linux-kernel - Re: [PATCH v10 1/3] PCI: microchip: Fix outbound address translation tables

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Message-ID: <20250116154253.GA584488@bhelgaas>
Date: Thu, 16 Jan 2025 09:42:53 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: daire.mcnamara@...rochip.com
Cc: linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
	conor.dooley@...rochip.com, lpieralisi@...nel.org, kw@...ux.com,
	robh@...nel.org, bhelgaas@...gle.com, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org, krzk+dt@...nel.org,
	conor+dt@...nel.org, ilpo.jarvinen@...ux.intel.com,
	kevin.xie@...rfivetech.com
Subject: Re: [PATCH v10 1/3] PCI: microchip: Fix outbound address translation
 tables

On Tue, Jan 14, 2025 at 06:13:10PM -0600, Bjorn Helgaas wrote:
> On Fri, Oct 11, 2024 at 03:00:41PM +0100, daire.mcnamara@...rochip.com wrote:
> > From: Daire McNamara <daire.mcnamara@...rochip.com>
> > 
> > On Microchip PolarFire SoC (MPFS) the PCIe Root Port can be behind one of
> > three general-purpose Fabric Interface Controller (FIC) buses that
> > encapsulate an AXI-M interface. That FIC is responsible for managing
> > the translations of the upper 32-bits of the AXI-M address. On MPFS,
> > the Root Port driver needs to take account of that outbound address
> > translation done by the parent FIC bus before setting up its own
> > outbound address translation tables.  In all cases on MPFS,
> > the remaining outbound address translation tables are 32-bit only.
> > 
> > Limit the outbound address translation tables to 32-bit only.
> 
> I don't quite understand what this is saying.  It seems like the code
> keeps only the low 32 bits of a PCI address and throws away any
> address bits above the low 32.
> 
> If that's what the FIC does, I wouldn't describe the FIC as
> "translating the upper 32 bits" since it sounds like the translation
> is just truncation.
> 
> I guess it must be more complicated than that?  I assume you can still
> reach BARs that have PCI addresses above 4GB using CPU loads/stores?
> 
> The apertures through the host bridge for MMIO access are described by
> DT ranges properties, so this must be something that can't be
> described that way?

Ping?  I'd really like to understand this before the v6.14 merge
window opens on Sunday.

Bjorn