linux-kernel - Re: [PATCH V5 2/2] scsi: ufs: Add configfs support for ufs provisioning

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Date:   Wed, 18 Jul 2018 10:56:51 +0200
From:   "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "evgreen@...omium.org" <evgreen@...omium.org>,
        "vinholikatti@...il.com" <vinholikatti@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "asutoshd@...eaurora.org" <asutoshd@...eaurora.org>,
        "sayalil@...eaurora.org" <sayalil@...eaurora.org>,
        "riteshh@...eaurora.org" <riteshh@...eaurora.org>,
        "cang@...eaurora.org" <cang@...eaurora.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "subhashj@...eaurora.org" <subhashj@...eaurora.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "vivek.gautam@...eaurora.org" <vivek.gautam@...eaurora.org>,
        "rnayak@...eaurora.org" <rnayak@...eaurora.org>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>
Subject: Re: [PATCH V5 2/2] scsi: ufs: Add configfs support for ufs
 provisioning

On Tue, Jul 17, 2018 at 09:06:35PM +0000, Bart Van Assche wrote:
> On Tue, 2018-07-17 at 13:23 -0700, Evan Green wrote:
> > On Mon, Jul 16, 2018 at 5:04 PM Bart Van Assche <Bart.VanAssche@....com> wrote:
> > > 
> > > On Mon, 2018-07-16 at 16:46 -0700, Evan Green wrote:
> > > > I see Bart has chimed in on the next series with a suggestion to break
> > > > out each field into individual files within configfs. Bart, what are
> > > > your feelings about converting to a binary attribute? I remember when
> > > > I did my sysfs equivalent of this patch, somebody chimed in indicating
> > > > a "commit" file might be needed so that the new configuration could be
> > > > written in one fell swoop. One advantage of the binary attribute is
> > > > that it writes the configuration atomically.
> > > 
> > > Hello Evan,
> > > 
> > > I may be missing some UFS background information. But since a configfs interface
> > > is being added I think the same rule applies as to all Linux kernel user space
> > > interfaces, namely that it should be backwards compatible. Additionally, if
> > > anyone ever will want to use this interface from a shell script, I think that
> > > it will be much easier to write multiple ASCII attributes than a single binary
> > > attribute.
> > > 
> > 
> > Hi Bart,
> > I'm unsure about the compatibility aspect for binary attributes that
> > essentially represent direct windows into hardware. I suppose this
> > comes down to who this interface is most useful to. Hypothetically
> > lets say a future revision of UFS adds fields to the configuration
> > descriptor, but is otherwise backwards compatible. If this interface
> > is primarily for OEMs initializing their devices in the factory, then
> > I'd argue they'd want the most direct window to the configuration
> > descriptor. These folks probably just have a configuration they want
> > to plunk into the hardware, and would prefer being able to write all
> > fields over having some sort of compatibility restriction. If, on the
> > other hand, this is used by long-running scripts that stick around for
> > years without modification, then yes, it seems like it would be more
> > important to stay compatible, and have smarts in the kernel to make
> > writes of old descriptors work in new devices.
> > 
> > At least for myself, I fall into the category of someone who just
> > needs to plunk a configuration descriptor in once, and would prefer
> > not to have to submit kernel changes if the descriptor evolves
> > slightly. It also seemed a little odd that this patch now spends a
> > bunch of energy converting ASCII into bytes, just to write it without
> > modification into the hardware, and convert back again to ASCII for
> > reads.
> > 
> > We plan to use a script for provisioning, and could easily handle
> > ASCII or rawbytes:
> > 
> > # Some bytes, ready to go with the interface today...
> > some_bytes="00 01 02 03"
> > 
> > # Same bytes, now in binary format
> > bytes_fmt=$(echo " $some_bytes" | sed 's/ /\\x/g')
> > /usr/bin/printf "$bytes_fmt" > /configfs/ufs_provision
> > 
> > I'm not dead set on binary, since as above I could do it either way,
> > but it seemed worth at least talking through. Let me know what you
> > think.
> 
> The configfs documentation (Documentation/filesystems/configfs/configfs.txt)
> is clear about this: "Preferably only one value per file should be used." So
> I would like to hear the opinion of someone who has more authority than I
> with regard to configfs.

Don't we have "binary" files for configfs?  We have them for sysfs, they
are for files that are not touched by the kernel and just "pass-through"
to the hardware.  Would that work here as well?

thanks,

greg k-h