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SubjectRe: [RFC PATCH 00/17] Drop uses of pci_read_config_*() return value
On Sun, Aug 02, 2020 at 07:28:00PM +0200, Saheed Bolarinwa wrote:
> Because the value ~0 has a meaning to some drivers and only

No, ~0 means that the PCI read failed. For *every* PCI device I know.

Here's me reading from 0xf0 offset of my hostbridge:

# setpci -s 00:00.0 0xf0.l
01000000

That device doesn't have extended config space, so the last valid byte
is 0xff. Let's read beyond that:

# setpci -s 00:00.0 0x100.l
ffffffff

> Again, only the drivers can determine if ~0 is a valid value. This
> information is not available inside pci_config_read*().

Of course it is.

*every* change you've done in 6/17 - this is the only patch I have
received - checks for == ~0. So that check can just as well be moved
inside pci_config_read_*().

Here's how one could do it:

#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) \
{ \
int res; \
unsigned long flags; \
u32 data = 0; \
if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER; \
pci_lock_config(flags); \
res = bus->ops->read(bus, devfn, pos, len, &data); \

/* Check we actually read something which is not all 1s.*/
if (data == ~0)
return PCIBIOS_READ_FAILED;

*value = (type)data; \
pci_unlock_config(flags); \
return res; \
}

Also, I'd prefer a function to *not* return void but return either
an error or success. In the success case, the @value argument can be
consumed by the caller and otherwise not.

In any case, that change is a step in the wrong direction and I don't
like it, sorry.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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