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

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Message-ID: <556F1A22.7050400@roeck-us.net>
Date:	Wed, 03 Jun 2015 08:15:46 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	"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

Bjorn,

On 06/02/2015 12:58 PM, Bjorn Helgaas wrote:
> On Tue, Jun 2, 2015 at 12:02 PM, Guenter Roeck <linux@...ck-us.net> wrote:
[ ... ]
>>>>
>>>> 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.
>
> I haven't researched this, but it sounds wrong that we skip
> pci_read_bridge_bases() if PCI_PROBE_ONLY is set.  I think
> PCI_PROBE_ONLY should mean "look, but don't touch."  So I think we
> should always look at the bridge windows, and my advice is to see if
> it looks reasonable to change this aspect of PCI_PROBE_ONLY.
>

Looks like Lorenzo's patch is going to take care of that problem.

How about the second problem mentioned above ? One option might be to
keep pci_bridge_check_ranges() but only use it to _enable_ the various
ranges (and maybe rename it accordingly) ?

Thanks,
Guenter

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