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

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Date:	Tue, 02 Jun 2015 10:02:49 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH] PCI: Only enable IO window if supported

On 06/02/2015 07:55 AM, Lorenzo Pieralisi wrote:
> Bjorn, Guenter,
>
> On Wed, May 27, 2015 at 10:04:47PM +0100, Bjorn Helgaas wrote:
>> [+cc Lorenzo, Suravee, Will]
>>
>> I cc'd Lorenzo, Suravee, and Will because Lorenzo is working on calling
>> pci_read_bases() from the PCI core instead of from arch code, and there are
>> likely some dependencies between these two things.
>>
>> On Fri, May 22, 2015 at 05:52:16PM -0700, Guenter Roeck wrote:
>>> The PCI subsystem always assumes that I/O is supported on PCIe bridges
>>> and tries to assign an I/O window to each port even if that is not
>>> the case.
>>>
>>> This may result in messages such as
>>>
>>> pcieport 0000:02:00.0: res[7]=[io  0x1000-0x0fff]
>>> 					get_res_add_size add_size 1000
>>> pcieport 0000:02:00.0: BAR 7: no space for [io  size 0x1000]
>>> pcieport 0000:02:00.0: BAR 7: failed to assign [io  size 0x1000]
>>>
>>> for each bridge port, even if a port or its parent does not support
>>> I/O in the first place.
>>>
>>> To avoid this message, check if a port supports I/O before trying to
>>> enable it. Also check if port's parent supports I/O, and only modify
>>> a port's I/O resource size if both the port and its parent support I/O.
>>>
>>> If IO is disabled after the initial port scan, the IO base and size
>>> registers are set to 0x00f0 to indicate that IO is disabled. A later
>>> rescan interprets this as "IO supported" and enables the IO range,
>>> even if the parent does not support IO. Handle this situation as well.
>>>
>>> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>>> ---
>>>   drivers/pci/probe.c     | 14 ++++++++++++++
>>>   drivers/pci/setup-bus.c |  4 ++--
>>>   include/linux/pci.h     |  9 +++++++++
>>>   3 files changed, 25 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>>> index 6675a7a1b9fc..f4944ef45148 100644
>>> --- a/drivers/pci/probe.c
>>> +++ b/drivers/pci/probe.c
>>> @@ -354,6 +354,20 @@ static void pci_read_bridge_io(struct pci_bus *child)
>>>   	base = (io_base_lo & io_mask) << 8;
>>>   	limit = (io_limit_lo & io_mask) << 8;
>>>
>>> +	/* If necessary, check if the bridge supports an I/O aperture */
>>> +	if (!io_base_lo && !io_limit_lo) {
>>> +		u16 io;
>>> +
>>> +		if (!pci_parent_supports_io(child))
>>> +			return;
>>> +
>>> +		pci_write_config_word(dev, PCI_IO_BASE, 0xe0f0);
>>> +		pci_read_config_word(dev, PCI_IO_BASE, &io);
>>> +		pci_write_config_word(dev, PCI_IO_BASE, 0x0);
>>> +		if (!io)
>>> +			return;
>>> +	}
>>
>> I really like the idea of pushing this into pci_read_bridge_io().
>>
>> I wonder if we can do the same with pci_read_bridge_mmio_pref(), and
>> somehow get rid of pci_bridge_check_ranges() altogether?
>>
>> I think I looked at doing that a while back, and it seems like there was
>> some wrinkle, but I don't remember what it was.
>

After looking into this some more, I think the wrinkle may be that
pci_read_bridge_bases() and thus pci_read_bridge_io() isn't called
on probe-only systems (if PCI_PROBE_ONLY is set). A secondary
problem is that pci_read_bridge_io() does not enable a resource
if it is explicitly disabled (base > limit), but the subsequent call
to pci_bridge_check_ranges() unconditionally enables it.

Not really sure how to address this; my current code checks IO support
in both pci_read_bridge_io() and pci_bridge_check_ranges(). And since
pci_read_bridge_io() is not always called, I don't see how it might
be possible to get rid of pci_bridge_check_ranges(), or even the check
for IO support in pci_bridge_check_ranges().

> While at it, do you think it is reasonable to also claim the bridge
> windows (resources) in the respective pci_read_bridge_* calls ?
>
> Is there a reason why we don't/can't do it ? I noticed that on
> PROBE_ONLY systems on ARM/ARM64 at the moment we do not claim
> the bridge apertures and this is not correct, see below:
>
> [5.980127] pcieport 0000:00:02.1: can't enable device: BAR 8
> [mem 0xbff00000 - 0xbfffffff] not claimed
> [5.988056] pcieport: probe of 0000:00:02.1 failed with error -22
>
Is this when trying my patches or with the current upstream code ?

Thanks,
Guenter

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