linux-kernel - Re: [PATCH 1/4] scsi: core: constify pointer to scsi_host

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Message-ID: <b6af3fe8-db9a-b5dc-199f-21c05d7664a2@huawei.com>
Date:   Tue, 12 Apr 2022 08:57:39 +0100
From:   John Garry <john.garry@...wei.com>
To:     "Ewan D. Milne" <emilne@...hat.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "Alim Akhtar" <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        "Doug Gilbert" <dgilbert@...erlog.com>,
        <linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     <james.smart@...adcom.com>
Subject: Re: [PATCH 1/4] scsi: core: constify pointer to scsi_host_template

On 08/04/2022 20:31, Ewan D. Milne wrote:
> 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.
>

The standard flow is:

shost = scsi_host_alloc(sht, )

// modify shost, like
shost->cmd_per_lun = 5;

scsi_add_host(shost)

Is there some reason for which those two drivers can't follow that?

>>
>> 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.
> 

Even so, I don't see why other drivers cannot declare their 
scsi_host_template as const. C would have no problem with sht not be 
being const for this:

struct Scsi_Host *scsi_host_alloc(const struct scsi_host_template *sht, )

thanks,
John