linux-kernel - Re: [PATCH] scsi: scsi_sysfs.c: Hide wwid sdev attr if VPD is not supported

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Message-ID: <850765d7-da85-3fc1-7bf4-f0edcb63f8d8@suse.com>
Date:   Wed, 19 Jun 2019 08:34:56 +0200
From:   Hannes Reinecke <hare@...e.com>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        Marcos Paulo de Souza <marcos.souza.org@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        linux-scsi@...r.kernel.org
Subject: Re: [PATCH] scsi: scsi_sysfs.c: Hide wwid sdev attr if VPD is not
 supported

On 6/19/19 5:35 AM, Martin K. Petersen wrote:
> 
> Marcos,
> 
>> WWID composed from VPD data from device, specifically page 0x83. So,
>> when a device does not have VPD support, for example USB storage
>> devices where VPD is specifically disabled, a read into <blk
>> device>/device/wwid file will always return ENXIO. To avoid this,
>> change the scsi_sdev_attr_is_visible function to hide wwid sysfs file
>> when the devices does not support VPD.
> 
> Not a big fan of attribute files that come and go.
> 
> Why not just return an empty string? Hannes?
> 
Actually, the intention of the 'wwid' attribute was to have a common
place where one could look up the global id.
As such it actually serves a dual purpose, namely indicating that there
_is_ a global ID _and_ that this kernel (version) has support for 'wwid'
attribute. This is to resolve one big issue we have to udev nowadays,
which is figuring out if a specific sysfs attribute is actually
supported on this particular kernel.
Dynamic attributes are 'nicer' on a conceptual level, but make the above
test nearly impossible, as we now have _two_ possibilities why a
specific attribute is not present.
So making 'wwid' conditional would actually defeat its very purpose, and
we should leave it blank if not supported.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@...e.com			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
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