linux-kernel - Re: 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:	Fri, 7 Oct 2011 10:09:49 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Amit Sahrawat <amit.sahrawat83@...il.com>
cc:	James Bottomley <James.Bottomley@...senpartnership.com>,
	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: Re: BUG in kernel: Wrong Handling of USB HDD’s in scsiglue(slave_configure) and scsi/sd(sd_read_cache_
 =?windows-1252?Q?type=29

On Fri, 7 Oct 2011, Amit Sahrawat wrote:

> Trying to set the cache type as "write back" through "echo "write
> back" > /sys/class/scsi_disk/<disk>/cache_type" does not work.
> Few logs :
> #> usb 2-1.4: new high speed USB device using ehci-sdp and address 5
> usb 2-1.4: New USB device found, idVendor=152d, idProduct=2339
> usb 2-1.4: New USB device strings: Mfr=1, Product=2, SerialNumber=5
> usb 2-1.4: Product: USB to ATA/ATAPI Bridge
> usb 2-1.4: Manufacturer: JMicron
> usb 2-1.4: SerialNumber: 3446184AA01C
> scsi1 : usb-storage 2-1.4:1.0
> scsi 1:0:0:0: Direct-Access     SAMSUNG  HM501IX               PQ: 0 ANSI: 2 CCS
> sd 1:0:0:0: [sda] 976773168 512-byte logical blocks: (500 GB/465 GiB)
> sd 1:0:0:0: [sda] Write Protect is off
> sd 1:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't
> support DPO or FUA
>  sda:
> 
> #> echo "write back" > /sys/class/scsi_disk/1\:0\:0\:0/cache_type
> sd 1:0:0:0: [sda] Sense Key : 0x5 [current]
> sd 1:0:0:0: [sda] ASC=0x20 ASCQ=0x0

This is what James was talking about.  When you write something to 
/sys/class/scsi_disk/.../cache_type, the value you specify is sent to 
the drive using a MODE SELECT command.  However, if a USB drive doesn't 
report the caching type properly, it's not very likely to support the 
appropriate MODE SELECT command either.

What you need to do is create a variant of the cache_type sysfs 
attribute, one that will set or clear the WCE flag directly, without 
trying to communicate with the drive.

Alan Stern

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