usb: xhci: correct debug message page size calculation

The ffs() function returns the index of the first set bit, starting from 1.
If no bits are set, it returns zero. This behavior causes an off-by-one
page size in the debug message, as the page size calculation [1]
is zero-based, while ffs() is one-based.

Fix this by subtracting one from the result of ffs(). Note that since
variable 'val' is unsigned, subtracting one from zero will result in the
maximum unsigned integer value. Consequently, the condition 'if (val < 16)'
will still function correctly.

[1], Page size: (2^(n+12)), where 'n' is the set page size bit.

Fixes: 81720ec ("usb: host: xhci: use ffs() in xhci_mem_init()")
Signed-off-by: Niklas Neronin <[email protected]>
Signed-off-by: Mathias Nyman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Epicurius

drivers/usb/host/xhci-mem.c

@@ -2391,10 +2391,10 @@ int xhci_mem_init(struct xhci_hcd *xhci, gfp_t flags)
 	page_size = readl(&xhci->op_regs->page_size);
 	xhci_dbg_trace(xhci, trace_xhci_dbg_init,
 			"Supported page size register = 0x%x", page_size);
-	i = ffs(page_size);
-	if (i < 16)
+	val = ffs(page_size) - 1;
+	if (val < 16)
 		xhci_dbg_trace(xhci, trace_xhci_dbg_init,
-			"Supported page size of %iK", (1 << (i+12)) / 1024);
+			"Supported page size of %iK", (1 << (val + 12)) / 1024);
 	else
 		xhci_warn(xhci, "WARN: no supported page size\n");
 	/* Use 4K pages, since that's common and the minimum the HC supports */