[PATCH 08/15] usb: xhci: correct debug message page size calculation

Mathias Nyman
2025-03-06T14:49:39Z

From: Niklas Neronin <niklas.neronin@linux.intel.com>

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: 81720ec5320c ("usb: host: xhci: use ffs() in xhci_mem_init()")
Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
---
drivers/usb/host/xhci-mem.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index 92703efda1f7..dc5bcd8db4c0 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/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 */
--
2.43.0