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

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Message-ID: <900f399d-cd97-eef8-56ba-ba06293a1a84@linux.intel.com>
Date:   Thu, 7 Feb 2019 13:54:18 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>,
        Mathias Nyman <mathias.nyman@...el.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 12:58, Felipe Balbi wrote:
> 
> Hi,
> 
> Mathias Nyman <mathias.nyman@...el.com> writes:
>>>>> 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.
> 
> Can it be zero in real life, or are you protecting against academic
> possibility that's never going to happen in HW?
> 

whole page_size check is not really doing much, just printing out
different debug messages.

-Mathias