Re: [PATCH v3 3/7] PCI: Do not disable memory decoding in pci_reassigndev_resource_alignment()

[Yongji Xie]

Hi Gavin,

On 2016/7/1 8:50, Gavin Shan wrote:

> On Thu, Jun 30, 2016 at 06:53:09PM +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 effect. For example, some fixup functions such as
> > quirk_e100_interrupt() read PCI_COMMAND_MEMORY to know whether
> > the devices has been initialized by the firmware or not. If we
> > disable memory decoding here, these functions will get a wrong
> > information that the devices was not initialized by the firmware
> > which may cause a wrong fixup. Besides, disabling memory decoding
> > may also break some devices that need to have memory decoding
> > always-on during probing.
> It seems the changelog isn't correct enough if it's talking about
> below check in code:
>
> if (!(command & PCI_COMMAND_MEMORY) || !pci_resource_start(dev, 0))
> 	return;

Could you give me more details?  It seems that we couldn't disable
e100 interrupts which enabled by some firmware if we disabling
memory decoding here.

> > Signed-off-by: Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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 6ae02de..6241cfc 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -4820,7 +4820,6 @@ void pci_reassigndev_resource_alignment(struct pci_dev *dev)
> > 	int i;
> > 	struct resource *r;
> > 	resource_size_t align, size;
> > -	u16 command;
> >
> > 	/* We should never try to reassign VF's alignment */
> > 	if (dev->is_virtfn)
> > @@ -4838,12 +4837,7 @@ 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");
> Is there a problem you found with PCI_COMMAND removed? If so, could you
> please share more details, thanks.

Disabling memory decoding may break some devices which
have flag mmio_always_on. Besides the mmio_always_on case,
I also found this would break some P2P bridge such as
PEX 8718 controller.

Thanks,
Yongji