Transcript RTL-8139 experimentation Setting up an environment for studying the Network Controller

RTL-8139 experimentation
Setting up an environment for
studying the Network Controller
Operating the NIC
• We saw that, with a suitable device-driver,
we could memory-map the NIC’s registers
into an application’s virtual address space
(our ‘mmap8139.c’ module did that for us)
• But operating the NIC requires two further
‘resources’ that are not directly accessible
from user-space: memory and interrupts
Kernel memory-buffer
• We can enhance of our device-driver so it
provides access to a region of physically
contiguous processor memory, by using
‘kmalloc()’ during module-initialization to
reserve a big enough buffer (80-KB), and
by equipping our driver with functions that
perform ‘read()’, ‘write()’, and ‘lseek()’ on
the allocated region of kernel memory
Avoid using interrupts
• We can temporarily avoid using interrupts
by performing ‘polling’ operations during
our initial exploratory work with the NIC
• Interrupts are disabled by doing a ‘reset’
• Later, after we have developed our basic
understanding of the NIC’s operations, we
can turn to the issue of how interrupts may
be employed for improved efficiency
Overview of our setup
user-space
kernel-space
standard
runtime libraries
Linux Kernel
‘user8139.ko’ device-driver
‘nicstudy’
application
mmap
llseek
8139 registers
write
read
memory buffer
The ‘transmit’ steps
• To transmit our first network packet, we
‘write’ it into our kernel buffer (at offset 0)
• We ‘enable’ transmission (bit #2 of CR)
• We ‘configure’ transmission, or use default
• To ‘initiate’ packet-transmission, we write
the packet’s length to ‘TxStatus0’ register
• The NIC sets bit 13 of TxStatus when the
transmission has been concluded
Did it work?
• Our ‘nicstudy.cpp’ application performs a
packet-transmission (using ‘user8139.c’)
• To verify that our packet was indeed sent
over the Local Area Network, we need to
setup another workstation that receives it
(and displays the transmitted information)
• Our ‘rxtester.cpp’ program can do this –
but it requires a ‘privileged’ user to run it
Multiple packets
• To send another packet, we need to use
the next ‘transmit descriptor’ among four
that the network controller supports:
– Descriptor 0: <TxStatus0, TxBuffer0>
– Descriptor 1: <TxStatus0, TxBuffer1>
– Descriptor 2: <TxStatus0, TxBuffer2>
– Descriptor 3: <TxStatus0, TxBuffer3>
• These descriptors must be used in ‘round
robin’ fashion (i.e., as a ‘circular queue’)
In-class exercise #1
• Modify the ‘nicstudy.cpp’ application so it
transmits fifteen packets in succession
• You can use your ‘rxtester’ workstation’s
MAC address in the destination-field for
your packet (first six bytes), to avoid your
15 packets being received by everyone’s
test-station (that would be very confusing)!
In-class exercise #2
• Find out the lengths for the smallest and
the largest packets you can successfully
transmit and receive
• Find out the maximum number of 80-byte
packets that you can successfully transmit
and receive during a one-second interval