linux-kernel - Re: [Patch] SCSI: Retrieve Cache Mode Using SG_ATA

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Message-ID: <CADDb1s1T8_BQA-mV3TGw7UjeuE-qdO_9V35xTNYE2DQcKrp_9A@mail.gmail.com>
Date:	Tue, 11 Oct 2011 21:30:30 +0530
From:	Amit Sahrawat <amit.sahrawat83@...il.com>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Douglas Gilbert <dgilbert@...erlog.com>,
	linux-usb@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	NamJae Jeon <linkinjeon@...il.com>
Subject: Re: [Patch] SCSI: Retrieve Cache Mode Using SG_ATA_16 if normal
 routine fails

On Tue, Oct 11, 2011 at 7:35 PM, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
> On Tue, 2011-10-11 at 13:56 +0530, Amit Sahrawat wrote:
>> SCSI: Retrieve Cache Mode Using SG_ATA_16 if normal routine fails
>>
>> It has been observed that a number of USB HDD's do not respond correctly
>> to SCSI mode sense command(retrieve caching pages) which results in their
>> Write Cache being discarded by queue requests i.e., WCE if left set to
>> '0'(disabled). So, in order to identify the devices correctly - give it
>> a last try using SG_ATA_16 after failure from normal routine.
>
> This is a non-starter, as I've said before.  Apart from the layering
> violation of trying to make sd ATA aware, ATA_16 is known to crash some
> USB devices (the bug reports are mostly about smartctl which can be made
> to use ATA_16 failing).
I agree that there might devices which crash on this response.
>
> The correct way to implement this is to have a user visible WCE variable
> that can be written to in the scsi_disk class sysfs files so the user
> can alter the caching type on the fly.
This is OK to add WCE variable, but again this "on the fly" is
confusing. I expect this to be initialized the moment I plug-in the
USB. If the user has to take this decision - then again there has to
be a supporting fact so as to change the "cache type" from sysfs.
Again, the point would be how to get this if the device do not respond
correctly. sd_store_cache_type() - is this main code point which needs
to be thoroughly checked for this? especially the scsi_mode_select()?
Please correct me if I am wrong.
>
> James
>
>
Thanks & Regards,
Amit Sahrawat
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