Re: [PATCH v3] efifb: avoid reconfiguration of BAR that covers the framebuffer

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On 10 April 2017 at 18:13, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> On 10 April 2017 at 17:53, Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> wrote:
>> On Mon, Apr 10, 2017 at 04:28:11PM +0100, Ard Biesheuvel wrote:
>>> On 2 April 2017 at 16:16, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> > On 30 March 2017 at 14:50, Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote:
>>> >> On 3/30/2017 9:38 AM, Ard Biesheuvel wrote:
>>> >>> On 30 March 2017 at 11:09, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> >>>> On 30 March 2017 at 11:05, Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> wrote:
>>> >>>>> On Thu, Mar 30, 2017 at 09:46:39AM +0100, Ard Biesheuvel wrote:
>>> >>>>>
>>> >>>>> [...]
>>> >>>>>
>>> >>>>>>> I'm asking why we don't fix the actual problem in PCIe ARM64 adaptation instead
>>> >>>>>>> of working around it by quirks.
>>> >>>>>>>
>>> >>>>>>> I don't see any reason why ACPI ARM64 should carry the burden of legacy systems.
>>> >>>>>>>
>>> >>>>>>> Legacy only applies to DT based systems.
>>> >>>>>>>
>>> >>>>>>
>>> >>>>>> I fully agree with this point: ACPI implies firmware, and so we should
>>> >>>>>> be able to rely on firmware to have initialized the PCIe subsystem by
>>> >>>>>> the time the kernel gets to access it.
>>> >>>>>
>>> >>>>> https://lkml.org/lkml/2016/3/3/458
>>> >>>>>
>>> >>>>
>>> >>>> I don't think the fact that at least one system existed over a year
>>> >>>> ago whose UEFI assigned resources incorrectly should prevent us from
>>> >>>> being normative in this case.
>>> >>>
>>> >>> In any case, given that EFIFB is enabled by default on some distros,
>>> >>> and the fact that DT boot is affected as well, I should get this patch
>>> >>> in to prevent serious potential issues that could arise when someone
>>> >>> with a graphical UEFI stack updates to such a new kernel.
>>> >>>
>>> >>> So I think we are in agreement that this is needed on both ARM and
>>> >>> arm64, since their PCI configuration is usually not preserved. The
>>> >>> open question is whether there is any harm in enabling it for x86 as
>>> >>> well.
>>> >>>
>>> >>
>>> >> Agreed, the other issue is about compatibility with UEFI and future
>>> >> proofing Linux for other potential issues like hotplug reservation.
>>> >>
>>> >
>>> > OK, given the lack of feedback regarding the suitability of this patch
>>> > for x86, I am going to rework it as a ARM/arm64 only patch, and queue
>>> > it as a fix for v4.11. This way, we can backport it to stable (which
>>> > is arguably appropriate, given that upgrading to a EFIFB enabled
>>> > kernel may cause severe breakage for existing systems that implement
>>> > the GOP protocol), and easily change the patch to apply to x86 going
>>> > forwards, by removing the #ifdefs
>>> >
>>>
>>> As it turns out, this patch does not solve the problem completely.
>>>
>>> For EFI framebuffers that are backed by a PCI bar of a device residing
>>> on bus 0, things work happily. However, claiming resources for devices
>>> behind bridges doesn't work.
>>
>> May I ask you to elaborate on this please ? It is because we do not
>> claim the bridge windows upstream the device and they are reassigned ?
>>
>
> The pci_claim_resource() call fails like this
>
> pci 0000:01:01.0: can't claim BAR 0 [mem 0x10000000-0x10ffffff pref]:
> no compatible bridge window
> pci 0000:01:01.0: BAR 0: failed to claim resource for efifb!
>
> which is caused by the fact that the parent resources are all zeroes.
> It appears the BAR configuration is never read from the bridges, so
> the only way we will ever have meaningful values in there is if we
> allocate them ourselves.
>
>>> Given that we have not made the situation worse, fixing it is less
>>> urgent than it was before. I.e., there is no longer a risk of
>>> inadvertently poking the wrong BAR when writing to the framebuffer.
>>> There is a regression in functionality, though, since EFI fb devices
>>> that happened to work (because the firmware BAR == the kernel BAR)
>>> have stopped working if they are behind a bridge, which is of course
>>> always the case for PCIe.
>>>
>>> So before starting the next round of hacking to work around this, I
>>> would like rekindle the discussion regarding the way we blindly
>>> reassign all resources on ACPI/arm64 systems, and whether there is a
>>> way imaginable to avoid doing that.
>>
>> There is a way if the whole ARM ecosystem work together to sort this
>> out and we think that's the right way to do; I am personally not
>> entirely convinced about that.
>>
>
> So what are the pros and cons here? EFI fb is not a hugely important
> use case, but it is one that relies on BARs staying where they are.
> Are there others like that?
>
>>> I suppose the state of the BARs as we inherit it from the firmware
>>> cannot be blindly trusted (and IIUC, this is Lorenzo's primary issue
>>> with it). So should there be some side channel (UEFI config table
>>> perhaps?) to describe this?
>>
>> PCI firmware specifications rev 3.1, 4.6.5, "_DSM for Ignoring PCI
>> Boot Configurations".
>>
>> Do we want to enforce it on ARM ? I do not know to be honest (and it
>> still would not solve the DT firmware case).
>>
>
> No, it doesn't. But that doesn't mean we shouldn't solve it on ACPI if
> the pros outweigh the cons.
>
>> Whatever we do, it is not going to be clean cut IMO. I think that
>> what x86 does is sensible (well, minus the link ordering dependency we
>> discovered), I can do it for ARM64 but get ready for regressions and
>> I still think we have no real FW binding support that would make this
>> behaviour robust.
>>
>> I can provide you with examples where, by simply claiming resources
>> on an ARM system you trigger resources allocation regressions by
>> preventing the kernel from allocating bigger bridge windows than
>> the ones set-up by FW.
>>
>
> So how is this specific to ARM then? If we are running the same
> resource allocation routines, why should we end up with a firmware
> allocation that the kernel cannot use?
>
>>> How do other architectures deal with this?
>>
>> On an arch specific basis that make things work.
>>
>>> Is this what the PCI bios accesses are for?
>>
>> Which ones :) ?
>>
>
> Well, some of them :-)
>
> I guess the question was if the overridden __weak methods are supposed
> to hook into tables or other BIOS structures that contain information
> about the PCI resource allocations by the firmware.
>
>>> In any case, I have updated the UEFI firmware we have for ARM Juno to
>>> use EDK2's generic PCI host bridge driver instead of one that was
>>> specially written for ARM Juno, and may deviate in the way it
>>> allocates PCI resources.
>>
>> As long as the kernel is free to reallocate them and that FW quiesces
>> devices at FW->OS handover I see no issues with that.
>>
>
> The reason is to eliminate another unknown from the discussion whether
> UEFI can be expected to leave the entire PCI hierarchy in a sane
> state.
>
>> If we want to try to claim the whole resource tree on boot (in ACPI)
>> I can send a patch for that but there will be regressions.
>>
>
> I would like to see it, yes.

