Re: [PATCH REPOST] Extend PCIE_BUS_PEER2PEER to set MRSS=128 to fix CNS3xxx BM DMA.

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Apr 21, 2016 at 10:42:34AM -0500, Bjorn Helgaas wrote:
> On Mon, Mar 21, 2016 at 10:39:52AM +0100, Krzysztof Halasa wrote:
> > I think this bug needs to be fixed, this way or another.
> > 
> > The platform in question is Cavium CNS3xxx, ARMv6.
> > 
> > A recent patch by Arnd Bergmann (498a92d42596 "ARM: cns3xxx: pci: avoid
> > potential stack overflow") converted an explicit setting of
> > PCI_EXP_DEVCTL_READRQ = 0 (i.e., max 128 bytes for bus-mastering PCIe DMA
> > read request) to:
> > +    pcie_bus_config = PCIE_BUS_PEER2PEER;
> 
> Any thoughts on this, Arnd?  I can (and will) look at this, but it's
> a complicated area and it will take me quite a while to dig into it.

What should we do here?  Should we revert 498a92d42596?  It appeared in
v4.4, so this has been broken for a while.

It looks like 498a92d42596 merely fixed a warning, at the expense of
breaking DMA on Cavium.  Reverting it would bring the warning back, but
that's better than broken DMA.

> > with the following commentary:
> >     "The second part is how the driver sets up the Max_Read_Request_Size
> >     value for the first device/function on bus 1, i.e. the device
> >     plugged directly into the PCIe root port.
> >     For all I can tell, this is in fact incomplete, as it does not
> >     perform the same setting on devices attached to a PCIe switch,
> >     or multi-function devices.
> >     The solution for this part fortunately is even easier: if we
> >     just set the global pcie_bus_config variable to PCIE_BUS_PEER2PEER,
> >     all PCIe devices in the system are limited to 128 byte MPS, which
> >     in turn limits the MRRS to 128 bytes for all devices, and we
> >     no longer even need to touch any devices."
> > 
> > The problem is the MRRS setting is never written to the hardware.
> > I propose the following, though I'm not sure if we can do this safely,
> > especially given the comments in probe.c. OTOH, this change may be
> > required in other (all?) cases when the user requests
> > PCIE_BUS_PEER2PEER.
> > 
> > On this Laguna GW-2388 the following patch fixes BM DMA with:
> > 0000:00:00.0 PCI bridge: Cavium Networks Device 3400 (rev 01)
> > 0000:01:00.0 PCI bridge: Texas Instruments XIO2001 PCI Express-to-PCI Bridge
> > 0000:02:0e.0 (PCI devices behind the bridge, these are doing actual BM xfers)
> > 0001:00:00.0 PCI bridge: Cavium Networks Device 3400 (rev 01 - this is
> > 	     the second lane from the CPU)
> > 
> > pci 0000:00:00.0: Max Payload Size set to  128/ 128 (was  128), Max Read Rq  128
> > pci 0000:01:00.0: Max Payload Size set to  128/ 512 (was  128), Max Read Rq  128
> > pci 0001:00:00.0: Max Payload Size set to  128/ 128 (was  128), Max Read Rq  128
> > 
> > Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
> > Fixes: 498a92d42596 ("ARM: cns3xxx: pci: avoid potential stack overflow")
> > 
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 6d7ab9b..91713b6 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -1919,7 +1919,8 @@ static void pcie_write_mrrs(struct pci_dev *dev)
> >  	/* In the "safe" case, do not configure the MRRS.  There appear to be
> >  	 * issues with setting MRRS to 0 on a number of devices.
> >  	 */
> > -	if (pcie_bus_config != PCIE_BUS_PERFORMANCE)
> > +	if (pcie_bus_config != PCIE_BUS_PERFORMANCE &&
> > +	    pcie_bus_config != PCIE_BUS_PEER2PEER)
> >  		return;
> >  
> >  	/* For Max performance, the MRRS must be set to the largest supported
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 2771625..6f5088a 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -756,7 +756,7 @@ enum pcie_bus_config_types {
> >  	PCIE_BUS_DEFAULT,	/* ensure MPS matches upstream bridge */
> >  	PCIE_BUS_SAFE,		/* use largest MPS boot-time devices support */
> >  	PCIE_BUS_PERFORMANCE,	/* use MPS and MRRS for best performance */
> > -	PCIE_BUS_PEER2PEER,	/* set MPS = 128 for all devices */
> > +	PCIE_BUS_PEER2PEER,	/* set MPS and MRSS to 128 for all devices */
> >  };
> >  
> >  extern enum pcie_bus_config_types pcie_bus_config;
> > --
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