linux-kernel - Re: [PATCH] scsi: use set_host_byte instead of open-coding it

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Date:   Wed, 11 Oct 2017 15:05:40 +0200
From:   Steffen Maier <maier@...ux.vnet.ibm.com>
To:     Johannes Thumshirn <jthumshirn@...e.de>,
        "Martin K . Petersen" <martin.petersen@...cle.com>
Cc:     Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Linux SCSI Mailinglist <linux-scsi@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>,
        Bart Van Assche <Bart.VanAssche@...disk.com>,
        Oliver Neukum <oneukum@...e.com>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] scsi: use set_host_byte instead of open-coding it

On 10/10/2017 05:29 PM, Johannes Thumshirn wrote:
> Call set_host_byte() instead of open-coding it.
> 
> Converted using this simple Coccinelle spatch
> 
> <SmPL>
> @@
> local idexpression struct scsi_cmnd *c;
> expression E1;
> @@
> 
> - c->result = E1 << 16;
> + set_host_byte(c, E1);

Maybe I misunderstand, but doesn't set_host_byte only set the host byte 
but leave the other 3 parts untouched in c->result?

static inline void set_host_byte(struct scsi_cmnd *cmd, char status)
{
	cmd->result = (cmd->result & 0xff00ffff) | (status << 16);
}

In contrast, assigning something to c->result resets all parts.
If so, the semantic patch would introduce a subtle semantic change.
Unless it's guaranteed that in all the touched cases, c->result always 
has 0 for status, message, and driver byte before calling set_host_byte().

Bart's suggestion also sounds nice.

FYI: Originally, I only thought about using set_host_byte in that one 
place fix of yours; I did not expect a full framework rework.

-- 
Mit freundlichen Grüßen / Kind regards
Steffen Maier

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