linux-kernel - Re: [PATCH] dw_dmac: don't wait for FIFO_EMPTY endlessly in dwc_chan

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Date:	Thu, 21 Mar 2013 15:51:28 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Vinod Koul <vinod.koul@...el.com>
Subject: Re: [PATCH] dw_dmac: don't wait for FIFO_EMPTY endlessly in dwc_chan_pause

On 21 March 2013 15:19, Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
> 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,
>     where
>         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@...ux.intel.com>
> ---
>  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);

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
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