linux-kernel - Re: [PATCH v9 3/7] PCI: Add parent_bus_offset to resource

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Message-ID: <20250227000817.GA565171@bhelgaas>
Date: Wed, 26 Feb 2025 18:08:17 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Frank Li <Frank.Li@....com>
Cc: Rob Herring <robh@...nel.org>, Saravana Kannan <saravanak@...gle.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Richard Zhu <hongxing.zhu@....com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, imx@...ts.linux.dev,
	Niklas Cassel <cassel@...nel.org>
Subject: Re: [PATCH v9 3/7] PCI: Add parent_bus_offset to resource_entry

On Tue, Jan 28, 2025 at 05:07:36PM -0500, Frank Li wrote:
> Introduce `parent_bus_offset` in `resource_entry` and a new API,
> `pci_add_resource_parent_bus_offset()`, to provide necessary information
> for PCI controllers with address translation units.
> 
> Typical PCI data flow involves:
>   CPU (CPU address) -> Bus Fabric (Intermediate address) ->
>   PCI Controller (PCI bus address) -> PCI Bus.
> 
> While most bus fabrics preserve address consistency, some modify addresses
> to intermediate values. 

s/modify/translate/

Specifically, they *translate* addresses, which means the same offset
is added to every address in the range, as opposed to masking or some
other transformation.

I think we can take advantage of this to simplify the callers of
.cpu_addr_fixup() later.

Ironically, most of the .cpu_addr_fixup() implementations *do* mask
the address, e.g., cdns_plat_cpu_addr_fixup() masks with 0x0fffffff.
But I think this is actually incorrect because masking results in a
many-to-one mapping, e.g.,

  0x42000000 & 0x0fffffff == 0x02000000
  0x52000000 & 0x0fffffff == 0x02000000

But presumably the addresses we pass to cdns_plat_cpu_addr_fixup()
don't cross a 256MB (0x10000000) boundary, so we could accomplish the
same by subtracting 0x40000000:

  0x42000000 - 0x40000000 == 0x02000000

> +++ b/drivers/pci/of.c
> @@ -402,7 +402,17 @@ static int devm_of_pci_get_host_bridge_resources(struct device *dev,
>  			res->flags &= ~IORESOURCE_MEM_64;
>  		}
>  
> -		pci_add_resource_offset(resources, res,	res->start - range.pci_addr);
> +		/*
> +		 * IORESOURCE_IO res->start is io space start address.
> +		 * IORESOURCE_MEM res->start is cpu start address, which is the
> +		 * same as range.cpu_addr.
> +		 *
> +		 * Use (range.cpu_addr - range.parent_bus_addr) to align both
> +		 * IO and MEM's parent_bus_offset always offset to cpu address.
> +		 */
> +
> +		pci_add_resource_parent_bus_offset(resources, res, res->start - range.pci_addr,
> +						   range.cpu_addr - range.parent_bus_addr);

Wrap to fit in 80 columns like the rest of the file.  Will have to
unindent the two lines of arguments to make this work.