RE: [PATCH v5 2/3] dmaengine: imx-sdma: fix dma freezes

[Robin Gong <yibin.gong@nxp.com>]

On 2019-9-23 21:58 Philipp Puschmann <philipp.puschmann@emlix.com> wrote:
> For some years and since many kernel versions there are reports that the RX
> UART SDMA channel stops working at some point. The workaround was to
> disable DMA for RX. This commit fixes the problem itself. Cyclic DMA transfers
> are used by uart and other drivers and these can fail in at least two cases
> where we can run out of descriptors available to the
> engine:
> - Interrupts are disabled for too long and all buffers are filled with
>   data, especially in a setup where many small dma transfers are being
>   executed only using a tiny part of a single buffer
> - DMA errors (such as generated by baud rate mismatch with imx-uart)
>   use up all descriptors before we can react.
> 
> In this case, SDMA stops the channel and no further transfers are done until
> the respective channel is disabled and re-enabled. We can check if the
> channel has been stopped and re-enable it then. To distinguish from the the
> case that the channel was stopped by upper-level driver we introduce new
> flag IMX_DMA_ACTIVE.
> 
> As sdmac->desc is constant we can move desc out of the loop.
> 
> Fixes: 1ec1e82f2510 ("dmaengine: Add Freescale i.MX SDMA support")
> Signed-off-by: Philipp Puschmann <philipp.puschmann@emlix.com>
> Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
> Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
> ---
> 
> Changelog v5:
>  - join with patch version from Jan Luebbe
>  - adapt comments and patch descriptions
> 
> Changelog v4:
>  - fixed the fixes tag
> 
> Changelog v3:
>  - use correct dma_wmb() instead of dma_wb()
>  - add fixes tag
> 
> Changelog v2:
>  - clarify comment and commit description
> 
>  drivers/dma/imx-sdma.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/dma/imx-sdma.c b/drivers/dma/imx-sdma.c index
> b42281604e54..0b1d6a62423d 100644
> --- a/drivers/dma/imx-sdma.c
> +++ b/drivers/dma/imx-sdma.c
> @@ -383,6 +383,7 @@ struct sdma_channel {  };
> 
>  #define IMX_DMA_SG_LOOP		BIT(0)
> +#define IMX_DMA_ACTIVE		BIT(1)
> 
>  #define MAX_DMA_CHANNELS 32
>  #define MXC_SDMA_DEFAULT_PRIORITY 1
> @@ -658,6 +659,9 @@ static int sdma_config_ownership(struct
> sdma_channel *sdmac,
> 
>  static void sdma_enable_channel(struct sdma_engine *sdma, int channel)  {
> +	struct sdma_channel *sdmac = &sdma->channel[channel];
> +
> +	sdmac->flags |= IMX_DMA_ACTIVE;
Add spin_lock_irq protect this flags.
>  	writel(BIT(channel), sdma->regs + SDMA_H_START);  }
> 
> @@ -774,16 +778,17 @@ static void sdma_start_desc(struct sdma_channel
> *sdmac)
> 
>  static void sdma_update_channel_loop(struct sdma_channel *sdmac)  {
> +	struct sdma_engine *sdma = sdmac->sdma;
>  	struct sdma_buffer_descriptor *bd;
> +	struct sdma_desc *desc = sdmac->desc;
>  	int error = 0;
> -	enum dma_status	old_status = sdmac->status;
> +	enum dma_status old_status = sdmac->status;
> 
>  	/*
>  	 * loop mode. Iterate over descriptors, re-setup them and
>  	 * call callback function.
>  	 */
> -	while (sdmac->desc) {
> -		struct sdma_desc *desc = sdmac->desc;
> +	while (desc) {
> 
>  		bd = &desc->bd[desc->buf_tail];
> 
> @@ -822,6 +827,18 @@ static void sdma_update_channel_loop(struct
> sdma_channel *sdmac)
>  		if (error)
>  			sdmac->status = old_status;
>  	}
> +
> +	/* In some situations it may happen that the sdma does not find any
> +	 * usable descriptor in the ring to put data into. The channel is
> +	 * stopped then and after having freed some buffers we have to restart
> +	 * it manually.
> +	 */
> +	if ((sdmac->flags & IMX_DMA_ACTIVE) &&
> +	    !(readl_relaxed(sdma->regs + SDMA_H_STATSTOP) &
> BIT(sdmac->channel))) {
Seems duplicate checking here, IMX_DMA_ACTIVE is enough.
> +		dev_err_ratelimited(sdma->dev, "SDMA channel %d: cyclic transfer
> disabled by HW, reenabling\n",
Would you change the print log to below:
"cyclic bds consumed all,reenableing".?
> +				    sdmac->channel);
> +			writel(BIT(sdmac->channel), sdma->regs + SDMA_H_START);
> +	};
>  }
> 
>  static void mxc_sdma_handle_channel_normal(struct sdma_channel *data)
> @@ -1051,7 +1068,8 @@ static int sdma_disable_channel(struct dma_chan
> *chan)
>  	struct sdma_engine *sdma = sdmac->sdma;
>  	int channel = sdmac->channel;
> 
> -	writel_relaxed(BIT(channel), sdma->regs + SDMA_H_STATSTOP);
> +	sdmac->flags &= ~IMX_DMA_ACTIVE;
> +	writel(BIT(channel), sdma->regs + SDMA_H_STATSTOP);
>  	sdmac->status = DMA_ERROR;
> 
>  	return 0;
> --
> 2.23.0