netdev - Re: [net-next 10/10] net/mlx5e: Add support for PCI relaxed ordering

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Message-ID: <20200709203351.2bj4vf5gehphy2ru@bsd-mbp.dhcp.thefacebook.com>
Date:   Thu, 9 Jul 2020 13:33:51 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, Aya Levin <ayal@...lanox.com>,
        David Miller <davem@...emloft.net>, kuba@...nel.org,
        saeedm@...lanox.com, mkubecek@...e.cz, linux-pci@...r.kernel.org,
        netdev@...r.kernel.org, tariqt@...lanox.com,
        alexander.h.duyck@...ux.intel.com
Subject: Re: [net-next 10/10] net/mlx5e: Add support for PCI relaxed ordering

On Thu, Jul 09, 2020 at 03:20:11PM -0300, Jason Gunthorpe wrote:
> On Thu, Jul 09, 2020 at 10:35:50AM -0700, Jonathan Lemon wrote:
> > On Wed, Jul 08, 2020 at 08:26:02PM -0300, Jason Gunthorpe wrote:
> > > On Wed, Jul 08, 2020 at 06:16:30PM -0500, Bjorn Helgaas wrote:
> > > >     I suspect there may be device-specific controls, too, because [1]
> > > >     claims to enable/disable Relaxed Ordering but doesn't touch the
> > > >     PCIe Device Control register.  Device-specific controls are
> > > >     certainly allowed, but of course it would be up to the driver, and
> > > >     the device cannot generate TLPs with Relaxed Ordering unless the
> > > >     architected PCIe Enable Relaxed Ordering bit is *also* set.
> > > 
> > > Yes, at least on RDMA relaxed ordering can be set on a per transaction
> > > basis and is something userspace can choose to use or not at a fine
> > > granularity. This is because we have to support historical
> > > applications that make assumptions that data arrives in certain
> > > orders.
> > > 
> > > I've been thinking of doing the same as this patch but for RDMA kernel
> > > ULPs and just globally turn it on if the PCI CAP is enabled as none of
> > > our in-kernel uses have the legacy data ordering problem.
> > 
> > If I'm following this correctly - there are two different controls being
> > discussed here:
> > 
> >     1) having the driver request PCI relaxed ordering, which may or may
> >        not be granted, based on other system settings, and
> 
> This is what Bjorn was thinking about, yes, it is some PCI layer
> function to control the global config space bit.
> 
> >     2) having the driver set RO on the transactions it initiates, which
> >        are honored iff the PCI bit is set.
> >
> > It seems that in addition to the PCI core changes, there still is a need
> > for driver controls?  Unless the driver always enables RO if it's capable?
> 
> I think the PCI spec imagined that when the config space RO bit was
> enabled the PCI device would just start using RO packets, in an
> appropriate and device specific way.
> 
> So the fine grained control in #2 is something done extra by some
> devices.
> 
> IMHO if the driver knows it is functionally correct with RO then it
> should enable it fully on the device when the config space bit is set.

Sounds reasonable to me.
-- 
Jonathan