Re: [PATCH v4 3/5] PCI: Do not disable memory decoding in pci_reassigndev_resource_alignment()

[Yongji Xie <xyjxie@linux.vnet.ibm.com>]

On 2016/9/7 0:56, Bjorn Helgaas wrote:

> 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 found that disabling memory decoding would not break
this kind of bridge.

However, it would cause some problems if we have a
VGA device using "vgaarb" driver behind the bridge.

The driver didn't call something like pci_enable_device()
when it is loaded or initialized. So when the driver issued
memory access to the VGA device, the access cannot be
supported by the bridge because its memory decoding is
still disabled.

Maybe we should drop this patch and try to fix the driver.

Thanks,
Yongji

> 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.
>
>> 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 <xyjxie@linux.vnet.ibm.com>
>> ---
>>   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
>>
>> --
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