linux-kernel - Re: [Outreachy kernel] [PATCH v2 2/7] staging: most: Using macro DIV_ROUND

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Date:   Wed, 22 Feb 2017 12:36:36 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     SIMRAN SINGHAL <singhalsimran0@...il.com>
cc:     outreachy-kernel <outreachy-kernel@...glegroups.com>,
        gregkh@...uxfoundation.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org
Subject: Re: [Outreachy kernel] [PATCH v2 2/7] staging: most: Using macro
 DIV_ROUND_UP



On Wed, 22 Feb 2017, SIMRAN SINGHAL wrote:

>
>
> On Wednesday, February 22, 2017 at 2:57:17 PM UTC+5:30, Julia Lawall wrote:
>       > @@ -117,7 +118,7 @@ static int alloc_dbr(u16 size)
>       >                  return DBR_SIZE; /* out of memory */
>       >
>       >          for (i = 0; i < DBR_MAP_SIZE; i++) {
>       > -                u32 const blocks = (size + DBR_BLOCK_SIZE -
>       1) / DBR_BLOCK_SIZE;
>       > +                u32 const blocks = DIV_ROUND_UP(size,
>       DBR_BLOCK_SIZE);
>       >                  u32 mask = ~((~(u32)0) << blocks);
>
>       Totally unrelated to DIV_ROUND_UP, would this code be a
>       candidate for
>       GENMASK?
>
>  
>       Not sure, what you are trying to say.

There was a discussion on the list recently about GENMASK, which allows to
create a mask containing a sequence of 1s, like 000011110000.  It looks
like that is what is being done here, because ~0 is 11111111 and then it
ie being shifted left and all the bits inverted.  But Joe had another
suggestion.  One could double check that his suggestion gives the right
result.

julia

>
>  
>
>       julia
>
>       >
>       >                  do {
>       > @@ -137,7 +138,7 @@ static int alloc_dbr(u16 size)
>       >  static void free_dbr(int offs, int size)
>       >  {
>       >          int block_idx = offs / DBR_BLOCK_SIZE;
>       > -        u32 const blocks = (size + DBR_BLOCK_SIZE - 1) /
>       DBR_BLOCK_SIZE;
>       > +        u32 const blocks = DIV_ROUND_UP(size,
>       DBR_BLOCK_SIZE);
>       >          u32 mask = ~((~(u32)0) << blocks);
>       >
>       >          mask <<= block_idx % 32;
>       > --
>       > 2.7.4
>       >
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