From 6c1c0ae59d7bc2e40c466c977ab1d55a5108149c Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Mon, 15 Sep 2014 16:40:06 +0200
Subject: [PATCH] dmaengine: edma: check for echan->edesc => NULL in
 edma_dma_pause()

I added book keeping of whether or not the 8250-dma driver has an RX
transfer pending or not so we don't BUG here if it calls
dmaengine_pause() on a channel which has not a pending transfer. Guess
what, this is not enough.
The following can be triggered with a busy RX channel and hackbench in
background:
- DMA transfer completes. The callback is delayed via
  vchan_cookie_complete() into a tasklet so it das not happen asap.
- hackbench keeps the system busy so the tasklet does not run "soon".
- the UART collected enough data and generates an "timeout"-interrupt.
  Since 8250-dma *thinks* the DMA-transfer is still pending it tries to
  cancel it via invoking dmaengine_pause() first. This causes the segfault
  because echan->edesc is NULL now that the transfer completed (however
  the callback did not run yet).

With this patch we don't BUG in the scenario described.

Cc: vinod.koul@intel.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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