linux-kernel - Re: mach-socfpga: PCIe Root IO TLP support for Cyclone V

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Date:   Mon, 6 Nov 2023 11:15:27 +0100
From:   David Epping <david.epping@...singlinkelectronics.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Krzysztof Wilczyński <kw@...ux.com>
Subject: Re: mach-socfpga: PCIe Root IO TLP support for Cyclone V

Hi David,

On Sun, Nov 05, 2023 at 11:20:03AM +0000, David Laight wrote:
> It isn't an ARM root complex ...
> I didn't think any of the Cyclone V had embedded arm cpu.
> I know some of the more recent Altera FPGA do, by the Cyclone V
> is pretty old now - although we are still using them in new cards.
> (Only as PCIe endpoints though.)

You are right, the root complex itself is (likely) not an ARM design.
I should have said "... unlike many other Root Complexes used in similar ARM CPU based SoCs ...".
The Cyclone V SoC sub-family of FPGAs features a dual core Cortex A9.
The "Cyclone V SoC Development Kit" is a development kit that exposes a readily usable PCIe root slot.

> If you are building the FPGA image then all the logic to convert the
> memory mapped slave cycles (into the fpga logic) is supplied as
> verilog source.

The CPU subsystem and the PCIe IP are hard IP in silicon and can not be altered.
They are connected via the FPGA logic, though, and I agree one approach could be to intercept their communication with custom HDL.
However, Linux uses the exact same PCIe hard IP registers required for IO TLPs to send Config TLPs.
Every TLP requires multiple accesses to multiple registers, so locking between FPGA logic and Linux transactions would be required.
I'm not saying that this is impossible, but I don't think it can be robust without a Linux software change.
A software only solution has the benefit of being available to all users of such an FPGA, without access to that special logic.

> I thought that all recent endpoints were required [1] to work with
> just memory BARs - even going back to the later PCI versions.
> So I'm surprised a PCIe endpoint need an IO BAR.

The AX99100 implements a so called "Legacy Endpoint" and is thus allowed to rely on support for IO space.
I guess this choice was made to stay driver-compatible to the PCI version, although I don't know the ancestry if this product.

Best regards, David