FWIW, the following minimal [naive] patch

diff --git a/arch/arm64/kernel/pci.c b/arch/arm64/kernel/pci.c
index 4f0e3ebfea4b..37c4d2f116a4 100644
--- a/arch/arm64/kernel/pci.c
+++ b/arch/arm64/kernel/pci.c
@@ -27,7 +27,7 @@
  */
 void pcibios_fixup_bus(struct pci_bus *bus)
 {
-       /* nothing to do, expected to be removed in the future */
+       pci_read_bridge_bases(bus);
 }

 /*
@@ -208,8 +208,8 @@ struct pci_bus *pci_acpi_scan_root(struct
acpi_pci_root *root)
        if (!bus)
                return NULL;

-       pci_bus_size_bridges(bus);
-       pci_bus_assign_resources(bus);
+       pci_assign_unassigned_root_bus_resources(bus);
+       pci_bus_claim_resources(bus);

        list_for_each_entry(child, &bus->children, node)
                pcie_bus_configure_settings(child);

running under QEMU+UEFI with the following PCI topology

-[0000:00]-+-00.0  Red Hat, Inc. Device 0008
           +-01.0-[01]----01.0  Device 1234:1111
           +-02.0-[02]--+-02.0  Red Hat, Inc Virtio RNG
           |            \-03.0  Red Hat, Inc Virtio block device
           \-03.0-[03]----04.0  Red Hat, Inc Virtio network device

