Messages in this thread | | | Date | Sat, 26 Sep 2015 14:49:59 +0800 | Subject | Re: [RFC] PCI: Unassigned Expansion ROM BARs | From | Daniel J Blueman <> |
| |
On Thursday, September 24, 2015 at 10:50:07 AM UTC+8, Myron Stowe wrote: > I've encountered numerous bugzilla reports related to platform BIOS' not > programming valid values into a PCI device's Type 0 Configuration space > "Expansion ROM Base Address" field (a.k.a. Expansion ROM BAR). The main > observed consequence being 'dmesg' entries like the following that get > customers excited enough to file reports against the kernel.
PCI option ROMs legitimately hold real-mode/EFI code needed to initialise devices; the problem is, we can't guarantee that the BIOS has initialised all devices with the option ROM code, so linux must ensure they are correctly accessible.
In addition to VMs as Alex points out, hotplug (eg Thunderbold GPUs) and PCI domains which may not be visible to the BIOS at early boot, may need the option ROM. Nvidia GPUs primarily have had a lot of encoder/connector (HDCP?) and product-specific voltage-frequency setup code and tables in the ROM.
As such, in my NumaConnect open firmware which maps the PCI domains of multiple servers into one, I have to also reallocate PCI option ROMs [1] to guarantee GPU VBIOS execution in linux. That said, option ROMs are a dying trend in favour of shipped binary blobs and open-coded initialisation for cross-platform support, and there are only 10 users of pci_map_rom().
Thanks, Daniel
[1] https://github.com/numascale/nc-utils/blob/master/bootloader/dnc-mmio.c -- Daniel J Blueman
| |