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Linux­PCI Support
Programming PCI­Devices under Linux
by Claus Schroeter
(clausi@chemie.fu­berlin.de)
Abstract
This document is intended to be a short tutorial about PCI Programming under
Linux. It describes the PCI basics and its implementation under Linux.
Introduction to PCI
CPU Memory
. . . . . . . . . . .
PCI­Bridge
PCI­Bus
PCI
#0
PCI
#n
PCI
#1
Figure 1: The Architecture of the PCI
Subsystem
The Peripheral Component Interconnect ­
Bus (PCI) today is present in a wide variety
of microcomputers ranging from Intel­based
PC architectures to DEC­Alpha­based work­
stations. The main difference between the
old fashioned ISA­Bus and PCI is the com­
plete separation of the Bus­subsystem from
the Memory Subsystem. The CPU communi­
cates with the PCI subsystem using a spe­
cial chipset known as PCI­Bridge. This is
an intelligent controller, that handles all nec­
essary tasks to transfer data from or to the
CPU or the Memory Subsystem. On PCI the
adresses and data are transferred as seper­
ate chunks over the bus because all bus­
lines can be used either as adress­ or as
data­lines. In special cases only one start ad­
dress is transferred followed by a whole data
block. To determine the address for each
single data­word the PCI­Bridge and the PCI­
Adapter increment internal counters, so the
address is calculated while the data words
are transmitted. This so called ''burst'' cy­
cle speeds up the transfer significantly (up to 266Mbyte/sec for a 64bit data­transfer).
Thanks to the intelligent handling of this tasks the programmer need not to take care
about all this stuff.
A schematic view of the PCI subsystem is depicted in Figure 1.
The PCI design forces Hardware designers to use a standardized interface for PCI­Board
access and control. Each bus device has its own 265byte space of Memory for config­
uration purposes that can be accessed through the CONFIG ADDRESS and the CON­
FIG DATA registers. On system startup the BIOS uses the device configuration block to
set the different board properties as IRQ line and base address for example. No jumpers
need to be placed in the right places, so IRQ or address conflicts are no longer a problem
with PCI. This feature is commonly known as Plug&Play functionality.
The Linux Lab Project whitepapers 11/1996 (http://www.llp.fu­berlin.de/)

Linux­PCI Support
First Contact with the PCI subsystem
To get an impression how linux sees the PCI bus in your computer try getting the PCI
bus configuration from the kernel with cat /proc/pci. If everything works OK you will
(hopefully) see:
PCI devices found:
Bus 0, device 12, function 0:
SCSI storage controller: Adaptec AIC­7881U (rev 0).
Medium devsel. Fast back­to­back capable. IRQ 11.
Master Capable. Latency=32. Min Gnt=8.Max Lat=8.
I/O at 0xd800.
Non­prefetchable 32 bit memory at 0xf7fff000.
Bus 0, device 11, function 0:
VGA compatible controller: S3 Inc. Vision 968 (rev 0).
Medium devsel. IRQ 10.
Non­prefetchable 32 bit memory at 0xf2000000.
Bus 0, device 10, function 0:
Ethernet controller: 3Com 3C590 10bT (rev 0).
Medium devsel. IRQ 12. Master Capable. Latency=248. Min Gnt=3.Max Lat=8.
I/O at 0xe000.
Bus 0, device 7, function 2:
Unknown class: Intel Unknown device (rev 0).
Vendor id=8086. Device id=7020.
Medium devsel. Fast back­to­back capable. IRQ 9. Master Capable. Latency=32.
I/O at 0xe400.
Bus 0, device 7, function 1:
IDE interface: Intel 82371SB Triton II PIIX (rev 0).
Medium devsel. Fast back­to­back capable. Master Capable. Latency=32.
I/O at 0xe800.
Bus 0, device 7, function 0:
ISA bridge: Intel 82371SB Triton II PIIX (rev 0).
Medium devsel. Fast back­to­back capable. Master Capable. No bursts.
Bus 0, device 0, function 0:
Host bridge: Intel 82439HX Triton II (rev 1).
Medium devsel. Master Capable. Latency=32.
All devices that are known to Linux you
will see at /proc/pci. Each device con­
figuration block is assigned to a device and
a function ID. To identify a certain device
while driver writing you will at least have
to know the vendor­ and the device­id that
is statically stored in the device configura­
tion block (as discussed later on). Driver
writers normally need to know only the
base address of the device and the IRQ
line that the device is using. As shown
above all device id's or device class fields
that cannot be resolved within the linux
PCI­subsystem are printed as plain values,
so driver writers have a simple chance to
identify their card of choice without writ­
ing a special tool to do this task.
The Device Configuration Block
As mentioned above each PCI­device
has its own assigned 256bytes PCI­
configuration memory block that is acces­
sible through PCI­BIOS routines. The first
64bytes of this reserved space are used
to identify and configure the basic proper­
ties for the PCI­Device. On system startup
some of the parameters in the configura­
tion block are replaced from the Plug&Play
BIOS that configures the I/O or Mem­
ory base addresses and IRQ lines for the
PCI­board to operate correctly with other
cards in the PCI or ISA slots. The config­
uration blocks of the PCI subsystem can
be accessed through the CONFIG ADDRESS
The Linux Lab Project whitepapers 11/1996 (http://www.llp.fu­berlin.de/)

