Re: [PATCH v4 3/5] PCI: Do not disable memory decoding in pci_reassigndev_resource_alignment()
From: Bjorn Helgaas <helgaas@kernel.org>
Date: 2016-09-06 16:56:58
Also in:
linux-pci
On Fri, Aug 12, 2016 at 01:42:24PM +0800, Yongji Xie wrote:
We should not disable memory decoding when we reassign alignment in pci_reassigndev_resource_alignment(). It's meaningless and have some side effects. For example, we found it would break this kind of P2P bridge: 0001:02:02.0 PCI bridge: PLX Technology, Inc. PEX 8718 16-Lane, 5-Port PCI Express Gen 3 (8.0 GT/s) Switch (rev aa)
I doubt that turning memory decode off breaks this bridge. I can believe that it could cause a problem, but I doubt it would be specific to this bridge. I also don't think it's meaningless. After your patch, we throw away our knowledge of what the BAR contains when we set "r->start = 0". But if you leave memory decoding enabled, the device will still respond at whatever address the BAR contains. That seems like a problem.
quoted hunk ↗ jump to hunk
And it may also potentially break the PCI devices with mmio_always_on bit set. Besides, disabling memory decoding is not expected in some fixup function such as fixup_vga(). The fixup_vga() read PCI_COMMAND_MEMORY to know whether the devices has been initialized by the firmware or not. Disabling memory decoding would cause the one initialized by firmware may not be set as the default VGA device when more than one graphics adapter is present. Signed-off-by: Yongji Xie <redacted> --- drivers/pci/pci.c | 8 +------- 1 file changed, 1 insertion(+), 7 deletions(-)diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c index b8357d7..caa0894 100644 --- a/drivers/pci/pci.c +++ b/drivers/pci/pci.c@@ -5046,7 +5046,6 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev) int i; struct resource *r; resource_size_t align, size; - u16 command; /* * VF BARs are RO zero according to SR-IOV spec 3.4.1.11. Their@@ -5069,12 +5068,7 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev) return; } - dev_info(&dev->dev, - "Disabling memory decoding and releasing memory resources.\n"); - pci_read_config_word(dev, PCI_COMMAND, &command); - command &= ~PCI_COMMAND_MEMORY; - pci_write_config_word(dev, PCI_COMMAND, command); - + dev_info(&dev->dev, "Releasing memory resources.\n"); for (i = 0; i < PCI_BRIDGE_RESOURCES; i++) { r = &dev->resource[i]; if (!(r->flags & IORESOURCE_MEM))-- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html