I apologize if this message is directed incorrectly; any hints you could
give on who else to ask on this would be appreciated.
I'm trying to get Linux running on a 405GP system with PCI. Actually
it's on the mambo simulator and I'm writing the PCI support. My
information about PCI comes from the PCI System Architecture
book by MindShare; also the PPC405GP User's Manual. My problem is
basically a big-endian/little-endian problem. All of the documents
agree that the Vendor ID is in bytes 0 and 1, and the Device ID
is in bytes 2 and 3. The header file include/linux/pci.h agrees:
#define PCI_VENDOR_ID 0x00 /* 16 bits */
#define PCI_DEVICE_ID 0x02 /* 16 bits */
But in drivers/pci/pci.c, when we are trying to discover
a particular device, in pci_scan_device, the code
reads out a dword (a 32-bit quantity) and then masks
out the two subfields:
if (pci_read_config_dword(temp, PCI_VENDOR_ID, &l))
return NULL;
....
memcpy(dev, temp, sizeof(*dev));
dev->vendor = l & 0xffff;
dev->device = (l >> 16) & 0xffff;
It seems to me this is incorrect for a big-endian machine
(like PowerPC). If we read the two 16-bit parts out of the
first 32-bit part, we will end up with:
0 1 2 3
vendor-id device-id
with a big-endian machine, but
0 1 2 3
device-id vendor-id
for a little endian machine. This would make the mask and
shift definition to the vendor and device field correct for little
endian, but swapped for big-endian.
There is a similar problem for the read of the Class Code
and Revision ID dword in pci_setup_device, where, again,
they read the dword and then shift to get the "upper 3 bytes"
while on a big-endian machine, you would actually want the
"lower 3 bytes" for the 3-byte Class Code field.
The fixes to this code is fairly simple -- always read the size
of field that is defined from the bytes where it is defined to be --
but I am more confused by why this has not shown up before.
This seems a fundamental problem -- incorrectly defining the
device and vendor fields for a PCI device -- which would have
prevented PCI devices from working on PowerPC systems
(or any other big-endian system), but I don't see #ifdef's for
it, or platform-specific code to correct it or anything else.
Yet since these fields are defined by hardware manufacturers,
I would expect they are always as defined by the PCI document,
that is always little-endian. So it seems that the code can't work,
and yet ...
I'd appreciate any comments or recommendations.
jim
James Peterson
IBM Austin Research Lab
Austin, Texas
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This archive was generated by hypermail 2b29 : Tue Apr 23 2002 - 22:00:25 EST