linux-kernel - Re: [PATCH] scsi:NCR5380: remove same check condition in NCR5380

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Message-ID: <alpine.LNX.2.21.1808031148320.16@nippy.intranet>
Date:   Fri, 3 Aug 2018 12:56:23 +1000 (AEST)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Michael Schmitz <schmitzmic@...il.com>
cc:     zhong jiang <zhongjiang@...wei.com>,
        Bart Van Assche <Bart.VanAssche@....com>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "john.garry@...wei.com" <john.garry@...wei.com>
Subject: Re: [PATCH] scsi:NCR5380: remove same check condition in
 NCR5380_select

On Thu, 2 Aug 2018, Michael Schmitz wrote:

> 
> This redundant load of the ICR has been in the driver code for a long 
> time. There's a small chance it is intentional,

Actually, it is intentional.

> so at least minimal testing might be in order.
> 

Minimal testing is almost useless if you are trying to prove the absence 
of race conditions. SCSI arbitration is a race between targets by design; 
so a race between the CPU and the 5380 is going to be hard to observe.

> Finn - does the ICR_ARBITRATION_LOST bit have to be cleared by a write 
> to the mode register?
> 

Something like that: the write to the mode register does clear the 
ICR_ARBITRATION_LOST bit, because it clears the MR_ARBITRATE bit.

> In that case, the first load would have been redundant and can be 
> omitted without changing driver behaviour?

This code is a faithful rendition of the arbitration flow chart in the 
datasheet, so even if you are right, I wouldn't want to change the code.

Besides, I think your argument assumes that ICR and MR are synchronized, 
and also assumes that targets are obeying the spec.

-- 

> Cheers,
> 
> 	Michael
> 
>