linux-kernel - Re: Re: Re: BUG in kernel: Wrong Handling of USB HDD’s in scsiglue(slave_configure) and scsi/sd(sd_read_cache

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Date:	Wed, 05 Oct 2011 13:08:15 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Amit Sahrawat <amit.sahrawat83@...il.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>
Subject: Re: Re: Re: BUG in kernel: Wrong Handling of USB
 HDD’s in scsiglue(slave_configure) and
 scsi/sd(sd_read_cache_type)

On Thu, 2011-10-06 at 01:21 +0530, Amit Sahrawat wrote:
> > The best we could probably offer is an interface to turn on the WCE bit
> > in software (technically, you can do this today
> > in /scsi/class/scsi_disk/<disk>/cache_type, it's just that it will try
> > to commit the change as a MODE_SELECT which will presumably fail).
> Thanks James, is this related with SCSI command MODE_SELECT? and this
> is to be passed when there is some failure?
> or simply doing an "echo <value>" to
> /scsi/class/scsi_disk/<disk>/cache_type? can you please elaborate a
> little

You tell me since you have the device.  What that echo does is that it
does try to make the change permanent with a mode select ... that likely
won't work and the cache change only takes if the revalidated disk says
the write back has been enabled (which I really think it won't).  So I
think you need an additional software bit to flip for the case where the
device lies.

James


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