linux-kernel - Re: [PATCH v6 1/5] PCI: dwc: Allow adjusting the number of ob/ib windows in glue driver

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Message-ID: <aO4bWRqX_4rXud25@ryzen>
Date: Tue, 14 Oct 2025 11:43:53 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Randolph Lin <randolph@...estech.com>
Cc: linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
	jingoohan1@...il.com, mani@...nel.org, lpieralisi@...nel.org,
	kwilczynski@...nel.org, robh@...nel.org, bhelgaas@...gle.com,
	krzk+dt@...nel.org, conor+dt@...nel.org, alex@...ti.fr,
	aou@...s.berkeley.edu, palmer@...belt.com, paul.walmsley@...ive.com,
	ben717@...estech.com, inochiama@...il.com,
	thippeswamy.havalige@....com, namcao@...utronix.de,
	shradha.t@...sung.com, pjw@...nel.org, randolph.sklin@...il.com,
	tim609@...estech.com
Subject: Re: [PATCH v6 1/5] PCI: dwc: Allow adjusting the number of ob/ib
 windows in glue driver

On Fri, Oct 03, 2025 at 10:35:23AM +0800, Randolph Lin wrote:
> The number of ob/ib windows is determined through write-read loops
> on registers in the core driver. Some glue drivers need to adjust
> the number of ob/ib windows to meet specific requirements,such as

Missing space after comma.

> hardware limitations. This change allows the glue driver to adjust
> the number of ob/ib windows to satisfy platform-specific constraints.
> The glue driver may adjust the number of ob/ib windows, but the values
> must stay within hardware limits.

Could we please get a better explaination than "satisfy platform-specific
constraints" ?

Your PCIe controller is synthesized with a certain number of {in,out}bound
windows, and I assume that dw_pcie_iatu_detect() correctly detects the number
of {in,out}bound windows, and initializes num_ob_windows/num_ib_windows
accordingly.

So, is the problem that because of some errata, you cannot use all the
{in,out}bound windows of the iATU?

Because it is hard to understand what kind of "hardware limit" that would
cause your SoC to not be able to use all the available {in,out}bound windows.

Because it is simply a mapping in the iATU (internal Address Translation Unit).

In fact, in many cases, e.g. the NVMe EPF driver, then number of {in,out}bound
windows is a major limiting factor of how many outstanding I/Os you can have,
so usually, you really want to be able to use the maximum that the hardware
supports.

TL;DR: to modify this common code, I think your reasoning has to be more
detailed.

Kind regards,
Niklas