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

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Date:   Tue, 14 Jun 2022 12:33:07 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     John Garry <john.garry@...wei.com>, 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 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.

> + * @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.

> + */
> +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?

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?

> +/*
> + * 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.

Thanks,

Bart.