linux-kernel - Re: [PATCH RFC v2 02/18] scsi: core: Resurrect scsi_{get,free}_host

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Message-ID: <56cd28b3-da05-7737-c053-3c28459581e4@huawei.com>
Date:   Wed, 15 Jun 2022 14:44:54 +0100
From:   John Garry <john.garry@...wei.com>
To:     Bart Van Assche <bvanassche@....org>, <axboe@...nel.dk>,
        <damien.lemoal@...nsource.wdc.com>, <jejb@...ux.ibm.com>,
        <martin.petersen@...cle.com>, <brking@...ibm.com>, <hare@...e.de>,
        <hch@....de>
CC:     <linux-block@...r.kernel.org>, <linux-ide@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-scsi@...r.kernel.org>,
        <chenxiang66@...ilicon.com>
Subject: Re: [PATCH RFC v2 02/18] scsi: core: Resurrect
 scsi_{get,free}_host_dev()

On 14/06/2022 20:33, Bart Van Assche wrote:

Hi Bart,

> On 6/9/22 03:29, John Garry wrote:
>> +/**
>> + * scsi_get_host_dev - Create a scsi_device that points to the host 
>> adapter itself
>                                                 
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> What does this mean? That part of the function description is not
> clear to me.
> 

Agreed, this text is just as it was before (it was originally deleted) 
but I can fix it up to make sense.

>> + * @shost: Host that needs a scsi_device
>                                ^^^^^^^^^^^^^
> This is not detailed enough. Consider changing "a scsi_device" into
> "a scsi device for allocating reserved commands from".
> 
>> + *
>> + * Lock status: None assumed.
>> + *
>> + * Returns:     The scsi_device or NULL
>> + *
>> + * Notes:
>> + *    Attach a single scsi_device to the Scsi_Host - this should
>> + *    be made to look like a "pseudo-device" that points to the
>> + *    HA itself.
>> + *
>> + *    Note - this device is not accessible from any high-level
>> + *    drivers (including generics), which is probably not
>> + *    optimal.  We can add hooks later to attach.
> 
> The "which is probably not optimal. We can add hooks later to attach."
> part probably should be moved to the patch description.

ok

> 
>> + */
>> +struct scsi_device *scsi_get_host_dev(struct Scsi_Host *shost)
>> +{
>> +    struct scsi_device *sdev = NULL;
>> +    struct scsi_target *starget;
>> +
>> +    mutex_lock(&shost->scan_mutex);
>> +    if (!scsi_host_scan_allowed(shost))
>> +        goto out;
>> +    starget = scsi_alloc_target(&shost->shost_gendev, 0, 
>> shost->this_id);
>                                                            
> ^^^^^^^^^^^^^^^^^^
> Is it guaranteed that this channel / id combination will not be used for
> any other SCSI device?

Does it matter if the parent device is different?

> 
> What if shost->this_id == -1?
> 
>> +    if (!starget)
>> +        goto out;
>> +
>> +    sdev = scsi_alloc_sdev(starget, 0, NULL);
>> +    if (sdev)
>> +        sdev->borken = 0;
>> +    else
>> +        scsi_target_reap(starget);
>> +    put_device(&starget->dev);
>> + out:
>> +    mutex_unlock(&shost->scan_mutex);
>> +    return sdev;
>> +}
>> +EXPORT_SYMBOL(scsi_get_host_dev);
> 
> Elsewhere in the SCSI core "get..dev" means increment the reference 
> count of
> a SCSI device. Maybe scsi_alloc_host_dev() is a better name?

I think that the intention is to only use this once for a shost, i.e. 
get or allocate that scsi_device once and use it for the lifetime of the 
shost. But I can rename if you think it's better.

> 
>> +/*
>> + * These two functions are used to allocate and free a pseudo device
>> + * which will connect to the host adapter itself rather than any
>> + * physical device.  You must deallocate when you are done with the
>> + * thing.  This physical pseudo-device isn't real and won't be available
>> + * from any high-level drivers.
>> + */
> 
> Please keep function comments in .c files because that makes it more likely
> that the comment and the implementation will remain in sync.
> 

fine, I can relocate this.

Thanks,
John