[PATCH] dw_dmac: don't wait for FIFO_EMPTY endlessly in dwc_chan_pause

From: Andy Shevchenko
Date: Thu Mar 21 2013 - 05:49:40 EST

When we pause the channel after transfer is completed we might stuck in the
dwc_chan_pause() because the FIFO_EMPTY flag will never be asserted. To avoid
the endless loop we introduce a timeout here (*). The proper solution is to
somehow get the residue in FIFO and avoid busyloop when transfer is done, but
this task is not simple and fast.

Unfortunately we can't use cpu_relax() in conjunction with jiffies checker, due
to we have interrupts disabled by spin_lock_irqsave() and there is a big chance
that no interrupts will come to update the jiffies..

(*) The worst case is
AHB write * FIFO size / hclk = 5.12 us,
AHB write = 2 cycles,
hclk = 100 MHz,
burst size = 1 byte,
FIFO size = 256 bytes.
The proposed 40us timeout might be considered as a big one, though we enter
to that state only when we have the transfer already completed.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
drivers/dma/dw_dmac.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/dw_dmac.c b/drivers/dma/dw_dmac.c
index 43a5329..43e2e89 100644
--- a/drivers/dma/dw_dmac.c
+++ b/drivers/dma/dw_dmac.c
@@ -1030,10 +1030,11 @@ set_runtime_config(struct dma_chan *chan, struct dma_slave_config *sconfig)
static inline void dwc_chan_pause(struct dw_dma_chan *dwc)
u32 cfglo = channel_readl(dwc, CFG_LO);
+ unsigned int count = 20; /* timeout iterations */

channel_writel(dwc, CFG_LO, cfglo | DWC_CFGL_CH_SUSP);
- while (!(channel_readl(dwc, CFG_LO) & DWC_CFGL_FIFO_EMPTY))
- cpu_relax();
+ while (!(channel_readl(dwc, CFG_LO) & DWC_CFGL_FIFO_EMPTY) && count--)
+ udelay(2);

dwc->paused = true;

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