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

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Date:   Thu, 7 Feb 2019 12:46:45 +0200
From:   Mathias Nyman <mathias.nyman@...el.com>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Andrey Smirnov <andrew.smirnov@...il.com>,
        linux-usb@...r.kernel.org
Cc:     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 11:06, Felipe Balbi wrote:
> 
> Hi,
> 
> Mathias Nyman <mathias.nyman@...ux.intel.com> writes:
>> 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
> 
> off-by-one, just use i = ffs() - 1. Or use __ffs().
> 

and handle the page_size == 0 case.

-Mathias