Linux­PCI Support
(0xcf8) and the CONFIG DATA (0xcfc)
registers
The Linux PCI­bios support is imple­
mented on top of the Bios32 routines and
defines some useful routines to handle the
PCI configuration block and configure the
PCI subsystem. Normally no changes in
the configuration block are necessary be­
cause the BIOS sets up all parameters to
operate correctly. Because Linux is in­
tended to run on many hardware architec­
tures it is recommended to use this func­
tions instead of accessing the configura­
tion registers directly.
Before a PCI­board can be used, the driver
has to determine the board specific param­
eters from the configuration block and set
all desired options to operate the board
correctly. Normally this task is done by a
autodetect routine in the kernel driver that
performs the following tasks:
ffl Check if the PCI­Bios Support is en­
abled in the kernel and the BIOS is
present with pcibios present()
ffl Get the configuration block
for the desired device using
pcibios find device()
ffl Get the desired parameters
from the configuration block us­
ing pcibios read config byte(),
pcibios read config word() or
pcibios read config dword() to
get 8,16 or 32bit parameter values.
ffl Set the desired parameters in
the configuration block using
pcibios write config byte(),
pcibios write config word() or
pcibios write config dword()
The following example from the ne2000
clone driver shows how this task is per­
formed in a real kernel network driver.
#if defined(CONFIG_PCI)
if (pcibios_present()) {
int pci_index;
for (pci_index = 0; pci_index < 8; pci_index++) {
unsigned char pci_bus, pci_device_fn;
unsigned int pci_ioaddr;
/* Currently only Realtek are making PCI ne2k clones. */
if (pcibios_find_device (PCI_VENDOR_ID_REALTEK,
PCI_DEVICE_ID_REALTEK_8029, pci_index,
&pci_bus, &pci_device_fn) != 0)
break; /* OK, now try to probe for std. ISA card */
pcibios_read_config_byte(pci_bus, pci_device_fn,
PCI_INTERRUPT_LINE, &pci_irq_line);
pcibios_read_config_dword(pci_bus, pci_device_fn,
PCI_BASE_ADDRESS_0, &pci_ioaddr);
/* Strip the I/O address out of the returned value */
pci_ioaddr &= PCI_BASE_ADDRESS_IO_MASK;
/* Avoid already found cards from previous ne_probe() calls */
if (check_region(pci_ioaddr, NE_IO_EXTENT))
continue;
printk("ne.c: PCI BIOS reports ne2000 clone at i/o %#x, irq %d.\n",
pci_ioaddr, pci_irq_line);
if (ne_probe1(dev, pci_ioaddr) != 0) { /* Shouldn't happen. */
printk(KERN_ERR "ne.c: Probe of PCI card at %#x failed.\n", pci_ioaddr);
break; /* Hrmm, try to probe for ISA card... */
}
The Linux Lab Project whitepapers 11/1996 (http://www.llp.fu­berlin.de/)

Linux­PCI Support
pci_irq_line = 0;
return 0;
}
}
#endif /* defined(CONFIG_PCI) */
This card uses one IRQ and one base ad­
dress in I/O space to communicate with
the driver. The lower bits of the returned
base address have different meanings de­
pending if the address is an I/O address
or a memory mapped area is returned. On
an I/O address bit 0 is always 1 and bit
2 is always zero. On a memory mapped
address type the lower 4 bits have the fol­
lowing meanings:
Bit Description
0 always zero
1­2 adress type:
00=arbitrary 32 bit,
01=below 1M,
10=arbitrary 64­bit
3 prefetchable
To keep things simple Linux defines two
macros PCI BASE ADDRESS MEM MASK and
PCI BASE ADDRESS IO MASK that can be
used to strip the type bits from the re­
turned address. The prefetch bit is set
if the PCI­bridge is allowed to read the
memory block in a prefetch buffer with­
out problems, this is normally the case if
the memory block is a memory area on the
PCI­device, if the memory area maps hard­
ware registers into memory the prefetch bit
should not be set.
There are a whole bunch of other setable
and readable parameters in the configura­
tion block, that can be accessed through
the linux pcibios XXX() routines. Linux
defines some useful Mnemonics to gain
simple access to the configuration block
parameter values that are defined in
include/linux/pci.h. The Tables 1,2
and 3 will describe the Macros in detail.
Reading and writing to a device
Reading and writing to a PCI device is as
easy as reading and writing to an ISA board
or reading and writing to a memory area
depending on the base address type of the
device.
If the device uses an I/O type base ad­
dress, normal I/O can be performed using
the usual inb()/outb(), inw()/outw()
or the 32bit inl()/outl() routines. To
get an impression how other linux drivers
use this access modes please take a look
through the network or scsi drivers.
If the device uses a memory mapped
area it is recommended to use the
readb()/writeb(), readw()/writew()
or the longword readl()/writel() rou­
tines to read or write to a single location
(see include/asm/io.h). memcpy() can also
be used to transfer a whole memory block
as this is the case on framegrabber de­
vices or whatever transfers large blocks
of data. In this case the PCI­Bridge han­
dles automatically which transfer mode is
optimal for the bus system. For this pur­
pose the PCI­bridge has internal prefetch
queues (one for the Bus side, one for the
CPU subsystem side) that are used to store
transfer data before a send or receive is ini­
tiated. If the PREF (prefetchable) bit is set
in the base address bit 3, the memory area
is non prefetchable, in other words data
and addressed are dumped directly to the
bus­subsystem. This mode is intended for
devices that maps special registers in a
memory area so the CPU can read or write
this registers without a PCI­Bridge related
delay.
Busmastering
With busmaster transfer mode a controller
can transfer its data to the memory with­
out any CPU action by driving the ap­
propriate bus­lines on a hardware basis.
This is similar to the DMA transfer on
a ISA bus with the only difference that
the busmaster­controller sits on the PCI­
adapter card and not on the motherboard.
If a device is busmaster capable the
PCI COMMAND MASTER bit should have
The Linux Lab Project whitepapers 11/1996 (http://www.llp.fu­berlin.de/)

