linux-kernel - Re: [PATCH] serial: vt8500_serial: Fix a parameter of find_first_zero

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Date:   Tue, 23 Aug 2016 22:24:22 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux@...sktech.co.nz, linux-serial@...r.kernel.org,
        jslaby@...e.com
Subject: Re: [PATCH] serial: vt8500_serial: Fix a parameter of
 find_first_zero_bit.

Le 23/08/2016 à 11:23, Arnd Bergmann a écrit :
> On Tuesday, August 23, 2016 6:20:28 AM CEST Christophe JAILLET wrote:
>> Le 22/08/2016 à 10:42, Arnd Bergmann a écrit :
>>> [...]
>>> Sorry, but I'm not following the logic here.
>>>
>>> [...]
>>> You argue that the two have the same meaning, which I see, but
>>> why is it better than the existing code?
>>>
>>> 	Arnd
>> Hi,
>>
>> sorry if my explanation was unclear.
>>
>> What I mean is that if "sizeof(unsigned long) = 4" (i.e. 32 bits systems
>> ?) then:
>>
>> 	port = find_first_zero_bit(&vt8500_ports_in_use, sizeof(vt8500_ports_in_use));
>> turns into:
>>    	port = find_first_zero_bit(&vt8500_ports_in_use, 4);
>>
>> find_first_zero_bit "Returns the bit number of the first set bit.  If no bits are set, returns @size."
>> So, in this case, it can return 1, 2, 3 or 4, if one of the 4 first bits is 0.
>> And will also return 4, if none of the 4 first bits is 0.
> Ah, got it.
>
>> Finally, what I meant by "Other options are possible:" is:
>>     - 'vt8500_ports_in_use' being a 'unsigned long', use ffz to reduce code verbosity
>> 	port = ffz(&vt8500_ports_in_use);
>>       would also work, because it is equivalent to:
>> 	port = find_first_zero_bit(&vt8500_ports_in_use, BITS_PER_LONG);
>>
>>     - VT8500_MAX_PORTS, in order to be consistent with the test below
>> 	port = find_first_zero_bit(&vt8500_ports_in_use, VT8500_MAX_PORTS);
>>       would also work and is maybe more logical in regard to the test "if (port >= VT8500_MAX_PORTS)"
>>
>>
>>
>> Now if "sizeof(unsigned long) = 8" (i.e. 64 bits systems ?), the actual code would work.
>> But using "sizeof(long)" to mean "more than VT8500_MAX_PORTS" is odd.
>> In other words, expressing a number of bits using something that gives a size in bytes is, IMHO, spurious.
>>    
>> All this is pure speculation.
>>
>> Hoping that it is clearer now ( and that my analysis is right :) )
> I misread the code in the same way the original author wrote it wrong,
> I guess it was meant to say
>
> 	port = find_first_zero_bit(&vt8500_ports_in_use, sizeof(vt8500_ports_in_use) * 8);
I guess so.

> to convert number of bytes into number of bits. Your patch is absolutely
> correct, but being more specific about the kind of mistake that was made
> is a good idea.
>
> Regarding which of the four alternatives to use, I'd probably use
> your third one, checking against VT8500_MAX_PORTS. To make this code
> absolutely foolproof, we can add this hunk too then:
Agreed for VT8500_MAX_PORTS. This documents the code.

Using DECLARE_BITMAP is also nice (even if I doubt that it will be 
useful one day in this particular case)
It would turn the vt8500_ports_in_use variable into a pointer. So some 
more code modification would be required.

Thk for your feedback and comments.

I'll send a v2.


CJ


> diff --git a/drivers/tty/serial/vt8500_serial.c b/drivers/tty/serial/vt8500_serial.c
> index 23cfc5e16b45..a68be66d2770 100644
> --- a/drivers/tty/serial/vt8500_serial.c
> +++ b/drivers/tty/serial/vt8500_serial.c
> @@ -118,7 +118,7 @@ struct vt8500_port {
>    * have been allocated as we can't use pdev->id in
>    * devicetree
>    */
> -static unsigned long vt8500_ports_in_use;
> +static DECLARE_BITMAP(vt8500_ports_in_use, VT8500_MAX_PORTS);
>   
>   static inline void vt8500_write(struct uart_port *port, unsigned int val,
>   			     unsigned int off)
>
>
>
> 	Arnd


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