pci_iounmap'2: Electric Boogaloo: try to make sense of it all

Nathan Chancellor reports that the recent change to pci_iounmap in
commit 9caea0007601 ("parisc: Declare pci_iounmap() parisc version only
when CONFIG_PCI enabled") causes build errors on arm64.

It took me about two hours to convince myself that I think I know what
the logic of that mess of #ifdef's in the <asm-generic/io.h> header file
really aim to do, and rewrite it to be easier to follow.

Famous last words.

Anyway, the code has now been lifted from that grotty header file into
lib/pci_iomap.c, and has fairly extensive comments about what the logic
is. It also avoids indirecting through another confusing (and badly
named) helper function that has other preprocessor config conditionals.

Let's see what odd architecture did something else strange in this area
to break things. But my arm64 cross build is clean.

Fixes: 9caea0007601 ("parisc: Declare pci_iounmap() parisc version only when CONFIG_PCI enabled")
Reported-by: Nathan Chancellor <[e-mail]>
Cc: Helge Deller <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Guenter Roeck <[e-mail]>
Cc: Ulrich Teichert <[e-mail]>
Cc: James Bottomley <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/asm-generic/io.h

···
 	port &= IO_SPACE_LIMIT;
 	return (port > MMIO_UPPER_LIMIT) ? NULL : PCI_IOBASE + port;
 }
-#define __pci_ioport_unmap __pci_ioport_unmap
-static inline void __pci_ioport_unmap(void __iomem *p)
-{
-	uintptr_t start = (uintptr_t) PCI_IOBASE;
-	uintptr_t addr = (uintptr_t) p;
-
-	if (addr >= start && addr < start + IO_SPACE_LIMIT)
-		return;
-	iounmap(p);
-}
+#define ARCH_HAS_GENERIC_IOPORT_MAP
 #endif
 
 #ifndef ioport_unmap
···
 #endif /* CONFIG_HAS_IOPORT_MAP */
 
 #ifndef CONFIG_GENERIC_IOMAP
-struct pci_dev;
-extern void __iomem *pci_iomap(struct pci_dev *dev, int bar, unsigned long max);
-
-#ifndef __pci_ioport_unmap
-static inline void __pci_ioport_unmap(void __iomem *p) {}
-#endif
-
 #ifndef pci_iounmap
-#define pci_iounmap pci_iounmap
-static inline void pci_iounmap(struct pci_dev *dev, void __iomem *p)
-{
-	__pci_ioport_unmap(p);
-}
+#define ARCH_WANTS_GENERIC_PCI_IOUNMAP
 #endif
-#endif /* CONFIG_GENERIC_IOMAP */
+#endif
 
 #ifndef xlate_dev_mem_ptr
 #define xlate_dev_mem_ptr xlate_dev_mem_ptr

lib/pci_iomap.c

···
 	return pci_iomap_wc_range(dev, bar, 0, maxlen);
 }
 EXPORT_SYMBOL_GPL(pci_iomap_wc);
+
+/*
+ * pci_iounmap() somewhat illogically comes from lib/iomap.c for the
+ * CONFIG_GENERIC_IOMAP case, because that's the code that knows about
+ * the different IOMAP ranges.
+ *
+ * But if the architecture does not use the generic iomap code, and if
+ * it has _not_ defined it's own private pci_iounmap function, we define
+ * it here.
+ *
+ * NOTE! This default implementation assumes that if the architecture
+ * support ioport mapping (HAS_IOPORT_MAP), the ioport mapping will
+ * be fixed to the range [ PCI_IOBASE, PCI_IOBASE+IO_SPACE_LIMIT [,
+ * and does not need unmapping with 'ioport_unmap()'.
+ *
+ * If you have different rules for your architecture, you need to
+ * implement your own pci_iounmap() that knows the rules for where
+ * and how IO vs MEM get mapped.
+ *
+ * This code is odd, and the ARCH_HAS/ARCH_WANTS #define logic comes
+ * from legacy <asm-generic/io.h> header file behavior. In particular,
+ * it would seem to make sense to do the iounmap(p) for the non-IO-space
+ * case here regardless, but that's not what the old header file code
+ * did. Probably incorrectly, but this is meant to be bug-for-bug
+ * compatible.
+ */
+#if defined(ARCH_WANTS_GENERIC_PCI_IOUNMAP)
+
+void pci_iounmap(struct pci_dev *dev, void __iomem *p)
+{
+#ifdef ARCH_HAS_GENERIC_IOPORT_MAP
+	uintptr_t start = (uintptr_t) PCI_IOBASE;
+	uintptr_t addr = (uintptr_t) p;
+
+	if (addr >= start && addr < start + IO_SPACE_LIMIT)
+		return;
+	iounmap(p);
+#endif
+}
+EXPORT_SYMBOL(pci_iounmap);
+
+#endif /* ARCH_WANTS_GENERIC_PCI_IOUNMAP */
+
 #endif /* CONFIG_PCI */