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

From: Niklas Cassel <cassel@kernel.org>
To: Randolph Lin <randolph@andestech.com>
Cc: linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-riscv@lists.infradead.org, devicetree@vger.kernel.org,
	jingoohan1@gmail.com, mani@kernel.org, lpieralisi@kernel.org,
	kwilczynski@kernel.org, robh@kernel.org, bhelgaas@google.com,
	krzk+dt@kernel.org, conor+dt@kernel.org, alex@ghiti.fr,
	aou@eecs.berkeley.edu, palmer@dabbelt.com,
	paul.walmsley@sifive.com, ben717@andestech.com,
	inochiama@gmail.com, thippeswamy.havalige@amd.com,
	namcao@linutronix.de, shradha.t@samsung.com, pjw@kernel.org,
	randolph.sklin@gmail.com, tim609@andestech.com,
	Samuel Holland <samuel.holland@sifive.com>
Subject: Re: [PATCH v6 1/5] PCI: dwc: Allow adjusting the number of ob/ib windows in glue driver
Date: Thu, 16 Oct 2025 13:54:35 +0200	[thread overview]
Message-ID: <aPDc-yclubiHbUcD@ryzen> (raw)
In-Reply-To: <aPDTJKwmpxolGEyj@swlinux02>

Hello Randolph,

On Thu, Oct 16, 2025 at 07:12:36PM +0800, Randolph Lin wrote:
> > 
> > Could we please get a better explaination than "satisfy platform-specific
> > constraints" ?
> > 
> 
> Due to this SoC design, only iATU regions with mapped addresses within the
> 32-bits address range need to be programmed. However, this SoC has a design
> limitation in which the maximum region size supported by a single iATU
> entry is restricted to 4 GB, as it is based on a 32-bits address region.
> 
> For most EP devices, we can only define one entry in the "ranges" property
> of the devicetree that maps an address within the 32-bit range,
> as shown below:
> 	ranges = <0x02000000 0x0 0x10000000 0x0 0x10000000 0x0 0xf0000000>;
> 
> For EP devices that require 64-bits address mapping (e.g., GPUs), BAR
> resources cannot be assigned.
> To support such devices, an additional entry for 64-bits address mapping is
> required, as shown below:
> 	ranges = <0x02000000 0x0 0x10000000 0x0 0x10000000 0x0 0xf0000000>,
> 		 <0x43000000 0x1 0x00000000 0x1 0x00000000 0x7 0x00000000>;
> 
> In the current common implementation, all ranges entries are programmed to
> the iATU. However, the size of entry for 64-bit address mapping exceeds the
> maximum region size that a single iATU entry can support. As a result, an
> error is reported during iATU programming, showing that the size of 64-bit
> address entry exceeds the region limit.

Note that each iATU can map up to IATU_LIMIT_ADDR_OFF_OUTBOUND_i +
IATU_UPPR_LIMIT_ADDR_OFF_OUTBOUND_i.

Some DWC controllers have this at 4G, others have this at 8G.

Samuel has submitted a patch to use multiple iATUs to support
a window size larger than the iATU limit of a single iATU:
https://lore.kernel.org/linux-pci/aPDObXsvMoz1OYso@ryzen/T/#m11c3d95215982411d0bbd36940e70122b70ae820

Perhaps this patch could be of use for you too?

Kind regards,
Niklas