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PCI: Add comments about ROM BAR updating

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pci_update_resource() updates a hardware BAR so its address matches the
kernel's struct resource UNLESS it's a disabled ROM BAR.  We only update
those when we enable the ROM.

It's not obvious from the code why ROM BARs should be handled specially.
Apparently there are Matrox devices with defective ROM BARs that read as
zero when disabled.  That means that if pci_enable_rom() reads the disabled
BAR, sets PCI_ROM_ADDRESS_ENABLE (without re-inserting the address), and
writes it back, it would enable the ROM at address zero.

Add comments and references to explain why we can't make the code look more
rational.

The code changes are from 755528c860 ("Ignore disabled ROM resources at
setup") and 8085ce084c ("[PATCH] Fix PCI ROM mapping").

Link: https://lkml.org/lkml/2005/8/30/138
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
This commit is contained in:
Bjorn Helgaas 2016-11-28 16:17:41 -06:00
parent 7a6d312b50
commit 0b457dde3c
2 changed files with 11 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
drivers/pci/rom.c
View File

@ -35,6 +35,11 @@ int pci_enable_rom(struct pci_dev *pdev)
if (res->flags & IORESOURCE_ROM_SHADOW)
return 0;
/*
* Ideally pci_update_resource() would update the ROM BAR address,
* and we would only set the enable bit here. But apparently some
* devices have buggy ROM BARs that read as zero when disabled.
*/
pcibios_resource_to_bus(pdev->bus, &region, res);
pci_read_config_dword(pdev, pdev->rom_base_reg, &rom_addr);
rom_addr &= ~PCI_ROM_ADDRESS_MASK;

6
drivers/pci/setup-res.c
View File

@ -72,6 +72,12 @@ static void pci_std_update_resource(struct pci_dev *dev, int resno)
if (resno < PCI_ROM_RESOURCE) {
reg = PCI_BASE_ADDRESS_0 + 4 * resno;
} else if (resno == PCI_ROM_RESOURCE) {
/*
* Apparently some Matrox devices have ROM BARs that read
* as zero when disabled, so don't update ROM BARs unless
* they're enabled. See https://lkml.org/lkml/2005/8/30/138.
*/
if (!(res->flags & IORESOURCE_ROM_ENABLE))
return;