results in the log below, preserving the configuration created by UEFI

pci_bus 0000:00: root bus resource [mem 0x10000000-0x3efeffff window]
pci_bus 0000:00: root bus resource [io  0x0000-0xffff window]
pci_bus 0000:00: root bus resource [mem 0x8000000000-0xffffffffff window]
pci_bus 0000:00: root bus resource [bus 00-0f]
pci 0000:00:00.0: [1b36:0008] type 00 class 0x060000
pci 0000:00:01.0: [1b36:0001] type 01 class 0x060400
pci 0000:00:01.0: reg 0x10: [mem 0x8000202000-0x80002020ff 64bit]
pci 0000:00:02.0: [1b36:0001] type 01 class 0x060400
pci 0000:00:02.0: reg 0x10: [mem 0x8000201000-0x80002010ff 64bit]
pci 0000:00:03.0: [1b36:0001] type 01 class 0x060400
pci 0000:00:03.0: reg 0x10: [mem 0x8000200000-0x80002000ff 64bit]
pci 0000:01:01.0: [1234:1111] type 00 class 0x030000
pci 0000:01:01.0: reg 0x10: [mem 0x10000000-0x10ffffff pref]
pci 0000:01:01.0: reg 0x18: [mem 0x11200000-0x11200fff]
pci 0000:01:01.0: reg 0x30: [mem 0xffff0000-0xffffffff pref]
pci 0000:01:01.0: can't claim BAR 0 [mem 0x10000000-0x10ffffff pref]:
no compatible bridge window
pci 0000:01:01.0: BAR 0: failed to claim resource for efifb!
pci 0000:00:01.0: PCI bridge to [bus 01]
pci 0000:00:01.0:   bridge window [mem 0x11200000-0x112fffff]
pci 0000:00:01.0:   bridge window [mem 0x10000000-0x10ffffff 64bit pref]
pci 0000:02:02.0: [1af4:1005] type 00 class 0x00ff00
pci 0000:02:02.0: reg 0x10: [io  0x1040-0x105f]
pci 0000:02:02.0: reg 0x20: [mem 0x8000004000-0x8000007fff 64bit pref]
pci 0000:02:03.0: [1af4:1001] type 00 class 0x010000
pci 0000:02:03.0: reg 0x10: [io  0x1000-0x103f]
pci 0000:02:03.0: reg 0x14: [mem 0x11100000-0x11100fff]
pci 0000:02:03.0: reg 0x20: [mem 0x8000000000-0x8000003fff 64bit pref]
pci 0000:00:02.0: PCI bridge to [bus 02]
pci 0000:00:02.0:   bridge window [io  0x1000-0x1fff]
pci 0000:00:02.0:   bridge window [mem 0x11100000-0x111fffff]
pci 0000:00:02.0:   bridge window [mem 0x8000000000-0x80000fffff 64bit pref]
pci 0000:03:04.0: [1af4:1000] type 00 class 0x020000
pci 0000:03:04.0: reg 0x10: [io  0x0000-0x001f]
pci 0000:03:04.0: reg 0x14: [mem 0x11000000-0x11000fff]
pci 0000:03:04.0: reg 0x20: [mem 0x8000100000-0x8000103fff 64bit pref]
pci 0000:03:04.0: reg 0x30: [mem 0xfffc0000-0xffffffff pref]
pci 0000:00:03.0: PCI bridge to [bus 03]
pci 0000:00:03.0:   bridge window [io  0x0000-0x0fff]
pci 0000:00:03.0:   bridge window [mem 0x11000000-0x110fffff]
pci 0000:00:03.0:   bridge window [mem 0x8000100000-0x80001fffff 64bit pref]
pci 0000:00:01.0: BAR 14: assigned [mem 0x10000000-0x117fffff]
pci 0000:00:01.0: BAR 15: assigned [mem 0x8000000000-0x8000ffffff 64bit pref]
pci 0000:00:02.0: BAR 14: assigned [mem 0x11800000-0x118fffff]
pci 0000:00:02.0: BAR 15: assigned [mem 0x8001000000-0x80010fffff 64bit pref]
pci 0000:00:03.0: BAR 14: assigned [mem 0x11900000-0x119fffff]
pci 0000:00:03.0: BAR 15: assigned [mem 0x8001100000-0x80011fffff 64bit pref]
pci 0000:00:02.0: BAR 13: assigned [io  0x1000-0x1fff]
pci 0000:00:03.0: BAR 13: assigned [io  0x2000-0x2fff]
