linux-kernel - Re: [v11,1/4] drivers: jtag: Add JTAG core driver

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Date:   Tue, 14 Nov 2017 12:10:46 +0100
From:   "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
To:     Oleksandr Shamray <oleksandrs@...lanox.com>
Cc:     'Chip Bilbrey' <chip@...brey.org>, "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "openbmc@...ts.ozlabs.org" <openbmc@...ts.ozlabs.org>,
        "joel@....id.au" <joel@....id.au>,
        "jiri@...nulli.us" <jiri@...nulli.us>,
        "tklauser@...tanz.ch" <tklauser@...tanz.ch>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "mec@...ut.net" <mec@...ut.net>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "openocd-devel-owner@...ts.sourceforge.net" 
        <openocd-devel-owner@...ts.sourceforge.net>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "mchehab@...nel.org" <mchehab@...nel.org>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [v11,1/4] drivers: jtag: Add JTAG core driver

On Tue, Nov 14, 2017 at 10:34:49AM +0000, Oleksandr Shamray wrote:
> > -----Original Message-----
> > From: Chip Bilbrey [mailto:chip@...brey.org]
> > Sent: Monday, November 6, 2017 12:33 AM
> > To: Oleksandr Shamray <oleksandrs@...lanox.com>
> > Cc: gregkh@...uxfoundation.org; arnd@...db.de; linux-
> > kernel@...r.kernel.org; linux-arm-kernel@...ts.infradead.org;
> > devicetree@...r.kernel.org; openbmc@...ts.ozlabs.org; joel@....id.au;
> > jiri@...nulli.us; tklauser@...tanz.ch; linux-serial@...r.kernel.org;
> > mec@...ut.net; Vadim Pasternak <vadimp@...lanox.com>; system-sw-low-
> > level <system-sw-low-level@...lanox.com>; robh+dt@...nel.org; openocd-
> > devel-owner@...ts.sourceforge.net; linux-api@...r.kernel.org;
> > davem@...emloft.net; mchehab@...nel.org; Jiri Pirko <jiri@...lanox.com>
> > Subject: Re: [v11,1/4] drivers: jtag: Add JTAG core driver
> > 
> > 
> > Oleksandr Shamray writes:
> 
> [..]
> 
> > I notice the single-open()-per-device lock was dropped by request in an earlier
> > revision of your patches, but multiple processes trying to drive a single JTAG
> > master could wreak serious havoc if transactions get interleaved. Would
> > something like an added JTAG_LOCKCHAIN/UNLOCKCHAIN
> > ioctl() for exclusive client access be reasonable to prevent this?
> > 
> 
> Yes, it dropped by recommendation of Greg KH <gregkh@...uxfoundation.org>. 

I asked to drop it as you didn't even implement it to work correctly :)

> Greg, what you can suggest about it. May be better to add again single-open()-per-device lock with right locking way like:
> 
> >if (mutex_lock_interruptible(&jtag->open_lock)) {

You would stall an open?  Why not just return saying you can't do that?

Anyway, if you want to only have one access to the device at a time,
great, but do it in a way that works properly.

thanks,

greg k-h