| Message ID | 20250113143037.2131346-2-niklas.neronin@linux.intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | usb: xhci: page size improvements | expand |
On Mon, 13 Jan 2025 16:30:36 +0200, Niklas Neronin wrote: > Fix this by subtracting one from the result of ffs(). Note that since > variable 'i' is unsigned, subtracting one from zero will result in the > maximum unsigned integer value. Consequently, the condition 'if (i < > 16)' will still function correctly. Is it unsigned? I see a simple 'int' here, so this wouldn't work. Michal
On 13/01/2025 18.00, Michał Pecio wrote: > On Mon, 13 Jan 2025 16:30:36 +0200, Niklas Neronin wrote: >> Fix this by subtracting one from the result of ffs(). Note that since >> variable 'i' is unsigned, subtracting one from zero will result in the >> maximum unsigned integer value. Consequently, the condition 'if (i < >> 16)' will still function correctly. > > Is it unsigned? I see a simple 'int' here, so this wouldn't work. > You are correct, it is not unsigned. Thank you for catching it, don't know how I missed it. Best Regards, Niklas Neronin
diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c index 92703efda1f7..66584aafc513 100644 --- a/drivers/usb/host/xhci-mem.c +++ b/drivers/usb/host/xhci-mem.c @@ -2391,7 +2391,7 @@ 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); + i = ffs(page_size) - 1; if (i < 16) xhci_dbg_trace(xhci, trace_xhci_dbg_init, "Supported page size of %iK", (1 << (i+12)) / 1024);
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 'i' is unsigned, subtracting one from zero will result in the maximum unsigned integer value. Consequently, the condition 'if (i < 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> --- drivers/usb/host/xhci-mem.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)