pci 0000:00:01.0: BAR 0: assigned [mem 0x8001200000-0x80012000ff 64bit]
pci 0000:00:02.0: BAR 0: assigned [mem 0x8001200100-0x80012001ff 64bit]
pci 0000:00:03.0: BAR 0: assigned [mem 0x8001200200-0x80012002ff 64bit]
pci 0000:01:01.0: BAR 0: assigned [mem 0x10000000-0x10ffffff pref]
pci 0000:01:01.0: BAR 6: assigned [mem 0x11000000-0x1100ffff pref]
pci 0000:01:01.0: BAR 2: assigned [mem 0x11010000-0x11010fff]
pci 0000:00:01.0: PCI bridge to [bus 01]
pci 0000:00:01.0:   bridge window [mem 0x10000000-0x117fffff]
pci 0000:00:01.0:   bridge window [mem 0x8000000000-0x8000ffffff 64bit pref]
pci 0000:02:02.0: BAR 4: assigned [mem 0x8001000000-0x8001003fff 64bit pref]
pci 0000:02:03.0: BAR 4: assigned [mem 0x8001004000-0x8001007fff 64bit pref]
pci 0000:02:03.0: BAR 1: assigned [mem 0x11800000-0x11800fff]
pci 0000:02:03.0: BAR 0: assigned [io  0x1000-0x103f]
pci 0000:02:02.0: BAR 0: assigned [io  0x1040-0x105f]
pci 0000:00:02.0: PCI bridge to [bus 02]
pci 0000:00:02.0:   bridge window [io  0x1000-0x1fff]
pci 0000:00:02.0:   bridge window [mem 0x11800000-0x118fffff]
pci 0000:00:02.0:   bridge window [mem 0x8001000000-0x80010fffff 64bit pref]
pci 0000:03:04.0: BAR 6: assigned [mem 0x11900000-0x1193ffff pref]
pci 0000:03:04.0: BAR 4: assigned [mem 0x8001100000-0x8001103fff 64bit pref]
pci 0000:03:04.0: BAR 1: assigned [mem 0x11940000-0x11940fff]
pci 0000:03:04.0: BAR 0: assigned [io  0x2000-0x201f]
pci 0000:00:03.0: PCI bridge to [bus 03]
pci 0000:00:03.0:   bridge window [io  0x2000-0x2fff]
pci 0000:00:03.0:   bridge window [mem 0x11900000-0x119fffff]
pci 0000:00:03.0:   bridge window [mem 0x8001100000-0x80011fffff 64bit pref]
pci_bus 0000:00: resource 4 [mem 0x10000000-0x3efeffff window]
pci_bus 0000:00: resource 5 [io  0x0000-0xffff window]
pci_bus 0000:00: resource 6 [mem 0x8000000000-0xffffffffff window]
pci_bus 0000:01: resource 1 [mem 0x10000000-0x117fffff]
pci_bus 0000:01: resource 2 [mem 0x8000000000-0x8000ffffff 64bit pref]
pci_bus 0000:02: resource 0 [io  0x1000-0x1fff]
pci_bus 0000:02: resource 1 [mem 0x11800000-0x118fffff]
pci_bus 0000:02: resource 2 [mem 0x8001000000-0x80010fffff 64bit pref]
pci_bus 0000:03: resource 0 [io  0x2000-0x2fff]
pci_bus 0000:03: resource 1 [mem 0x11900000-0x119fffff]
pci_bus 0000:03: resource 2 [mem 0x8001100000-0x80011fffff 64bit pref]
pci 0000:00:01.0: PCI bridge to [bus 01]
pci 0000:00:01.0:   bridge window [mem 0x10000000-0x117fffff]
pci 0000:00:01.0:   bridge window [mem 0x8000000000-0x8000ffffff 64bit pref]
pci 0000:00:02.0: PCI bridge to [bus 02]
pci 0000:00:02.0:   bridge window [io  0x1000-0x1fff]
pci 0000:00:02.0:   bridge window [mem 0x11800000-0x118fffff]
pci 0000:00:02.0:   bridge window [mem 0x8001000000-0x80010fffff 64bit pref]
pci 0000:00:03.0: PCI bridge to [bus 03]
pci 0000:00:03.0:   bridge window [io  0x2000-0x2fff]
pci 0000:00:03.0:   bridge window [mem 0x11900000-0x119fffff]
pci 0000:00:03.0:   bridge window [mem 0x8001100000-0x80011fffff 64bit pref]
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