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

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Date:   Tue, 17 Jul 2018 21:06:35 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "evgreen@...omium.org" <evgreen@...omium.org>
CC:     "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>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.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, 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.

Bart.