Re: [PATCH] VGA arbitration: draft of kernel side

From: Jon Smirl
Date: Tue Mar 08 2005 - 23:37:05 EST

This is from /linux-2.5/Documentation/filesystems/sysfs-pci.txt. It
describes how ia64 is achieving legacy IO. The VGA control code
probably needs to be coordinated with this.


Accessing legacy resources through sysfs

Legacy I/O port and ISA memory resources are also provided in sysfs if the
underlying platform supports them. They're located in the PCI class heirarchy,

|-- bridge -> ../../../devices/pci0000:17
|-- cpuaffinity
|-- legacy_io
`-- legacy_mem

The legacy_io file is a read/write file that can be used by applications to
do legacy port I/O. The application should open the file, seek to the desired
port (e.g. 0x3e8) and do a read or a write of 1, 2 or 4 bytes. The legacy_mem
file should be mmapped with an offset corresponding to the memory offset
desired, e.g. 0xa0000 for the VGA frame buffer. The application can then
simply dereference the returned pointer (after checking for errors of course)
to access legacy memory space.

Supporting PCI access on new platforms

In order to support PCI resource mapping as described above, Linux platform
code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
Platforms are free to only support subsets of the mmap functionality, but
useful return codes should be provided.

Legacy resources are protected by the HAVE_PCI_LEGACY define. Platforms
wishing to support legacy functionality should define it and provide
pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.

Jon Smirl
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at