Linux­PCI Support
been set in the PCI COMMAND regis­
ter. If a busmaster transfer has been
aborted by the busmaster device, the
PCI STATUS REC MASTER ABORT bit in the
PCI STATUS register is set, on the receiving
end an abort can be notified by looking on
the PCI STATUS REC TARGET ABORT bit.
Macro Width Description
PCI VENDOR ID 16bit Unique Vendor ID (see pci.h) This ID is defined by
the PCI consortium
PCI DEVICE ID 16bit Unique Device ID (see pci.h) this is defined by the
vendor unique for each device
PCI COMMAND 16bit This field is used for device specific configuration
commands (see table 2)
PCI STATUS 16bit This is used for board specific status results (see
table 3)
PCI CLASS REVISION 32bit The high 24bits are used to determine the device's
class type the low 8bit for revision code
PCI CLASS DEVICE The device's class code (see pci.h)
PCI BIST 8bit If PCI BIST CAPABLE is set the device can perform
a Build­in self test that can be started by writing
PCI BIST START to this field. The result mask is
PCI BIST CODE MASK
PCI HEADER TYPE 8bit Specifies the Layout type for the following 48 bytes
in PCI configuration (currently only 0x0)
PCI LATENCY TIMER 8bit This is the maximal time a PCI cycle may consume
(time=latency+8cycles)
PCI CACHE LINE SIZE 8bit Specifies the Cache­Line Size in units of 32bytes,
PCI BASE ADDRESS [0­5] 32bit This are up to 6 memory locations the device can
map its memory area(s) to. The lower 4 bits are
used to specify the type and the access mode of the
memory area.
PCI ROM ADDRESS 32bit bit 11­31 is the start address of the device's ROM
area. (write bit 1 to enable ROM)
PCI MIN GNT 8bit minimal latency time (vendor specific)
PCI MIN LAT 8bit maximal latency time (vendor specific)
PCI INTERRUPT PIN 8bit This entry denotes the IRQ pin that should be used
1=INTA 2=INTB 0=disabled
PCI INTERRUPT LINE 8bit This entry specifies the interrupt line on which the
device IRQ is mapped (usually IRQ 0­15)
Table 1: The Linux PCI­Configuration macros
The Linux Lab Project whitepapers 11/1996 (http://www.llp.fu­berlin.de/)

Linux­PCI Support
Macro Description
PCI COMMAND IO Enable I/O area
PCI COMMAND MEMORY Enable Memory area
PCI COMMAND MASTER Enable Busmastering
PCI COMMAND SPECIAL Enable response to special cycles
PCI COMMAND INVALIDATE Use memory write and invalidate
PCI COMMAND VGA PALETTE Enable video palette access
PCI COMMAND PARITY Enable parity checking
PCI COMMAND WAIT Enable address/data stepping
PCI COMMAND SERR Enable SERR
PCI COMMAND FAST BACK Enable back­to­back writes
Table 2: The Linux PCI­Command Bit­Settings
Macro Description
PCI STATUS 66MHZ Support 66 Mhz PCI 2.1 bus
PCI STATUS UDF Support User Definable Features
PCI STATUS FAST BACK Accept fast­back to back
PCI STATUS PARITY Detected parity error
PCI STATUS DEVSEL [MASK|FAST|MEDIUM|SLOW] DEVSEL timing
PCI STATUS SIG TARGET ABORT Set on target abort
PCI STATUS REC TARGET ABORT Master ack of
PCI STATUS REC MASTER ABORT Set on master abort
PCI STATUS SIG SYSTEM ERROR Set when we drive SERR
PCI STATUS DETECTED PARITY Set on parity error
Table 3: The Linux PCI­Status Bit­Settings
The Linux Lab Project whitepapers 11/1996 (http://www.llp.fu­berlin.de/)