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

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Date:   Sat, 27 Apr 2019 09:00:42 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Serge Semin <fancer.lancer@...il.com>
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 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.

greg k-h