Guide to Implementing Packet-Transmit Capability with Intel 82573L NIC, Slides of Computer Applications

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Our ‘xmit1000.c’ driver

Implementing a ‘packet-transmit’

capability with the Intel 82573L

network interface controller

Remenber ‘echo’ and ‘cat’?

• Your device-driver module (named ‘uart.c’)

was supposed to allow two programs that

are running on a pair of adjacent PCs to

communicate via a “null-modem” cable

$ echo Hello > /dev/uart $ _ $ cat /dev/uart Hello _

Transmitting… Receiving…

This function will program the actual data-transfer

Driver’s components

write

my_fops

my_write()

module_init() module_exit()

This function will allow us to inspect the transmit-descriptors

This function will detect and configure the hardware, define page-mappings, allocate and initialize the descriptors, start the ‘transmit’ engine, create the pseudo-file and register ‘my_fops’

This function will do needed ‘cleanup’ when it’s time to unload our driver – turn off the ‘transmit’ engine, free the memory, delete page-table entries, the pseudo-file, and the ‘my_fops’

‘struct’ holds one function-pointer

my_get_info()

Kzalloc()

• Linux kernels since 2.6.13 offer this convenient

function for allocating pre-zeroed kernel memory

• It has the same syntax as the ‘kmalloc()’ function

(described in our texts), but adds the after-effect

of zeroing out the newly-allocated memory-area

• Thus it does two logically distinct actions (often

coupled anyway) within a single function-call

void kmem = kmalloc ( region_size, GFP_KERNEL ); memset ( kmem, 0x00, region_size ); / can be replaced with */ void *kmem = kzalloc ( region_size, GFP_KERNEL );

Our Tx-Descriptor ring

descriptor 0

Our ‘reusable’ transmit-buffer (1536 bytes)

descriptor 1 descriptor 2 descriptor 3 descriptor 4 descriptor 5 descriptor 6 descriptor 7 one packet-buffer Array of 8 transmit-descriptors

TAIL HEAD

After writing the data into our packet-buffer, and writing its length to the the current TAIL descriptor, our driver will advance the TAIL index; the NIC responds by reading the current HEAD descriptor, fetching its data, then advancing the HEAD index as it sends our data out over the wire.

‘/proc/xmit1000’

• This pseudo-file can be examined anytime

to find out what values (if any) the NIC has

‘written back’ into the transmit-descriptors

(i.e., the descriptor-status information) and

current values in registers TDH and TDT:

$ cat /proc/xmit

Configuration registers

TIPG

TCTL

TDBAL

TDBAH

TDLEN

TDH

TDT

TXDCTL

CTRL

CTRL_EXT

Device Control Extended Device Control

Transmit Inter-Packet Gap

Transmit Control

Transmit Descriptor-queue Base-Address (LOW)

Transmit Descriptor-queue Base-Address (HIGH)

Transmit Descriptor-queue Length

Transmit Descriptor-queue HEAD

Transmit Descriptor-queue TAIL

Transmit Descriptor-queue Control

The ‘initialization’ sequence

• Detect the network interface controller

• Obtain its i/o-memory address and size

• Remap the i/o-memory into kernel-space

• Allocate memory for buffer and descriptors

• Initialize the array of transmit-descriptors

• Reset the NIC and configure its operations

• Create the ‘/proc/xmit1000’ pseudo-file

• Register our ‘write()’ driver-method

Our ‘write()’ algorithm

• Get index of the current TAIL descriptor

• Confine the amount of user-data

• Copy user-data into the packet-buffer

• Setup the packet’s Ethernet Header

• Setup packet-length in the TAIL descriptor

• Now hand over this descriptor to the NIC

(by advancing the value in register TDT)

• Tell the kernel how many bytes were sent

Recall Tx-Descriptor Layout

special

0x

0x

0x

0xC

CMD

Buffer-Address high (bits 63..32)

Buffer-Address low (bits 31..0)

31 0

CSO Packet Length (in bytes)

CSS reserved=0 status

Buffer-Address = the packet-buffer’s 64-bit address in physical memory Packet-Length = number of bytes in the data-packet to be transmitted CMD = Command-field CSO/CSS = Checksum Offset/Start (in bytes) STA = Status-field

Transmit IPG (0x0410)

82573L

IPG

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

R =

IPG After Deferral (Recommended value = 7)

IPG Part 1 (Recommended value = 8)

IPG Back-To-Back (Recommended value = 8)

IPG = Inter-Packet Gap

This register controls the Inter-Packet Gap timer for the Ethernet controller.

Note that the recommended TIPG register-value to achieve IEEE 802. compliant minimum transfer IPG values in full- and half-duplex operations would be 00702008 (hexadecimal), equal to (7<<20) | (8<<10) | (8<<0).

Transmit Control (0x0400)

R

=

R

=

R

=

MULR TXCSCMT UNORTX RTLC R

=

SW XOFF

COLD (upper 6-bits) (COLLISION DISTANCE)

COLD (lower 4-bits) (COLLISION DISTANCE)^0 ASDV

I L O S

S

L

U

TBI mode

P S P

0 0 R =

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

R =

E N

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16

SPEED

CT (COLLISION THRESHOLD)

EN = Transmit Enable SWXOFF = Software XOFF Transmission PSP = Pad Short Packets RLTC = Retransmit on Late Collision CT = Collision Threshold (=0xF) UNORTX = Underrun No Re-Transmit COLD = Collision Distance (=0x3F) TXCSCMT = TxDescriptor Minimum Threshold MULR = Multiple Request Support

82573L

An ‘e1000.c’ anomaly?

• The official Linux kernel is delivered with a

device-driver supporting Intel’s ‘Pro/1000’

gigabit ethernet controllers (several)

• Often this driver will get loaded by default

during the system’s startup procedures

• But it will interfere with your own driver if

you try to write a substitute for ‘e1000.ko’

• So you will want to remove it with ‘ rmmod ’

Side-effect of ‘rmmod’

• We’ve observed an unexpected side-effect

of ‘unloading’ the ‘e1000.ko’ device-driver

• The PCI Configuration Space’s command

register gets modified in a way that keeps

the NIC from working with your own driver

• Specifically, the Bus Mastering capability

gets disabled (by clearing bit #2 in the PCI

Configuration Space’s word at address 4)