linux-kernel - Re: [PATCH v2] usb: usb251xb: Lock i2c-bus segment the hub resides

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Message-ID: <20190427073940.6yzhklfus4qd6ver@mobilestation>
Date:   Sat, 27 Apr 2019 10:39:41 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Richard Leitner <richard.leitner@...data.com>,
        Serge Semin <Sergey.Semin@...latforms.ru>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] usb: usb251xb: Lock i2c-bus segment the hub resides

On Sat, Apr 27, 2019 at 09:32:00AM +0200, Greg Kroah-Hartman wrote:
> On Sat, Apr 27, 2019 at 10:10:27AM +0300, Serge Semin wrote:
> > On Sat, Apr 27, 2019 at 09:00:42AM +0200, Greg Kroah-Hartman wrote:
> > > On Fri, Apr 26, 2019 at 01:46:50PM +0300, Serge Semin wrote:
> > > > SMBus slave configuration is activated by CFG_SEL[1:0]=0x1 pins
> > > > state. This is the mode the hub is supposed to be to let this driver
> > > > work correctly. But a race condition might happen right after reset
> > > > is cleared due to CFG_SEL[0] pin being multiplexed with SMBus SCL
> > > > function. In case if the reset pin is handled by a i2c GPIO expander,
> > > > which is also placed at the same i2c-bus segment as the usb251x
> > > > SMB-interface connected to, then the hub reset clearance might
> > > > cause the CFG_SEL[0] being latched in unpredictable state. So
> > > > sometimes the hub configuration mode might be 0x1 (as expected),
> > > > but sometimes being 0x0, which doesn't imply to have the hub SMBus-slave
> > > > interface activated and consequently causes this driver failure.
> > > > 
> > > > In order to fix the problem we must make sure the GPIO-reset chip doesn't
> > > > reside the same i2c-bus segment as the SMBus-interface of the hub. If
> > > > it doesn't, we can safely block the segment for the time the reset is
> > > > cleared to prevent anyone generating a traffic at the i2c-bus SCL lane
> > > > connected to the CFG_SEL[0] pin. But if it does, nothing we can do, so
> > > > just return an error. If we locked the i2c-bus segment and tried to
> > > > communicate with the GPIO-expander, it would cause a deadlock. If we didn't
> > > > lock the i2c-bus segment, it would randomly cause the CFG_SEL[0] bit flip.
> > > > 
> > > > Signed-off-by: Serge Semin <fancer.lancer@...il.com>
> > > > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > > 
> > > I signed off on this?  Where?  When?
> > > 
> > > never add a s-o-b line that you did not create, that implies a legal
> > > agreement.
> > > 
> > 
> > Ah, shit. Sorry. I should have added Acked-by. That's what I was going to do,
> > since you already added the first version of this into linux-next tree. But
> > apparently copy-pasted and left as is...
> 
> If I have already applied a patch, I can't apply it again (or a
> different version.)  You need to send a fix-up patch for the reported
> issue, not a whole new one as I can not go back in time and rewrite
> history.
> 
> thanks,
> 
> greg k-h

Ok. Should I send it in reply to this patch or as a completely separate one?

-Sergey