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Date:   Tue, 28 May 2019 02:39:24 -0500
From:   Shawn Anastasio <shawn@...stas.io>
To:     Alexey Kardashevskiy <aik@...abs.ru>, Oliver <oohall@...il.com>
Cc:     Sam Bobroff <sbobroff@...ux.ibm.com>, linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        rppt@...ux.ibm.com, Paul Mackerras <paulus@...ba.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, xyjxie@...ux.vnet.ibm.com,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v3 1/3] PCI: Introduce pcibios_ignore_alignment_request



On 5/28/19 1:27 AM, Alexey Kardashevskiy wrote:
> 
> 
> On 28/05/2019 15:36, Oliver wrote:
>> On Tue, May 28, 2019 at 2:03 PM Shawn Anastasio <shawn@...stas.io> wrote:
>>>
>>> Introduce a new pcibios function pcibios_ignore_alignment_request
>>> which allows the PCI core to defer to platform-specific code to
>>> determine whether or not to ignore alignment requests for PCI resources.
>>>
>>> The existing behavior is to simply ignore alignment requests when
>>> PCI_PROBE_ONLY is set. This is behavior is maintained by the
>>> default implementation of pcibios_ignore_alignment_request.
>>>
>>> Signed-off-by: Shawn Anastasio <shawn@...stas.io>
>>> ---
>>>   drivers/pci/pci.c   | 9 +++++++--
>>>   include/linux/pci.h | 1 +
>>>   2 files changed, 8 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>> index 8abc843b1615..8207a09085d1 100644
>>> --- a/drivers/pci/pci.c
>>> +++ b/drivers/pci/pci.c
>>> @@ -5882,6 +5882,11 @@ resource_size_t __weak pcibios_default_alignment(void)
>>>          return 0;
>>>   }
>>>
>>> +int __weak pcibios_ignore_alignment_request(void)
>>> +{
>>> +       return pci_has_flag(PCI_PROBE_ONLY);
>>> +}
>>> +
>>>   #define RESOURCE_ALIGNMENT_PARAM_SIZE COMMAND_LINE_SIZE
>>>   static char resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
>>>   static DEFINE_SPINLOCK(resource_alignment_lock);
>>> @@ -5906,9 +5911,9 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
>>>          p = resource_alignment_param;
>>>          if (!*p && !align)
>>>                  goto out;
>>> -       if (pci_has_flag(PCI_PROBE_ONLY)) {
>>> +       if (pcibios_ignore_alignment_request()) {
>>>                  align = 0;
>>> -               pr_info_once("PCI: Ignoring requested alignments (PCI_PROBE_ONLY)\n");
>>> +               pr_info_once("PCI: Ignoring requested alignments\n");
>>>                  goto out;
>>>          }
>>
>> I think the logic here is questionable to begin with. If the user has
>> explicitly requested re-aligning a resource via the command line then
>> we should probably do it even if PCI_PROBE_ONLY is set. When it breaks
>> they get to keep the pieces.
>>
>> That said, the real issue here is that PCI_PROBE_ONLY probably
>> shouldn't be set under qemu/kvm. Under the other hypervisor (PowerVM)
>> hotplugged devices are configured by firmware before it's passed to
>> the guest and we need to keep the FW assignments otherwise things
>> break. QEMU however doesn't do any BAR assignments and relies on that
>> being handled by the guest. At boot time this is done by SLOF, but
>> Linux only keeps SLOF around until it's extracted the device-tree.
>> Once that's done SLOF gets blown away and the kernel needs to do it's
>> own BAR assignments. I'm guessing there's a hack in there to make it
>> work today, but it's a little surprising that it works at all...
> 
> 
> The hack is to run a modified qemu-aware "/usr/sbin/rtas_errd" in the
> guest which receives an event from qemu (RAS_EPOW from
> /proc/interrupts), fetches device tree chunks (and as I understand it -
> they come with BARs from phyp but without from qemu) and writes "1" to
> "/sys/bus/pci/rescan" which calls pci_assign_resource() eventually:

Interesting. Does this mean that the PHYP hotplug path doesn't
call pci_assign_resource? If so it means the patch may not
break that platform after all, though it still may not be
the correct way of doing things.

> 
> [c000000006e6f960] [c0000000005f62d4] pci_assign_resource+0x44/0x360
> 
> [c000000006e6fa10] [c0000000005f8b54]
> assign_requested_resources_sorted+0x84/0x110
> [c000000006e6fa60] [c0000000005f9540] __assign_resources_sorted+0xd0/0x750
> [c000000006e6fb40] [c0000000005fb2e0]
> __pci_bus_assign_resources+0x80/0x280
> [c000000006e6fc00] [c0000000005fb95c]
> pci_assign_unassigned_bus_resources+0xbc/0x100
> [c000000006e6fc60] [c0000000005e3d74] pci_rescan_bus+0x34/0x60
> 
> [c000000006e6fc90] [c0000000005f1ef4] rescan_store+0x84/0xc0
> 
> [c000000006e6fcd0] [c00000000068060c] bus_attr_store+0x3c/0x60
> 
> [c000000006e6fcf0] [c00000000037853c] sysfs_kf_write+0x5c/0x80
> 
> 
> 
> 
> 
>>
>> IIRC Sam Bobroff was looking at hotplug under pseries recently so he
>> might have something to add. He's sick at the moment, but I'll ask him
>> to take a look at this once he's back among the living
>>
>>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>>> index 4a5a84d7bdd4..47471dcdbaf9 100644
>>> --- a/include/linux/pci.h
>>> +++ b/include/linux/pci.h
>>> @@ -1990,6 +1990,7 @@ static inline void pcibios_penalize_isa_irq(int irq, int active) {}
>>>   int pcibios_alloc_irq(struct pci_dev *dev);
>>>   void pcibios_free_irq(struct pci_dev *dev);
>>>   resource_size_t pcibios_default_alignment(void);
>>> +int pcibios_ignore_alignment_request(void);
>>>
>>>   #ifdef CONFIG_HIBERNATE_CALLBACKS
>>>   extern struct dev_pm_ops pcibios_pm_ops;
>>> --
>>> 2.20.1
>>>
> 

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