linux-kernel - Re: [PATCH v3 04/11] PCI: brcmstb: Expand inbound size calculation helper

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Message-ID: <20241016193802.GA645895@bhelgaas>
Date: Wed, 16 Oct 2024 14:38:02 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Jim Quinlan <jim2101024@...il.com>
Cc: Florian Fainelli <florian.fainelli@...adcom.com>,
	Stanimir Varbanov <svarbanov@...e.de>, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-rpi-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Thomas Gleixner <tglx@...utronix.de>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Nicolas Saenz Julienne <nsaenz@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>, kw@...ux.com,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Andrea della Porta <andrea.porta@...e.com>,
	Phil Elwell <phil@...pberrypi.com>,
	Jonathan Bell <jonathan@...pberrypi.com>
Subject: Re: [PATCH v3 04/11] PCI: brcmstb: Expand inbound size calculation
 helper

On Wed, Oct 16, 2024 at 01:09:00PM -0400, Jim Quinlan wrote:
> On Mon, Oct 14, 2024 at 1:25 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > On Mon, Oct 14, 2024 at 10:10:11AM -0700, Florian Fainelli wrote:
> > > On 10/14/24 09:57, Bjorn Helgaas wrote:
> > > > On Mon, Oct 14, 2024 at 04:07:03PM +0300, Stanimir Varbanov wrote:
> > > > > BCM2712 memory map can supports up to 64GB of system
> > > > > memory, thus expand the inbound size calculation in
> > > > > helper function up to 64GB.
> > > >
> > > > The fact that the calculation is done in a helper isn't important
> > > > here.  Can you make the subject line say something about supporting
> > > > DMA for up to 64GB of system memory?
> > > >
> > > > This is being done specifically for BCM2712, but I assume it's safe
> > > > for *all* brcmstb devices, right?
> > >
> > > It is safe in the sense that all brcmstb devices with this PCIe
> > > controller will adopt the same encoding of the size, all of the
> > > currently supported brcmstb devices have a variety of
> > > limitations when it comes to the amount of addressable DRAM
> > > however. Typically we have a hard limit at 4GB of DRAM per
> > > memory controller, some devices can do 2GB x3, 4GB x2, or 4GB
> > > x1.
> > >
> > > Does that answer your question?
> >
> > I'd like something in the commit log to the effect that while
> > we're doing this to support more system memory on BCM2712, this
> > change is safe for other SoCs that don't support as much system
> > memory.
> 
> This setting configures the size of an RC's inbound window to system
> memory.  Any inbound access outside of all of the inbound windows
> will be discarded.
> 
> Some existing SoCs cannot support the 64GB size.  Configuring such
> an SoC to 64GB will effectively disable the entire window.

So I *think* you're saying that this patch will break existing SoCs
that don't support the 64GB size, right?

Bjorn