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From:   "chanho61.park" <chanho61.park@samsung.com>
To:     "'Bart Van Assche'" <bvanassche@acm.org>, <jejb@linux.ibm.com>,
        <martin.petersen@oracle.com>
Cc:     <alim.akhtar@samsung.com>, <avri.altman@wdc.com>,
        <linux-scsi@vger.kernel.org>, <linux-kernel@vger.kernel.org>
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Subject: RE: [PATCH] scsi: ufs: make sure scan sequence for multiple hosts
Date:   Wed, 21 Oct 2020 13:23:32 +0900
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Hi,

> -----Original Message-----
> From: Bart Van Assche <bvanassche@acm.org>
> Sent: Wednesday, October 21, 2020 12:15 PM
> To: Chanho Park <chanho61.park@samsung.com>; jejb@linux.ibm.com;
> martin.petersen@oracle.com
> Cc: alim.akhtar@samsung.com; avri.altman@wdc.com; linux-
> scsi@vger.kernel.org; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH] scsi: ufs: make sure scan sequence for multiple
hosts
> 
> On 10/20/20 12:05 AM, Chanho Park wrote:
> > By doing scan as asynchronous way, scsi device scannning can be out of
> > order execution. It is no problem if there is a ufs host but the scsi
> > device name of each host can be changed according to the scan
sequences.
> >
> > Ideal Case) host0 scan first
> > host0 will be started from /dev/sda
> >  -> /dev/sdb (BootLUN0 of host0)
> >  -> /dev/sdc (BootLUN1 of host1)
> > host1 will be started from /dev/sdd
> >
> > This might be an ideal case and we can easily find the host device by
> > this mappinng.
> >
> > However, Abnormal Case) host1 scan first,
> > host1 will be started from /dev/sda and host0 will be followed later.
> >
> > To make sure the scan sequences according to the host, we can use a
> > bitmap which hosts are scanned and wait until previous hosts are
> > finished to scan.
> 
> This sounds completely wrong to me. No code should depend on the order in
> which LUNs are scanned. Please use the soft links created by udev instead
> of serializing LUN scanning.
> 

Thanks for your review.
Did you mean /dev/disk/by-[part]label/ symlink? It's quite reasonable to
use them by udev in userspace such as initramfs but some cases does not use
initramfs or initrd. In that case, we need to load the root
device(/dev/sda[N]) directly from kernel.
Anyway, scsi disk(sd) case, the scan order will not be changed until the
port has changed so they'll have permanent device names. I'd like to make
permanent UFS device names.

Best Regards,
Chanho Park

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