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

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Message-ID: <87mun8klxj.fsf@linux.intel.com>
Date:   Thu, 07 Feb 2019 11:06:00 +0200
From:   Felipe Balbi <felipe.balbi@...ux.intel.com>
To:     Mathias Nyman <mathias.nyman@...ux.intel.com>,
        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()


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().

-- 
balbi

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