dmaengine: at_hdmac: Don't allow CPU to reorder channel enable

at_hdmac uses __raw_writel for register writes. In the absence of a
barrier, the CPU may reorder the register operations.
Introduce a write memory barrier so that the CPU does not reorder the
channel enable, thus the start of the transfer, without making sure that
all the pre-required register fields are already written.

Fixes: dc78baa2 ("dmaengine: at_hdmac: new driver for the Atmel AHB DMA Controller")
Reported-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/lkml/13c6c9a2-6db5-c3bf-349b-4c127ad3496a@axentia.se/Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20221025090306.297886-1-tudor.ambarus@microchip.com
Link: https://lore.kernel.org/r/20221025090306.297886-14-tudor.ambarus@microchip.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/at_hdmac.c

@@ -256,6 +256,8 @@ static void atc_dostart(struct at_dma_chan *atchan, struct at_desc *first)
 		       ATC_SPIP_BOUNDARY(first->boundary));
 	channel_writel(atchan, DPIP, ATC_DPIP_HOLE(first->dst_hole) |
 		       ATC_DPIP_BOUNDARY(first->boundary));
+	/* Don't allow CPU to reorder channel enable. */
+	wmb();
 	dma_writel(atdma, CHER, atchan->mask);
 
 	vdbg_dump_regs(atchan);