linux-kernel - Re: [PATCH V2] lib/scatterlist: Add SG_CHAIN and SG

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Date:   Thu, 15 Feb 2018 07:36:57 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        bart.vanassche@....com, jthumshirn@...e.de, axboe@...nel.dk,
        tvrtko.ursulin@...el.com
Cc:     khandual@...ux.vnet.ibm.com
Subject: Re: [PATCH V2] lib/scatterlist: Add SG_CHAIN and SG_END macros for LSB
 encodings

Quoting Anshuman Khandual (2018-02-15 03:33:56)
> This replaces scatterlist->page_link LSB encodings with SG_CHAIN and
> SG_END definitions without any functional change.
> 
> Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
> ---
> Changes in V2:
> - Changed SG_EMARK as SG_END as per Johannes and Tvrtko
> - Added 'UL' to the constants as per Bart
> 
>  include/linux/scatterlist.h | 23 +++++++++++++----------
>  1 file changed, 13 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 22b2131bcdcd..b6fe1815f5c4 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -65,16 +65,18 @@ struct sg_table {
>   */
>  
>  #define SG_MAGIC       0x87654321
> +#define SG_CHAIN       0x01UL
> +#define SG_END         0x02UL

The alternative would be BIT(0) and BIT(1), which by the time you write
the constant out in full, as above, become clearer.

I think you also want whitespace between the MAGIC and CHAIN,END so that
the reader doesn't think they refer to some magic encoding of SG_MAGIC.
-Chris