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

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Date:	Wed, 3 Jun 2015 11:07:26 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:	Bjorn Helgaas <bhelgaas@...gle.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

On Wed, Jun 03, 2015 at 05:55:35PM +0100, Lorenzo Pieralisi wrote:
> On Wed, Jun 03, 2015 at 04:12:24PM +0100, Guenter Roeck wrote:
> 
> [...]
> 
> > >> 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
> > >
> > > That's what we would like to change :)
> > >
> > > https://lkml.org/lkml/2015/5/21/359
> > 
> > Yes, that should help. I had a brief look last night and concluded
> > that this would require changes all over the place, which your patch
> > pretty much confirms. Glad that you are tackling it - changes all over
> > the place spell trouble and would probably require more time than I have
> > available to spend on the problem.
> 
> Eh, trouble did not even start because we have just tested it on ARM/ARM64
> systems (that's all I can do no sign of testing on any other arch), so I do
> not expect it will be merged quickly, it will take me time to get all the
> required acks.
> 
> I should be able to send a v2 beginning of next week.
> 
Ok.

> > >> 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 ?
> > >
> > > It is upstream code with a couple of ARM64 related patches not yet
> > > merged. Still, it shows an issue that must be tackled.
> > >
> > > It is not caused by your patches but it can be solved by them.
> > > On PROBE_ONLY systems, all resources must be claimed (since they
> > > are not reassigned, hence not claimed by the code that reassigns them),
> > > otherwise we can't enable a device resources (ie pcibios_enable_device
> > > calls pci_enable_resources that fails, since resources are not claimed).
> > >
> > > That's why we are suggesting claiming the bridge apertures as soon
> > > as they are read from the base registers, even on PROBE_ONLY systems.
> > >
> > > I think that's the only approach Bjorn would accept, otherwise
> > > we will have to fiddle with PROBE_ONLY on ARM64, and either avoid calling
> > > pci_enable_resources or avoid checking if a resource is claimed in
> > > pci_enable_resources, neither solution seems sane to me.
> > >
> > 
> > Looks like I'll need one of those arm64 systems at some point ;-).
> > 
> > Where is your patch in respect to acceptance ? Would it make sense to
> > merge it into my code and base my patch(es) on it, or do you expect
> > major changes which would make that difficult ?
> 
> I have a tweak to v1, I will post v2 next week and copy you in.

Thanks!

> Acceptance, I think it received review only from ARM guys/platforms
> so we are still far from merging it.
> 
I should be able to test it on powerpc (p2020/p5020) and x86.

Guenter
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