linux-kernel - Re: [PATCH] PCI: Fix endianness issues in pci_bus_read

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Message-ID: <aI0CupiFvyOvgNQY@kbusch-mbp>
Date: Fri, 1 Aug 2025 12:08:58 -0600
From: Keith Busch <kbusch@...nel.org>
To: Hans Zhang <18255117159@....com>
Cc: Gerd Bayer <gbayer@...ux.ibm.com>,
	Manivannan Sadhasivam <mani@...nel.org>,
	Hans Zhang <hans.zhang@...tech.com>,
	Arnd Bergmann <arnd@...nel.org>, Bjorn Helgaas <helgaas@...nel.org>,
	bhelgaas@...gle.com, Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	jingoohan1@...il.com,
	Krzysztof Wilczy´nski <kwilczynski@...nel.org>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-next <linux-next@...r.kernel.org>, linux-pci@...r.kernel.org,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Niklas Schnelle <schnelle@...ux.ibm.com>, geert@...ux-m68k.org
Subject: Re: [PATCH] PCI: Fix endianness issues in pci_bus_read_config()

On Sat, Aug 02, 2025 at 12:54:27AM +0800, Hans Zhang wrote:
> As I mentioned in my reply to Mani's email, the data ultimately read here is
> also a forced type conversion.
> 
> #define PCI_OP_READ(size, type, len) \
> int noinline pci_bus_read_config_##size \
> 	(struct pci_bus *bus, unsigned int devfn, int pos, type *value)	\
> {									\
> 	unsigned long flags;						\
> 	u32 data = 0;							\
> 	int res;							\
> 									\
> 	if (PCI_##size##_BAD)						\
> 		return PCIBIOS_BAD_REGISTER_NUMBER;			\
> 									\
> 	pci_lock_config(flags);						\
> 	res = bus->ops->read(bus, devfn, pos, len, &data);		\
> 	if (res)							\
> 		PCI_SET_ERROR_RESPONSE(value);				\
> 	else								\
> 		*value = (type)data;					\
> 	pci_unlock_config(flags);					\
> 									\
> 	return res;							\
> }
> 
> And this function. Could it be that I misunderstood something?

The above macro retains the caller's type for "value". If the caller
passes a "u8 *", the value is deferenced as a u8.

The function below promotes everything to a u32 pointer and deferences
it as such regardless of what type the user passed in.
 
> int pci_generic_config_read(struct pci_bus *bus, unsigned int devfn,
> 			    int where, int size, u32 *val)
> {
> 	void __iomem *addr;
> 
> 	addr = bus->ops->map_bus(bus, devfn, where);
> 	if (!addr)
> 		return PCIBIOS_DEVICE_NOT_FOUND;
> 
> 	if (size == 1)
> 		*val = readb(addr);
> 	else if (size == 2)
> 		*val = readw(addr);
> 	else
> 		*val = readl(addr);
> 
> 	return PCIBIOS_SUCCESSFUL;
> }