netdev - Re: [PATCH net-next v2 0/2] sfc: Add EF100 BAR config support

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Date:   Mon, 11 Jul 2022 11:48:06 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Martin Habets <habetsm.xilinx@...il.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, davem@...emloft.net,
        pabeni@...hat.com, edumazet@...gle.com, netdev@...r.kernel.org,
        ecree.xilinx@...il.com, linux-pci@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net-next v2 0/2] sfc: Add EF100 BAR config support

On Mon, 11 Jul 2022 14:38:54 +0100 Martin Habets wrote:
> > Normally drivers rely on the PCI Vendor and Device ID to learn the
> > number of BARs and their layouts.  I guess this series implies that
> > doesn't work on this device?  And the user needs to manually specify
> > what kind of device this is?  
> 
> When a new PCI device is added (like a VF) it always starts of with
> the register layout for an EF100 network device. This is hardcoded,
> i.e. it cannot be customised.
> The layout can be changed after bootup, and only after the sfc driver has
> bound to the device.
> The PCI Vendor and Device ID do not change when the layout is changed.
> 
> For vDPA specifically we return the Xilinx PCI Vendor and our device ID
> to the vDPA framework via struct vdpa_config_opts.

So it's switching between ethernet and vdpa? Isn't there a general
problem for configuring vdpa capabilities (net vs storage etc) and
shouldn't we seek to solve your BAR format switch in a similar fashion
rather than adding PCI device attrs, which I believe is not done for
anything vDPA-related?

> > I'm confused about how this is supposed to work.  What if the driver
> > is built-in and claims a device before the user can specify the
> > register layout?  
> 
> The bar_config file will only exist once the sfc driver has bound to
> the device. So in fact we count on that driver getting loaded.
> When a new value is written to bar_config it is the sfc driver that
> instructs the NIC to change the register layout.

When you say "driver bound" you mean the VF driver, right?

> > What if the user specifies the wrong layout and the
> > driver writes to the wrong registers?  
> 
> We have specific hardware and driver requirements for this sort of
> situation. For example, the register layouts must have some common
> registers (to ensure some compatibility).
> A layout that is too different will require a separate device ID.
> A driver that writes to the wrong register is a bug.
> 
> Maybe the name "bar_config" is causing most of the confusion here.
> Internally we also talk about "function profiles" or "personalities",
> but we thought such a name would be too vague.