Re: [PATCH v3 04/10] PCI/RCEC: Add pcie_walk_rcec() to walk associated RCiEPs

From: Sean V Kelley
Date: Fri Sep 11 2020 - 19:16:32 EST

On 4 Sep 2020, at 19:23, Bjorn Helgaas wrote:

On Fri, Sep 04, 2020 at 10:18:30PM +0000, Kelley, Sean V wrote:
Hi Bjorn,

Quick question below...

On Wed, 2020-09-02 at 14:55 -0700, Sean V Kelley wrote:
Hi Bjorn,

On Wed, 2020-09-02 at 14:00 -0500, Bjorn Helgaas wrote:
On Wed, Aug 12, 2020 at 09:46:53AM -0700, Sean V Kelley wrote:
From: Qiuxu Zhuo <qiuxu.zhuo@xxxxxxxxx>

When an RCEC device signals error(s) to a CPU core, the CPU core
needs to walk all the RCiEPs associated with that RCEC to check
errors. So add the function pcie_walk_rcec() to walk all RCiEPs
associated with the RCEC device.

Co-developed-by: Sean V Kelley <sean.v.kelley@xxxxxxxxx>
Signed-off-by: Sean V Kelley <sean.v.kelley@xxxxxxxxx>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@xxxxxxxxx>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
---
drivers/pci/pci.h | 4 +++
drivers/pci/pcie/rcec.c | 76
+++++++++++++++++++++++++++++++++++++++++
2 files changed, 80 insertions(+)

diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index bd25e6047b54..8bd7528d6977 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -473,9 +473,13 @@ static inline void pci_dpc_init(struct
pci_dev
*pdev) {}
#ifdef CONFIG_PCIEPORTBUS
void pci_rcec_init(struct pci_dev *dev);
void pci_rcec_exit(struct pci_dev *dev);
+void pcie_walk_rcec(struct pci_dev *rcec, int (*cb)(struct
pci_dev
*, void *),
+ void *userdata);
#else
static inline void pci_rcec_init(struct pci_dev *dev) {}
static inline void pci_rcec_exit(struct pci_dev *dev) {}
+static inline void pcie_walk_rcec(struct pci_dev *rcec, int
(*cb)(struct pci_dev *, void *),
+ void *userdata) {}
#endif

#ifdef CONFIG_PCI_ATS
diff --git a/drivers/pci/pcie/rcec.c b/drivers/pci/pcie/rcec.c
index 519ae086ff41..405f92fcdf7f 100644
--- a/drivers/pci/pcie/rcec.c
+++ b/drivers/pci/pcie/rcec.c
@@ -17,6 +17,82 @@

#include "../pci.h"

+static int pcie_walk_rciep_devfn(struct pci_bus *bus, int
(*cb)(struct pci_dev *, void *),
+ void *userdata, const unsigned long
bitmap)
+{
+ unsigned int devn, fn;
+ struct pci_dev *dev;
+ int retval;
+
+ for_each_set_bit(devn, &bitmap, 32) {
+ for (fn = 0; fn < 8; fn++) {
+ dev = pci_get_slot(bus, PCI_DEVFN(devn, fn));

Wow, this is a lot of churning to call pci_get_slot() 256 times per
bus for the "associated bus numbers" case where we pass a bitmap of
0xffffffff. They didn't really make it easy for software when they
added the next/last bus number thing.

Just thinking out loud here. What if we could set dev->rcec during
enumeration, and then use that to build pcie_walk_rcec()?

I think follow what you are doing.

As we enumerate an RCEC, use the time to discover RCiEPs and
associate
each RCiEP's dev->rcec. Although BIOS already set the bitmap for this
specific RCEC, it's more efficient to simply discover the devices
through the bus walk and verify each one found against the bitmap.

Further, while we can be certain that an RCiEP found with a matching
device no. in a bitmap for an associated RCEC is correct, we cannot
be
certain that any RCiEP found on another bus range is correct unless
we
verify the bus is within that next/last bus range.

Finally, that's where find_rcec() callback for rcec_assoc_rciep()
does
double duty by also checking on the "on-a-separate-bus" case captured
potentially by find_rcec() during an RCiEP's bus walk.


bool rcec_assoc_rciep(rcec, rciep)
{
if (rcec->bus == rciep->bus)
return (rcec->bitmap contains rciep->devfn);

return (rcec->next/last contains rciep->bus);
}

link_rcec(dev, data)
{
struct pci_dev *rcec = data;

if ((dev is RCiEP) && rcec_assoc_rciep(rcec, dev))
dev->rcec = rcec;
}

find_rcec(dev, data)
{
struct pci_dev *rciep = data;

if ((dev is RCEC) && rcec_assoc_rciep(dev, rciep))
rciep->rcec = dev;
}

pci_setup_device
...

I just noticed your use of pci_setup_device(). Are you suggesting
moving the call to pci_rcec_init() out of pci_init_capabilities() and
move it into pci_setup_device()? If so, would pci_rcec_exit() still
remain in pci_release_capabilities()?

I'm just wondering if it could just remain in pci_init_capabilities().

Yeah, I didn't mean in pci_setup_device() specifically, just somewhere
in the callchain of pci_setup_device(). But you're right, it probably
would make more sense in pci_init_capabilities(), so I *should* have
said pci_scan_single_device() to be a little less specific.

Bjorn

I’ve done some experimenting with this approach, and I think there may be a problem of just walking the busses during enumeration pci_init_capabilities(). One problem is where one has an RCEC on a root bus: 6a(00.4) and an RCiEP on another root bus: 6b(00.0). They will never find each other in this approach through a normal pci_bus_walk() call using their respective root_bus.

+-[0000:6b]-+-00.0
| +-00.1
| +-00.2
| \-00.3
+-[0000:6a]-+-00.0
| +-00.1
| +-00.2
| \-00.4

While having a lot of slot calls per bus is unfortunate, unless I’m mistaken you would have to walk every peer root_bus with your RCiEP in this example until you hit on the right RCEC, unless of course you have a bitmap associated RCEC on dev->bus.

Conversely, if you are enumerating the above RCEC at 6a(00.4) and you attempt to link_rcec() through calls to pci_walk_bus(), the walk will still be limited to 6a and below; never encountering 6b(00.0). So you would then need an additional walk for each of the associated bus ranges, excluding the same bus as the RCEC.

pci_init_capabilities()
…
pci_init_rcec() // Cached

if (RCEC)
Walk the dev->bus for bitmap associated RCiEP
Walk all associated bus ranges for RCiEP

else if (RCiEP)
Walk the dev->bus for bitmap associated RCEC
Walk all peer root_bus for RCEC, confirm if own dev->bus falls within discovered RCEC associated ranges

The other problem here is temporal. I’m wondering if we may be trying to find associated devices at the pci_init_capabilities() stage prior to them being fully enumerated, i.e., RCEC has not been cached but we are searching with a future associated RCiEP. So one could encounter a race condition where one is checking on an RCiEP whose associated RCEC has not been enumerated yet.

So let’s say one throws out RCiEP entirely and just relies upon RCEC to find the associations because one knows that an encountered RCEC (in pci_init_capabilities()) has already been cached. In that case you end up with the original implementation being done with this patch series…

if (RCEC)
Walk the dev->bus for bitmap associated RCiEP
Walk all associated bus ranges for RCiEP

Perhaps I’ve muddled some things here but it doesn’t look like the twain will meet unless I cover multiple peer root_bus and even then you may have an issue because the devices don’t yet fully exist from the perspective of the OS.

Thanks,

Sean