[v4,7/7] PCI: Add comments about ROM BAR updating

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 755528c860b0 ("Ignore disabled ROM resources at
setup") and 8085ce084c0f ("[PATCH] Fix PCI ROM mapping").

Link: https://lkml.org/lkml/2005/8/30/138
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
---
 drivers/pci/rom.c       |    5 +++++
 drivers/pci/setup-res.c |    6 ++++++
 2 files changed, 11 insertions(+)

On Mon, Nov 28, 2016 at 10:16:48PM -0600, Bjorn Helgaas wrote:
>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 755528c860b0 ("Ignore disabled ROM resources at
>setup") and 8085ce084c0f ("[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>

>---
> drivers/pci/rom.c       |    5 +++++
> drivers/pci/setup-res.c |    6 ++++++
> 2 files changed, 11 insertions(+)
>
>diff --git a/drivers/pci/rom.c b/drivers/pci/rom.c
>index 06663d3..b6edb18 100644
>--- a/drivers/pci/rom.c
>+++ b/drivers/pci/rom.c
>@@ -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;
>diff --git a/drivers/pci/setup-res.c b/drivers/pci/setup-res.c
>index 09bdff7..57d7041 100644
>--- a/drivers/pci/setup-res.c
>+++ b/drivers/pci/setup-res.c
>@@ -67,6 +67,12 @@ 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;
>
>
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