linux-kernel - Re: [PATCH] staging/comedi: fix sparse warning: shift too big

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Message-ID: <5363FF10.306@mev.co.uk>
Date:	Fri, 2 May 2014 21:24:48 +0100
From:	Ian Abbott <abbotti@....co.uk>
To:	Hartley Sweeten <HartleyS@...ionengravers.com>,
	Brilliantov Kirill Vladimirovich <brilliantov@...ox.ru>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging/comedi: fix sparse warning: shift too big

On 02/05/14 19:28, Hartley Sweeten wrote:
> On Friday, May 02, 2014 9:50 AM, Ian Abbott wrote:
>> On 2014-05-02 15:57, Brilliantov Kirill Vladimirovich wrote:
>>> Signed-off-by: Brilliantov Kirill Vladimirovich <brilliantov@...ox.ru>
>>> ---
>>>    drivers/staging/comedi/drivers/ni_stc.h | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/staging/comedi/drivers/ni_stc.h b/drivers/staging/comedi/drivers/ni_stc.h
>>> index f0630b78..197b568 100644
>>> --- a/drivers/staging/comedi/drivers/ni_stc.h
>>> +++ b/drivers/staging/comedi/drivers/ni_stc.h
>>> @@ -717,7 +717,7 @@ enum AI_AO_Select_Bits {
>>>    static inline unsigned ni_stc_dma_channel_select_bitfield(unsigned channel)
>>>    {
>>>    	if (channel < 4)
>>> -		return 1 << channel;
>>> +		return 1 << (channel & 0x3);
>>>    	if (channel == 4)
>>>    		return 0x3;
>>>    	if (channel == 5)
>>>
>>
>> The warning seems a bit spurious!
>
> The whole mite.c driver and it's users are all a bit of a mess.
>
> I did find the information about the "AI AO Select Register" in the National
> Instruments PCI E Series manual:
>
> -----
>
> The AI AO Select Register contains 8 bits that control the logical DMA selection for the
> analog input and analog output resources. The contents of this register are cleared upon power
> up and after a reset condition.
> 	Address:	base address + 0x09
> 	Type:		write-only
> 	Word size:	8-bit
>
> 	Bit	Name		Description
> 	7-4	Reserved	Reserved-Always write 0 to these bits (for PCI-6032E
> 				and PCI-6033E only).
> 	7-4	Output<D..A>	Analog Output Logical Channel D through A-These four
> 				bits select the logical channels of the MITE to be used by
> 				the analog output. You can only set one of these bits at a
> 				time (except for the PCI-6032E and PCI-6033E).
> 	3-0	Input<D..A>	Analog Input Logical Channel D through A-These four
> 				bits select the logical channels to be used by the analog
> 				input. You can only set one of these bits at a time.
>
> -----
>
> Based on this the ni_stc_dma_channel_select_bitfield() function appears to be
> incorrect. The "channel"s should only be in the range 0 to 3 and result in a returned
> bitfield of:
>
> 	channel	bitfield	Description
> 	0		0x01	Analog Input or Output Channel A
> 	1		0x02 	Analog Input or Output Channel B
> 	2		0x04 	Analog Input or Output Channel C
> 	3		0x08 	Analog Input or Output Channel D
>
> Just my two cents...

Apparently, the M-series cards have six DMA channels - two more than the 
E-series cards.  They must have decided to keep the existing bitfield 
encoding for the first four channels for backwards compatibility, and 
use special bitfield values for the extra channels.

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@....co.uk>        )=-
-=( Tel: +44 (0)161 477 1898   FAX: +44 (0)161 718 3587         )=-
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