linux-kernel - Re: [PATCH] synclink_gt fix transmit race and timeout

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Message-Id: <20090623092739.5a8e9dd5.akpm@linux-foundation.org>
Date:	Tue, 23 Jun 2009 09:27:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Fulghum <paulkf@...rogate.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] synclink_gt fix transmit race and timeout

On Tue, 23 Jun 2009 10:16:01 -0500 Paul Fulghum <paulkf@...rogate.com> wrote:

> On Mon, 2009-06-22 at 23:19 -0700, Andrew Morton wrote:
> > On Tue, 16 Jun 2009 14:44:11 -0500 Paul Fulghum <paulkf@...rogate.com> wrote:
> > 
> > > Fix race condition when adding transmit data to active DMA buffer ring
> > > that can cause transmit stall.
> 
> > It's not a terribly good changelog, sorry.
> > It fails to describe the race condition?
> 
> I attempted to say what was done in the change log and
> let the patch describe the details of how it was done.
> But I can see that might not be enough.
> 
> How's this? :
> 
> If after adding transmit data to the transmit DMA ring the
> transmit DMA controller has become inactive before reading
> the new data and the serial transmitter is still active sending
> data already in the transmit FIFO and/or shift register,
> then wait for the serial transmitter to become idle before
> reactivating the transmit DMA controller.
> 
> Otherwise the transmit DMA controller may present the
> new data to the serial transmitter in a small timing window
> just as the serial transmitter transitions to the idle state.
> Hitting this window results in the serial transmitter entering
> an inconsistent state where it does not process the new data
> and does not signal the transition to the idle state.
> 
> >From the driver perspective, this condition appears as an
> active transmit DMA controller (correct) with unsent DMA buffers 
> (correct) and an active serial transmitter (wrong), even though
> no data is actually being sent. When all the DMA buffers are full,
> no further data is accepted from a user application calling
> write() until a timeout occurs and the transmitter and
> DMA controller are reset.
> 

That covers it ;) But I stil lcan't work out whether we shoul backport
this into -stable.

> 
> If this is good enough, I will resubmit the patch with
> the updated change log.

I updated the patch.

I did have a couple of other comments on the patch which seem to have
been missed?

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