dmaengine: imx-sdma: fix context cache

There is a DMA problem with the serial ports on i.MX6.

When the following sequence is performed:

1) Open a port
2) Write some data
3) Close the port
4) Open a *different* port
5) Write some data
6) Close the port

The second write sends nothing and the second close hangs.
If the first close() is omitted it works.

Adding logs to the the UART driver shows that the DMA is being setup but
the callback is never invoked for the second write.

This used to work in 4.19.

Git bisect leads to:
	ad0d92d7: "dmaengine: imx-sdma: refine to load context only once"

This commit adds a "context_loaded" flag used to avoid unnecessary context
setups.
However the flag is only reset in sdma_channel_terminate_work(),
which is only invoked in a worker triggered by sdma_terminate_all() IF
there is an active descriptor.

So, if no active descriptor remains when the channel is terminated, the
flag is not reset and, when the channel is later reused the old context
is used.

Fix the problem by always resetting the flag in sdma_free_chan_resources().

Cc: stable@vger.kernel.org
Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Fixes: ad0d92d7 ("dmaengine: imx-sdma: refine to load context only once")
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/1580305274-27274-1-git-send-email-martin.fuzzey@flowbird.groupSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/imx-sdma.c

@@ -1338,6 +1338,7 @@ static void sdma_free_chan_resources(struct dma_chan *chan)
 
 	sdmac->event_id0 = 0;
 	sdmac->event_id1 = 0;
+	sdmac->context_loaded = false;
 
 	sdma_set_channel_priority(sdmac, 0);