linux-kernel - Re: [PATCH repost 3] [SCSI] Retrieve the Caching mode page

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Message-ID: <549123.75632.qm@web31809.mail.mud.yahoo.com>
Date:	Wed, 24 Nov 2010 01:02:28 -0800 (PST)
From:	Luben Tuikov <ltuikov@...oo.com>
To:	Matthew Dharm <mdharm-kernel@...-eyed-alien.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-usb@...r.kernel.org, Greg KH <greg@...ah.com>,
	James Bottomley <James.Bottomley@...e.de>
Subject: Re: [PATCH repost 3] [SCSI] Retrieve the Caching mode page

--- On Tue, 11/23/10, Matthew Dharm <mdharm-kernel@...-eyed-alien.net> wrote:

> Not really.  In your example case of the write-protect
> bit, either the
> device would be improperly marked as write-protected when
> it is not, or we
> would improperly send write commands to a protected
> device.  Neither is a
> great tradgedy; commands will fail, errors will be
> generated, and life will
> go on.

Yes, indeed.

> However, I don't know what is in the Caching Mode
> Page.

It's described in SBC-3.

> I don't know how
> those bits are used to determine the behavior of the rest
> of the kernel.

I see.

> Maybe one of those bits means "only store data in Japanese"
> or something
> equally disturbing.

I doubt it. (And I don't find that disturbing.)

> The old code used to make a "safe" default choice if the
> caching mode page
> was not available (for whatever
> reason).   What are the implications of not
> making the "safe" choice improperly (due to mode page
> corruption)?

Let's see... the old code made the ``"safe" default choice'' of believing that the write cache is disabled and will thus never synchronize the cache, i.e. send SYNCHRONIZE CACHE command, reading drivers/scsi/sd.c. It also believes that the read cache is enabled, harmless as it is not used by the kernel. It also assumes that DPOFUA is not supported and will thus never set FUA in the CDB, in sd_revalidate_disk() (blk dev ops).

Quoting you, s/write commands/cdbs with FUA set or SYNCHRONIZE CACHE command/g:

> we
> would improperly send write commands to a protected
> device.  Neither is a
> great tradgedy; commands will fail, errors will be
> generated, and life will
> go on.

But I believe the discussion has gone astray. Bringing it back to the original topic:

I doubt this as very unlikely. Has anyone actually seen a device that sends mode parameter data with faux Caching mode page or corrupted data that is in fact interpreted as a Caching mode page? Is such a device fully operational sans the faux Caching mode page, or does it just not work? Is it common to have devices having a faux Caching mode page or corrupted mode parameter data resulting in a Caching mode page with random data?

Undoubtedly, as the usb-storage maintainer, you must have variety of devices, some broken some not. Could you apply this patch to your tree and test some of the devices you have? My tests indicate a stable behavior.

    Luben

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