# pcimem tool ## License - SPDX-FileCopyrightText: 2010 Bill Farrow - SPDX-FileCopyrightText: 2023 Institute for Automation of Complex Power Systems, RWTH Aachen University - SPDX-FileCopyrightText: 2000 Jan-Derk Bakker - SPDX-License-Identifier: GPL-2.0-or-later ## Overview The pcimem application provides a simple method of reading and writing to memory registers on a PCI card. Usage: ./pcimem { sys file } { offset } [ type [ data ] ] sys file: sysfs file for the pci resource to act on offset : offset into pci memory region to act upon type : access operation type : [b]yte, [h]alfword, [w]ord data : data to be written ## Platform Support **WARNING!** This method is platform dependent and may not work on your particular target architecture. Refer to the PowerPC section below. ## Example bash# ./pcimem /sys/devices/pci0001\:00/0001\:00\:07.0/resource0 0 w /sys/devices/pci0001:00/0001:00:07.0/resource0 opened. Target offset is 0x0, page size is 4096 mmap(0, 4096, 0x3, 0x1, 3, 0x0) PCI Memory mapped to address 0x4801f000. Value at offset 0x0 (0x4801f000): 0xC0BE0100 ## Why do this at all? When I start working on a new PCI device driver I generally go through a discovery phase of reading and writing to certain registers on the PCI card. Over the years I have written lots of small kernel modules to probe addresses within the PCI memory space, constantly iterating: modify code, recompile, scp to target, load module, unload module, dmesg. Urk! There has to be a better way - sysfs and mmap() to the rescue. ## Sysfs Let's start at with the PCI files under sysfs: ```bash bash# ls -l /sys/devices/pci0001\:00/0001\:00\:07.0/ total 0 -rw-r--r-- 1 root root     4096 Jul  2 20:13 broken_parity_status lrwxrwxrwx 1 root root        0 Jul  2 20:13 bus -> ../../../bus/pci -r--r--r-- 1 root root     4096 Jul  2 20:13 class -rw-r--r-- 1 root root      256 Jul  2 20:13 config -r--r--r-- 1 root root     4096 Jul  2 20:13 device -r--r--r-- 1 root root     4096 Jul  2 20:13 devspec -rw------- 1 root root     4096 Jul  2 20:13 enable -r--r--r-- 1 root root     4096 Jul  2 20:13 irq -r--r--r-- 1 root root     4096 Jul  2 20:13 local_cpus -r--r--r-- 1 root root     4096 Jul  2 20:13 modalias -rw-r--r-- 1 root root     4096 Jul  2 20:13 msi_bus -r--r--r-- 1 root root     4096 Jul  2 20:13 resource -rw------- 1 root root     4096 Jul  2 20:13 resource0 -rw------- 1 root root    65536 Jul  2 20:13 resource1 -rw------- 1 root root 16777216 Jul  2 20:13 resource2 lrwxrwxrwx 1 root root        0 Jul  2 20:13 subsystem -> ../../../bus/pci -r--r--r-- 1 root root     4096 Jul  2 20:13 subsystem_device -r--r--r-- 1 root root     4096 Jul  2 20:13 subsystem_vendor -rw-r--r-- 1 root root     4096 Jul  2 20:13 uevent -r--r--r-- 1 root root     4096 Jul  2 20:13 vendor ``` The vendor and device files report the PCI vendor ID and device ID: ```bash bash# cat device 0x0001 ``` This info is also available from lspci ````bash bash# lspci -v 0001:00:07.0 Class 0680: Unknown device bec0:0001 (rev 01)     Flags: bus master, 66MHz, medium devsel, latency 128, IRQ 31     Memory at 8d010000 (32-bit, non-prefetchable) [size=4K]     Memory at 8d000000 (32-bit, non-prefetchable) [size=64K]     Memory at 8c000000 (32-bit, non-prefetchable) [size=16M] ``` This PCI card makes 3 seperate regions of memory available to the host computer. The sysfs resource0 file corresponds to the first memory region. The PCI card lets the host computer know about these memory regions using the BAR registers in the PCI config. ## `mmap()` These sysfs resource can be used with mmap() to map the PCI memory into a userspace applications memory space.  The application then has a pointer to the start of the PCI memory region and can read and write values directly. (There is a bit more going on here with respect to memory pointers, but that is all taken care of by the kernel). ```c fd = open("/sys/devices/pci0001\:00/0001\:00\:07.0/resource0", O_RDWR | O_SYNC); ptr = mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); printf("PCI BAR0 0x0000 = 0x%4x\n",  *((unsigned short *) ptr); ``` ## PowerPC To make this work on a PowerPC architecture you also need to make a small change to the pci core. My example is from kernel 2.6.34, and hopefully this will be fixed for us in a later kernel version. ```bash bash# vi arch/powerpc/kernel/pci-common.c     /* If memory, add on the PCI bridge address offset */      if (mmap_state == pci_mmap_mem) { -#if 0 /* See comment in pci_resource_to_user() for why this is disabled */ +#if 1 /* See comment in pci_resource_to_user() for why this is disabled */          *offset += hose->pci_mem_offset;  #endif          res_bit = IORESOURCE_MEM;            /* We pass a fully fixed up address to userland for MMIO instead of          * a BAR value because X is lame and expects to be able to use that          * to pass to /dev/mem !          *          * That means that we'll have potentially 64 bits values where some          * userland apps only expect 32 (like X itself since it thinks only          * Sparc has 64 bits MMIO) but if we don't do that, we break it on          * 32 bits CHRPs :-(          *          * Hopefully, the sysfs insterface is immune to that gunk. Once X          * has been fixed (and the fix spread enough), we can re-enable the          * 2 lines below and pass down a BAR value to userland. In that case          * we'll also have to re-enable the matching code in          * __pci_mmap_make_offset().          *          * BenH.          */ -#if 0 +#if 1         else if (rsrc->flags & IORESOURCE_MEM)                 offset = hose->pci_mem_offset; #endif ```