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Date:   Fri, 31 Jan 2020 15:29:06 +0100
From:   Andrew Lunn <andrew@lunn.ch>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Jon Nettleton <jon@solid-run.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Marc Zyngier <maz@kernel.org>,
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        nleeder@codeaurora.org, Ioana Ciornei <ioana.ciornei@nxp.com>,
        Cristi Sovaiala <cristian.sovaiala@nxp.com>,
        Hanjun Guo <guohanjun@huawei.com>,
        Will Deacon <will@kernel.org>,
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        Pankaj Bansal <pankaj.bansal@nxp.com>,
        Russell King <linux@armlinux.org.uk>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
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Subject: Re: [EXT] Re: [PATCH] bus: fsl-mc: Add ACPI support for fsl-mc
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> > But by design SFP, SFP+, and QSFP cages are not fixed function network
> > adapters.  They are physical and logical devices that can adapt to
> > what is plugged into them.  How the devices are exposed should be
> > irrelevant to this conversation it is about the underlying
> > connectivity.
> 
> Apologies - I was under the impression that SFP and friends were a
> physical-layer thing and that a MAC in the SoC would still be fixed such
> that its DMA and interrupt configuration could be statically described
> regardless of what transceiver was plugged in (even if some configurations
> might not use every interrupt/stream ID/etc.) If that isn't the case I shall
> go and educate myself further.

Hi Robin

It gets interesting with QSFP cages. The Q is quad, there are 4 SERDES
lanes. You can use them for 1x 40G link, or you can split them into 4x
10G links. So you either need one MAC or 4 MACs connecting to the
cage, and this can change on the fly when a modules is ejected and
replaced with another module. There are only one set of control pins
for i2c, loss of signal, TX disable, module inserted. So where the
interrupt/stream ID/etc are mapped needs some flexibility.

There is also to some degree a conflict with hiding all this inside
firmware. This is complex stuff. It is much better to have one core
implementing in Linux plus some per hardware driver support, than
having X firmware blobs, generally closed source, each with there own
bugs which nobody can fix.

     Andrew