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

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Date:   Mon, 06 Jul 2020 12:49:47 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     ayal@...lanox.com
Cc:     kuba@...nel.org, saeedm@...lanox.com, mkubecek@...e.cz,
        linux-pci@...r.kernel.org, helgaas@...nel.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

From: Aya Levin <ayal@...lanox.com>
Date: Mon, 6 Jul 2020 16:00:59 +0300

> Assuming the discussions with Bjorn will conclude in a well-trusted
> API that ensures relaxed ordering in enabled, I'd still like a method
> to turn off relaxed ordering for performance debugging sake.
> Bjorn highlighted the fact that the PCIe sub system can only offer a
> query method. Even if theoretically a set API will be provided, this
> will not fit a netdev debugging - I wonder if CPU vendors even support
> relaxed ordering set/unset...
> On the driver's side relaxed ordering is an attribute of the mkey and
> should be available for configuration (similar to number of CPU
> vs. number of channels).
> Based on the above, and binding the driver's default relaxed ordering
> to the return value from pcie_relaxed_ordering_enabled(), may I
> continue with previous direction of a private-flag to control the
> client side (my driver) ?

I don't like this situation at all.

If RO is so dodgy that it potentially needs to be disabled, that is
going to be an issue not just with networking devices but also with
storage and other device types as well.

Will every device type have a custom way to disable RO, thus
inconsistently, in order to accomodate this?

That makes no sense and is a terrible user experience.

That's why the knob belongs generically in PCI or similar.