linux-kernel - Re: [PATCH] vme: 8-bit status/id takes 256 values, not 255

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Date:	Sun, 18 Oct 2015 13:07:08 -0400
From:	Dmitry Kalinkin <dmitry.kalinkin@...il.com>
To:	Martyn Welch <martyn@...chs.me.uk>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Manohar Vanga <manohar.vanga@...il.com>,
	devel@...verdev.osuosl.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vme: 8-bit status/id takes 256 values, not 255

On Sun, Oct 18, 2015 at 9:30 AM, Martyn Welch <martyn@...chs.me.uk> wrote:
> On 10 October 2015 at 23:00, Dmitry Kalinkin <dmitry.kalinkin@...il.com> wrote:
>> Fixes an off by one array size.
>>
>
> The Status/ID is an 8-bit value (OK, the standard states it can be a
> 8, 16 or 32-bit value, however both of the chipsets we support at this
> point in time support an 8-bit value), why do we need a 256th entry in
> the array?
Hi Martyn,

8 bit signal spans 256 values from 8'b0000_0000 to 8'b1111_1111.
In order to serve that we need a C array of size 256.
callback[255]; in the original code means that array has 255 elements
with indices from 0 to 254, there would be no way to register callback
for a valid Status/ID value of 255 (aka 8'b1111_1111). For that we
need a 256th element.

Dmitri

>
> Martyn
>
>> Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@...il.com>
>> ---
>>  drivers/vme/vme_bridge.h | 4 +++-
>>  include/linux/vme.h      | 3 +++
>>  2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vme/vme_bridge.h b/drivers/vme/vme_bridge.h
>> index 397578a..b59cbee 100644
>> --- a/drivers/vme/vme_bridge.h
>> +++ b/drivers/vme/vme_bridge.h
>> @@ -1,6 +1,8 @@
>>  #ifndef _VME_BRIDGE_H_
>>  #define _VME_BRIDGE_H_
>>
>> +#include <linux/vme.h>
>> +
>>  #define VME_CRCSR_BUF_SIZE (508*1024)
>>  /*
>>   * Resource structures
>> @@ -91,7 +93,7 @@ struct vme_callback {
>>
>>  struct vme_irq {
>>         int count;
>> -       struct vme_callback callback[255];
>> +       struct vme_callback callback[VME_NUM_STATUSID];
>>  };
>>
>>  /* Allow 16 characters for name (including null character) */
>> diff --git a/include/linux/vme.h b/include/linux/vme.h
>> index c013135..71e4a6d 100644
>> --- a/include/linux/vme.h
>> +++ b/include/linux/vme.h
>> @@ -81,6 +81,9 @@ struct vme_resource {
>>
>>  extern struct bus_type vme_bus_type;
>>
>> +/* Number of VME interrupt vectors */
>> +#define VME_NUM_STATUSID       256
>> +
>>  /* VME_MAX_BRIDGES comes from the type of vme_bus_numbers */
>>  #define VME_MAX_BRIDGES                (sizeof(unsigned int)*8)
>>  #define VME_MAX_SLOTS          32
>> --
>> 1.8.3.1
>>
--
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