illumos: manual page: mc

MC_TX(9E) Driver Entry Points MC_TX(9E)

NAME

mc_tx, mri_tx - transmit a message block chain

SYNOPSIS

#include <sys/mac_provider.h>

mblk_t *
prefix_m_tx(void *driver, mblk_t *mp_chain);

mblk_t *
prefix_ring_tx(void *driver_rh, mblk_t *mp_chain);

INTERFACE LEVEL

illumos DDI specific

The mri_tx() entry point is Uncommitted - API and ABI stability is not
guaranteed.

PARAMETERS

driver A pointer to the driver's private data that was passed in via
the m_pdata member of the mac_register(9S) structure to the
mac_register(9F) function.

driver_rh A pointer to the driver's private data for the ring that was
passed in via the mri_driver member of the mac_ring_info(9S)
structure. This is initialized by the driver when its
mr_rget(9E) is called by MAC.

mp_chain A series of mblk(9S) structures that may have multiple
independent packets linked together on their b_next member.

DESCRIPTION

The mc_tx() entry point is called when the system requires a device driver
to transmit data. The device driver will receive a chain of message
blocks. The mp_chain argument represents the first frame. The frame may
be spread out across one or more mblk(9S) structures that are linked
together by the b_cont member. There may be multiple frames, linked
together by the b_next pointer of the mblk(9S).

For each frame, the driver should allocate the required resources and
prepare it for being transmitted on the wire. The driver may opt to copy
those resources to a DMA buffer or it may bind them. For more information
on these options, see the MBLKS AND DMA section of mac(9E).

As it processes each frame in the chain, if the device driver has
advertised either of the MAC_CAPAB_HCKSUM or MAC_CAPAB_LSO flags, it must
check whether either apply for the given frame using the mac_hcksum_get(9F)
and mac_lso_get(9F) functions respectively. If either is enabled for the
given frame, the hardware must arrange for that to be taken care of.

For each frame that the device driver processes it is responsible for doing
one of three things with it:

1. Transmit the frame.

2. Drop the frame by calling freemsg(9F) on the individual mblk_t.

3. Return the frames to indicate that resources are not available.

The device driver is in charge of the memory associated with mp_chain. If
the device driver does not return the message blocks to the MAC framework,
then it must call freemsg(9F) on the frames. If it does not, the memory
associated with them will be leaked. When a frame is being transmitted, if
the device driver performed DMA binding, it should not free the message
block until after it is guaranteed that the frame has been transmitted. If
the message block was copied to a DMA buffer, then it is allowed to call
freemsg(9F) at any point.

In general, the device driver should not drop frames without transmitting
them unless it has no other choice. Times when this happens may include
the device driver being in a state where it can't transmit, an error was
found in the frame while trying to establish the checksum or LSO state, or
some other kind of error that represents an issue with the passed frame.

The device driver should not free the chain when it does not have enough
resources. For example, if entries in a device's descriptor ring fill up,
then it should not drop those frames and instead should return all of the
frames that were not transmitted. This indicates to the stack that the
device is full and that flow control should be asserted. Back pressure
will be applied to the rest of the stack, allowing most systems to behave
better.

Once a device driver has returned unprocessed frames from its mc_tx() entry
point, then the device driver will not receive any additional calls to its
mc_tx() entry point until it calls the mac_tx_update(9F) function to
indicate that resources are available again. Note that because it is the
device driver that is calling this function to indicate resources are
available, it is very important that it only return frames in cases where
the device driver itself will be notified that resources are available
again. For example, when it receives an interrupt indicating that the data
that it transmitted has been completed so it can use entries in its
descriptor ring or other data structures again.

The device driver can obtain access to its soft state through the driver
member. It should cast it to the appropriate structure. The device driver
should employ any necessary locking to access the transmit related data
structures. Note that the device driver should expect that it may have its
transmit endpoints called into from other threads while it's servicing
device interrupts related to them.

The mri_tx() entry point is similar to the mc_tx() entry point, except that
it is used by device drivers that have negotiated the MAC_CAPAB_RINGS
capability with transmit rings. The driver should follow all of the same
rules described earlier, except that it will access a ring-specific data
structure through driver_rh and when it needs to update that there is
additional space available, it must use mac_tx_ring_update(9F) and not
mac_tx_update(9F).

When the mri_tx() entry point is called, the ring that should be used has
been specified. The driver must not attempt to use any other ring than the
one specified by driver_rh for any reason, including a lack of resources or
an attempt to perform its own hashing.

CONTEXT

The mc_tx() entry point may be called from kernel or interrupt context.

RETURN VALUES

Upon successful completion, the device driver should return NULL.
Otherwise, it should return all unprocessed message blocks and ensure that
it calls either mac_tx_update(9F) or mac_tx_ring_update(9F) some time in
the future.