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

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Date:   Thu, 9 Jul 2020 12:47:26 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jonathan Lemon <jonathan.lemon@...il.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Aya Levin <ayal@...lanox.com>,
        "David Miller" <davem@...emloft.net>, <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, 9 Jul 2020 15:20:11 -0300 Jason Gunthorpe wrote:
> >     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.
> 
> I'm not sure there is a reason to allow users to finely tune RO, at
> least I haven't heard of cases where RO is a degredation depending on
> workload.
> 
> If some platform doesn't work when RO is turned on then it should be
> globally black listed like is already done in some cases.
> 
> If the devices has bugs and uses RO wrong, or the driver has bugs and
> is only stable with !RO and Intel, then the driver shouldn't turn it
> on at all.
> 
> In all of these cases it is not a user tunable.
> 
> Development and testing reasons, like 'is my crash from a RO bug?' to
> tune should be met by the device global setpci, I think.

+1