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From:   Felipe Balbi <felipe.balbi@linux.intel.com>
To:     Mathias Nyman <mathias.nyman@intel.com>,
        Mathias Nyman <mathias.nyman@linux.intel.com>,
        Andrey Smirnov <andrew.smirnov@gmail.com>,
        linux-usb@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] xhci: Use ffs() to find page size in xhci_mem_init()
In-Reply-To: <74208350-eba5-c299-376e-9df194fe71b5@intel.com>
References: <20190207000349.7816-1-andrew.smirnov@gmail.com> <1ecb0604-3e27-810e-9fae-18d9d1bf7ff9@linux.intel.com> <87mun8klxj.fsf@linux.intel.com> <74208350-eba5-c299-376e-9df194fe71b5@intel.com>
Date:   Thu, 07 Feb 2019 12:58:26 +0200
Message-ID: <87k1iblval.fsf@linux.intel.com>
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Hi,

Mathias Nyman <mathias.nyman@intel.com> writes:
>>>> Get page size order using ffs() instead of open coding it with a loop.
>>>>
>>>> Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
>>>> Cc: Mathias Nyman <mathias.nyman@intel.com>
>>>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>> Cc: linux-usb@vger.kernel.org
>>>> Cc: linux-kernel@vger.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 =3D readl(&xhci->op_regs->page_size);
>>>>    	xhci_dbg_trace(xhci, trace_xhci_dbg_init,
>>>>    			"Supported page size register =3D 0x%x", page_size);
>>>> -	for (i =3D 0; i < 16; i++) {
>>>> -		if ((0x1 & page_size) !=3D 0)
>>>> -			break;
>>>> -		page_size =3D page_size >> 1;
>>>> -	}
>>>> +	i =3D 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 th=
an the loop.
>>>
>>> *old loop
>>>     valid page_size value if i < 16
>>> *ffs()
>>>     valid page_size value if i >=3D 1 and i < 17
>>=20
>> off-by-one, just use i =3D ffs() - 1. Or use __ffs().
>
> and handle the page_size =3D=3D 0 case.

Can it be zero in real life, or are you protecting against academic
possibility that's never going to happen in HW?

=2D-=20
balbi

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