Re: [PATCH 5/8] PCI PCIe portdrv: Fix allocation of interrupts

[Rafael J. Wysocki]

On Thursday 08 January 2009, Kenji Kaneshige wrote:
> Rafael J. Wysocki wrote:
> > On Thursday 08 January 2009, Kenji Kaneshige wrote:
> >> Rafael J. Wysocki wrote:
> >>> From: Rafael J. Wysocki <rjw@xxxxxxx>
> >>>
> >>> If MSI-X interrupt mode is used by the PCI Express port driver, too
> >>> many vectors are allocated and it is not ensured that the right
> >>> vectors will be used for various services.  Namely, the PCI Express
> >>> specification states that both PCI Express native PME and PCI Express
> >>> hotplug will always use the same MSI or MSI-X message for signalling
> >>> interrupts, which implies that the same vector will be used by both
> >>> of them.  Also, the VC service does not use interrupts at all.
> >>> Moreover, is not clear which of the vectors allocated by
> >>> pci_enable_msix() will be used for PME and hotplug and which of them
> >>> will be used for AER if all of these services are configured.
> >>>
> >>> For these reasons, rework the allocation of interrupts for PCI
> >>> Express ports so that at most two vectors are allocated and ensure
> >>> that the right vector will be used for the right purpose.
> >>>
> >>> Signed-off-by: Rafael J. Wysocki <rjw@xxxxxxx>
> >>> ---
> >>>  drivers/pci/pcie/portdrv.h      |    1 
> >>>  drivers/pci/pcie/portdrv_core.c |  155 +++++++++++++++++++++++++++++-----------
> >>>  2 files changed, 117 insertions(+), 39 deletions(-)
> >>>
> >>> Index: linux-2.6/drivers/pci/pcie/portdrv.h
> >>> ===================================================================
> >>> --- linux-2.6.orig/drivers/pci/pcie/portdrv.h
> >>> +++ linux-2.6/drivers/pci/pcie/portdrv.h
> >>> @@ -25,6 +25,7 @@
> >>>  #define PCIE_CAPABILITIES_REG		0x2
> >>>  #define PCIE_SLOT_CAPABILITIES_REG	0x14
> >>>  #define PCIE_PORT_DEVICE_MAXSERVICES	4
> >>> +#define PORT_MSI_VECTOR_MASK		0x1f
> >>>  
> >>>  #define get_descriptor_id(type, service) (((type - 4) << 4) | service)
> >>>  
> >>> Index: linux-2.6/drivers/pci/pcie/portdrv_core.c
> >>> ===================================================================
> >>> --- linux-2.6.orig/drivers/pci/pcie/portdrv_core.c
> >>> +++ linux-2.6/drivers/pci/pcie/portdrv_core.c
> >>> @@ -30,55 +30,120 @@ static void release_pcie_device(struct d
> >>>  }
> >>>  
> >>>  /**
> >>> + * fix_up_vectors - ensure the right ordering of MSI-X interrupt vectors
> >>> + * @dev: PCI Express port that is going to use the vectors
> >>> + * @vectors: Array of interrupt vectors to check (2 entries)
> >>> + *
> >>> + * Return value:
> >>> + * 0 on success, error code if the values read from config registers are not as
> >>> + * expected
> >>> + *
> >>> + * If this function is called, we are going to use two interrupt vectors which
> >>> + * may be different, but we have to make sure they are in the right order such
> >>> + * that the vector to be used for PME and hotplug signalling is the first one.
> >>> + *
> >>> + * NOTE: The assumption here is that MSI message offset (with respect to the
> >>> + * base Message Data) equal to N corresponds to index N in the array of vectors
> >>> + * returned by pci_enable_msix().
> >>> + */
> >> I've posted the similar patch recently, which doesn't have this
> >> assumption.
> > 
> > Actually, this assumption is in agreement with PCI Express Base Specification
> > 2.0, which very clearly states in Section 7.8.2 that:
> > 
> > "[...] Interrupt Message Number – This field indicates which
> > MSI/MSI-X vector is used for the interrupt message generated in
> > association with any of the status bits of this Capability structure.
> > 
> > [...]
> > 
> > For MSI-X, the value in this field indicates which MSI-X Table
> > entry is used to generate the interrupt message. The entry must
> > be one of the first 32 entries even if the Function implements
> > more than 32 entries. For a given MSI-X implementation, the
> > entry must remain constant."
> > 
> 
> Though I might be misunderstanding something, my understanding is
> that this implies that there can be more MSI-X table entries than
> number of interrupts, and PME/HotPlug or AER interrupts can be
> mapped to other than first two entries.

We are requesting two vectors with my patch and they will be assigned to the
first two entries in the MSI-X table.  If my understanding of the MSI-X code is
corrtect, there won't be more MSI-X table entries set up for the port.

> If my understanding is correct, your patch would not work if PME/HotPlug or
> AER interrupts were mapped to other than first two.

In this case it's going to fall back to MSI w/ one vector, because we're going
to find that offset > 1 in one of the tests in fix_up_vectors().

> Regardless of my understanding is correct or not, I have another
> concern. I think there are two interrupts for PCIe port service
> currently as your patch indicates. But new interrupts can be added
> in the future. With the assumption, PME/HotPlug and/or AER interrupts
> would not work if the first several entries are mapped to other new
> interrupts. Therefore, with the assumption, we need to fix MSI-X
> initialization code whenever new interrupts are defined.

In that case we'll need to allocate more vectors, so we'll need to change the
code anyway.  Also, we'll have to add the tests for the new interrupts to
the code, because there will have to be new registers defined to read the
MSI offsets or MSI-X table indices from.

I very much prefer to keep the code as simple as possible as long as we can.

> > Well, I have to update the comment. :-)
> > 
> >> Please take a look.
> >>
> >> http://marc.info/?l=linux-pci&m=122992792507715&w=2
> > 
> > OK, thanks.  I can easily change my patch to follow this behavior, although
> > I don't really think it's necessary given the above.
> > 
> 
> Please see above mentioned my concerns.
> 
> >> NOTE: I've confirmed MSI and INTx still work with my patch, but it
> >> is not tested on the machine with MSI-X capable port, since I don't
> >> have such environment.
> > 
> > I also have tested my patch with both MSI and INTx, but I also don't have
> > MSI-X capable root ports in any of my test boxes.
> > 
> 
> Though the patch is not tested with MSI-X capable ports, I think it
> is much better than the current code.

Well, I can only agree here. :-)

Thanks a lot for your comments.

Best,
Rafael
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