vtag)

me device send with vtag (write)

Parameters

struct mei_cl_device *cldev
me client device
const u8 *buf
buffer to send
size_t length
buffer length
u8 vtag
virtual tag

Return

  • written size in bytes
  • < 0 on error
ssize_t mei_cldev_recv_vtag(struct mei_cl_device *cldev, u8 *buf, size_t length, u8 *vtag)

client receive with vtag (read)

Parameters

struct mei_cl_device *cldev
me client device
u8 *buf
buffer to receive
size_t length
buffer length
u8 *vtag
virtual tag

Return

  • read size in bytes
  • < 0 on error
ssize_t mei_cldev_recv_nonblock_vtag(struct mei_cl_device *cldev, u8 *buf, size_t length, u8 *vtag)

non block client receive with vtag (read)

Parameters

struct mei_cl_device *cldev
me client device
u8 *buf
buffer to receive
size_t length
buffer length
u8 *vtag
virtual tag

Return

  • read size in bytes
  • -EAGAIN if function will block.
  • < 0 on other error
ssize_t mei_cldev_send(struct mei_cl_device *cldev, const u8 *buf, size_t length)

me device send (write)

Parameters

struct mei_cl_device *cldev
me client device
const u8 *buf
buffer to send
size_t length
buffer length

Return

  • written size in bytes
  • < 0 on error
ssize_t mei_cldev_recv(struct mei_cl_device *cldev, u8 *buf, size_t length)

client receive (read)

Parameters

struct mei_cl_device *cldev
me client device
u8 *buf
buffer to receive
size_t length
buffer length

Return

read size in bytes of < 0 on error

ssize_t mei_cldev_recv_nonblock(struct mei_cl_device *cldev, u8 *buf, size_t length)

non block client receive (read)

Parameters

struct mei_cl_device *cldev
me client device
u8 *buf
buffer to receive
size_t length
buffer length

Return

read size in bytes of < 0 on error
-EAGAIN if function will block.
int mei_cldev_register_rx_cb(struct mei_cl_device *cldev, mei_cldev_cb_t rx_cb)

register Rx event callback

Parameters

struct mei_cl_device *cldev
me client devices
mei_cldev_cb_t rx_cb
callback function

Return

0 on success
-EALREADY if an callback is already registered <0 on other errors
int mei_cldev_register_notif_cb(struct mei_cl_device *cldev, mei_cldev_cb_t notif_cb)

register FW notification event callback

Parameters

struct mei_cl_device *cldev
me client devices
mei_cldev_cb_t notif_cb
callback function

Return

0 on success
-EALREADY if an callback is already registered <0 on other errors
void * mei_cldev_get_drvdata(const struct mei_cl_device *cldev)

driver data getter

Parameters

const struct mei_cl_device *cldev
mei client device

Return

driver private data

void mei_cldev_set_drvdata(struct mei_cl_device *cldev, void *data)

driver data setter

Parameters

struct mei_cl_device *cldev
mei client device
void *data
data to store
const uuid_le * mei_cldev_uuid(const struct mei_cl_device *cldev)

return uuid of the underlying me client

Parameters

const struct mei_cl_device *cldev
mei client device

Return

me client uuid

u8 mei_cldev_ver(const struct mei_cl_device *cldev)

return protocol version of the underlying me client

Parameters

const struct mei_cl_device *cldev
mei client device

Return

me client protocol version

bool mei_cldev_enabled(const struct mei_cl_device *cldev)

check whether the device is enabled

Parameters

const struct mei_cl_device *cldev
mei client device

Return

true if me client is initialized and connected

int mei_cldev_enable(struct mei_cl_device *cldev)

enable me client device create connection with me client

Parameters

struct mei_cl_device *cldev
me client device

Return

0 on success and < 0 on error

int mei_cldev_disable(struct mei_cl_device *cldev)

disable me client device disconnect form the me client

Parameters

struct mei_cl_device *cldev
me client device

Return

0 on success and < 0 on error

Example

As a theoretical example let’s pretend the ME comes with a “contact” NFC IP. The driver init and exit routines for this device would look like:

#define CONTACT_DRIVER_NAME "contact"

static struct mei_cl_device_id contact_mei_cl_tbl[] = {
        { CONTACT_DRIVER_NAME, },

        /* required last entry */
        { }
};
MODULE_DEVICE_TABLE(mei_cl, contact_mei_cl_tbl);

static struct mei_cl_driver contact_driver = {
        .id_table = contact_mei_tbl,
        .name = CONTACT_DRIVER_NAME,

        .probe = contact_probe,
        .remove = contact_remove,
};

static int contact_init(void)
{
        int r;

        r = mei_cl_driver_register(&contact_driver);
        if (r) {
                pr_err(CONTACT_DRIVER_NAME ": driver registration failed\n");
                return r;
        }

        return 0;
}

static void __exit contact_exit(void)
{
        mei_cl_driver_unregister(&contact_driver);
}

module_init(contact_init);
module_exit(contact_exit);

And the driver’s simplified probe routine would look like that:

int contact_probe(struct mei_cl_device *dev, struct mei_cl_device_id *id)
{
        [...]
        mei_cldev_enable(dev);

        mei_cldev_register_rx_cb(dev, contact_rx_cb);

        return 0;
}

In the probe routine the driver first enable the MEI device and then registers an rx handler which is as close as it can get to registering a threaded IRQ handler. The handler implementation will typically call mei_cldev_recv() and then process received data.

#define MAX_PAYLOAD 128
#define HDR_SIZE 4
static void conntact_rx_cb(struct mei_cl_device *cldev)
{
        struct contact *c = mei_cldev_get_drvdata(cldev);
        unsigned char payload[MAX_PAYLOAD];
        ssize_t payload_sz;

        payload_sz = mei_cldev_recv(cldev, payload,  MAX_PAYLOAD)
        if (reply_size < HDR_SIZE) {
                return;
        }

        c->process_rx(payload);

}

MEI Client Bus Drivers