linux-kernel - Re: [PATCH 1/1] block: Use bits.h macros to improve readability

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Message-ID: <a41b5530-f2e1-0932-1f39-0ce66ce902ae@kernel.dk>
Date:   Fri, 2 Aug 2019 21:18:10 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Leonardo Bras <leonardo@...ux.ibm.com>,
        linux-kernel@...r.kernel.org
Cc:     Dennis Zhou <dennis@...nel.org>, Hannes Reinecke <hare@...e.com>,
        Damien Le Moal <damien.lemoal@....com>,
        Tejun Heo <tj@...nel.org>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Sagi Grimberg <sagi@...mberg.me>
Subject: Re: [PATCH 1/1] block: Use bits.h macros to improve readability

On 8/1/19 6:00 PM, Leonardo Bras wrote:
> Applies some bits.h macros in order to improve readability of
> linux/blk_types.h.
> 
> Signed-off-by: Leonardo Bras <leonardo@...ux.ibm.com>
> ---
>   include/linux/blk_types.h | 55 ++++++++++++++++++++-------------------
>   1 file changed, 28 insertions(+), 27 deletions(-)
> 
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 95202f80676c..31c8c6d274f6 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -9,6 +9,7 @@
>   #include <linux/types.h>
>   #include <linux/bvec.h>
>   #include <linux/ktime.h>
> +#include <linux/bits.h>
>   
>   struct bio_set;
>   struct bio;
> @@ -101,13 +102,13 @@ static inline bool blk_path_error(blk_status_t error)
>   #define BIO_ISSUE_SIZE_BITS     12
>   #define BIO_ISSUE_RES_SHIFT     (64 - BIO_ISSUE_RES_BITS)
>   #define BIO_ISSUE_SIZE_SHIFT    (BIO_ISSUE_RES_SHIFT - BIO_ISSUE_SIZE_BITS)
> -#define BIO_ISSUE_TIME_MASK     ((1ULL << BIO_ISSUE_SIZE_SHIFT) - 1)
> +#define BIO_ISSUE_TIME_MASK     GENMASK_ULL(BIO_ISSUE_SIZE_SHIFT - 1, 0)

Not sure why we even have these helpers, I'd argue that patches like
this HURT readability, not improve it. When I see

((1ULL << SOME_SHIFT) - 1)

I know precisely what that does, whereas I have to think about the other
one, maybe even look it up to be sure. For instance, without looking
now, I have no idea what the second argument is. Looking at the git log,
I see numerous instances of:

"xxx: Fix misuses of GENMASK macro

 Arguments are supposed to be ordered high then low."

Hence it seems GENMASK_ULL is easy to misuse or get wrong, the very
opposite of what you'd want in a helper. How is it helping readability
if the helper is easy to misuse?

Ditto with

(1ULL << SOME_SHIFT)

vs BIT_ULL. But at least that one doesn't have a mysterious 2nd
argument.

Hence I'm not inclined to apply this patch.

-- 
Jens Axboe