linux-kernel - Re: [PATCH v3] PCI: Only enable IO window if supported

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Message-ID: <55C2E79E.3090809@roeck-us.net>
Date:	Wed, 05 Aug 2015 21:50:38 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Bjorn Helgaas <bhelgaas@...gle.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH v3] PCI: Only enable IO window if supported

On 08/05/2015 09:25 PM, Yinghai Lu wrote:
> On Wed, Aug 5, 2015 at 7:22 PM, Guenter Roeck <linux@...ck-us.net> wrote:
>> On 08/05/2015 06:38 PM, Yinghai Lu wrote:
>>>
>>> On Wed, Aug 5, 2015 at 6:14 PM, Yinghai Lu <yinghai@...nel.org> wrote:
>>>>
>>>>
>>>> It only can avoid warning with bridge, and still have warning on
>>>> devices under the bridge.
>>>>
>>>> also would have problem on transparent bridges, like
>>>>
>>>> BRIDGE_A ---- BRIDGE_AA----DEVICE_AA
>>>>                      |
>>>>                      \-- BRIDGE_AB ---DEVICE_AB
>>>>
>>>> if only BRIDGE_A and BRIDGE_AA are transparent, and BRIDGE_AB is not.
>>>>
>>>> and if pci_bridge_supports_io() return false for BRIDGE_A.
>>>>
>>>> We will have all three bridges have PCI_BUS_FLAGS_SUPPORTS_IO cleared.
>>>> so all will not been sized and will not get assigned io port resource.
>>>>
>>>> later DEVICE_AA could root bus io port as parent, and get io resource
>>>> assigned.
>>>> but DEVICE_AB will not get assigned.
>>>>
>>>> so we should get rid of pci_bridge_supports_io(), and just check root
>>>> resource IO port.
>>>
>>>
>>> I would suggest this version instead:
>>>
>>> Subject: [PATCH] PCI: Only try to assign io port only if root bus support
>>> that
>>>
>>
>> Bjorn had asked me to move the IO support check out of
>> pci_bridge_check_ranges().
>> It looks like you want to keep it there.
>>
>> Can you explain your reasons ? Sorry, I just don't understand.
>
> in pci_alloc_child_bus(), child bus's bus_flags will be assigned with
> parent's bus_flags.
>
> so if one parent bus has bridge's io bar can not changed, then all
> children bus will also
> have same bus_flags, and their bridge io bar will not used.
>
> then if BRIDGE_AB is not transparent, we do need to use it's bridge io bar.
>
If I understand you correctly, we might need to change the code
in pci_read_bridge_io() as follows.

From

	if (!(child->bus_flags & PCI_BUS_FLAGS_SUPPORTS_IO))
		return;

to something like

	if (!(child->bus_flags & PCI_BUS_FLAGS_SUPPORTS_IO)) {
		if (dev->transparent)
			return;
		child->bus_flags |= PCI_BUS_FLAGS_SUPPORTS_IO;
	}

Would that solve the problem you are concerned about ?

Thanks,
Guenter

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