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Message-ID: <c121430b1b5c8f5816b2b42b9178d00889260c90.camel@redhat.com>
Subject: Re: [PATCH 1/4] scsi: core: constify pointer to scsi_host_template
From:   "Ewan D. Milne" <emilne@redhat.com>
To:     John Garry <john.garry@huawei.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Avri Altman <avri.altman@wdc.com>,
        Doug Gilbert <dgilbert@interlog.com>,
        linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     james.smart@broadcom.com
Date:   Fri, 08 Apr 2022 15:31:35 -0400
In-Reply-To: <7b3885e3-dbae-ff0b-21dc-c28d635d950b@huawei.com>
References: <20220408103027.311624-1-krzysztof.kozlowski@linaro.org>
         <2a88a992-641a-b3ff-fe39-7a61fff87cb6@huawei.com>
         <4c3be5b6-50ef-9e9a-6cee-9642df943342@linaro.org>
         <7b3885e3-dbae-ff0b-21dc-c28d635d950b@huawei.com>
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On Fri, 2022-04-08 at 13:57 +0100, John Garry wrote:
> On 08/04/2022 13:32, Krzysztof Kozlowski wrote:
> > On 08/04/2022 14:14, John Garry wrote:
> > > On 08/04/2022 11:30, Krzysztof Kozlowski wrote:
> > > > Several pointers to 'struct scsi_host_template' do not modify it, so
> > > > made them const for safety.
> > > > 
> > > Is this standard practice? What is so special here?
> > This is standard practice and there is nothing special here. Pointers to
> > const are preferred because:
> > 1. They add safety if data is actually const. This is not yet the case,
> > but scsi_host_template allocation could be made const with some effort.

This seems unlikely, because some drivers, e.g. vmw_pvscsi and scsi_debug,
modify the scsi_host_template based on things like module parameters.

> 
> To me this seems better, but I think that some drivers might modify 
> their scsi_host_template (so not possible)

Several drivers modify scsi_host_template, e.g. .can_queue, .cmd_per_lun

There is also code in lpfc_create_port() that initializes a scsi_host_template
that is embedded in the lpfc_hba struct.  I don't think it gets modified after
scsi_add_host() but it seems like driver maintainers might expect to be able
to do so, in general.

-Ewan

> 
> > 2. The more const variables, the easier function contents and its impact
> > is to understand. This is actually the biggest benefit when dealing with
> > code touching different structures.
> > 
> > In general, constifying is a common practice everywhere in the kernel.
> 
> Thanks,
> John
>