linux-kernel - Re: [PATCH] xhci: Use ffs() to find page size in xhci_mem

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Date:   Thu, 7 Feb 2019 11:04:28 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Andrey Smirnov <andrew.smirnov@...il.com>,
        linux-usb@...r.kernel.org
Cc:     Mathias Nyman <mathias.nyman@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xhci: Use ffs() to find page size in xhci_mem_init()

On 07.02.2019 02:03, Andrey Smirnov wrote:
> Get page size order using ffs() instead of open coding it with a loop.
> 
> Signed-off-by: Andrey Smirnov <andrew.smirnov@...il.com>
> Cc: Mathias Nyman <mathias.nyman@...el.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: linux-usb@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> ---
>   drivers/usb/host/xhci-mem.c | 6 +-----
>   1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
> index 36a3eb8849f1..44b43c3d819f 100644
> --- a/drivers/usb/host/xhci-mem.c
> +++ b/drivers/usb/host/xhci-mem.c
> @@ -2362,11 +2362,7 @@ 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);
> -	for (i = 0; i < 16; i++) {
> -		if ((0x1 & page_size) != 0)
> -			break;
> -		page_size = page_size >> 1;
> -	}
> +	i = ffs(page_size);
>   	if (i < 16)
>   		xhci_dbg_trace(xhci, trace_xhci_dbg_init,
>   			"Supported page size of %iK", (1 << (i+12)) / 1024);

Hi

using ffs() is a welcome change, but it will give different a result than the loop.

*old loop
   valid page_size value if i < 16
*ffs()
   valid page_size value if i >= 1 and i < 17
  
-Mathias