Re: [PATCH v3 1/3] PCI: Introduce pcibios_ignore_alignment_request

[Shawn Anastasio <shawn@anastas.io>]

On 5/28/19 12:36 AM, Oliver wrote:
> On Tue, May 28, 2019 at 2:03 PM Shawn Anastasio <shawn@anastas.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@anastas.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...
Interesting, I wasn't aware that hotplugged devices are configured
by the hypervisor on PowerVM. That at least means that this patch is
wrong as-is since it won't handle that properly. Definitely seems like
there will need to be different behavior here depending on the hypervisor.

That being said, wouldn't PCI_PROBE_ONLY still be set on pseries/kvm
(at least for initial boot) to observe SLOF's original BAR assignments?
Perhaps it should be un-set after initial PCI init?

> 
> 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

Good to know. I'll await his comments before continuing here.

>> 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
>>