Re: [PATCH] ARM: edma: fix residue race for cyclic

[Peter Ujfalusi]

On 10/15/2015 01:46 PM, John Ogness wrote:
> When retrieving the residue value for cyclic transfers, the DST
> field of the active PaRAM is read. However, the AM335x Technical
> Reference Manual states:
> 
>   11.3.3.6 Parameter Set Updates
> 
>   After the TR is read from the PaRAM (and is in the process of
>   being submitted to the EDMA3TC), the following fields are
>   updated as needed: ... DST
> 
> This means the DST is incremented even though the DMA transfer
> may not have started yet or is in progress. Thus if the reader
> of the residue accesses the DMA buffer too quickly, the CPU
> will read where data is not yet written.
> 
> The CCSTAT.ACTV register is a boolean that is set if any TR is
> being processed by either the EMDA3CC or EDMA3TC. By polling
> this register it is possible to ensure that the residue value
> returned is valid for immediate processing.
> 
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
> ---
>  arch/arm/common/edma.c             |   17 +++++++++++++++++

The arch/arm/common/edma.c does not exist anymore - see linux-next.

It is a good practice to include the maintainers for patches, like Sekhar and
Vinod in case of eDMA/dmaengine...

>  drivers/dma/edma.c                 |    8 ++++++++
>  include/linux/platform_data/edma.h |    1 +
>  3 files changed, 26 insertions(+)
> 
> diff --git a/arch/arm/common/edma.c b/arch/arm/common/edma.c
> index 873dbfc..86ce980 100644
> --- a/arch/arm/common/edma.c
> +++ b/arch/arm/common/edma.c
> @@ -995,6 +995,23 @@ void edma_set_dest(unsigned slot, dma_addr_t dest_port,
>  }
>  EXPORT_SYMBOL(edma_set_dest);
>  
> +#define EDMA_CCSTAT_ACTV (1 << 4)
> +
> +/**
> + * edma_is_actv - report if any transfer requests are active
> + * @slot: parameter RAM slot being examined
> + *
> + * Returns true if any transfer requests are active on the slot
> + */
> +bool edma_is_actv(unsigned slot)
> +{
> +	u32 ctlr = EDMA_CTLR(slot);
> +	unsigned int ccstat;
> +
> +	ccstat = edma_read(ctlr, EDMA_CCSTAT);
> +	return (ccstat & EDMA_CCSTAT_ACTV);
> +}
> +
>  /**
>   * edma_get_position - returns the current transfer point
>   * @slot: parameter RAM slot being examined
> diff --git a/drivers/dma/edma.c b/drivers/dma/edma.c
> index 3e5d4f1..493c774 100644
> --- a/drivers/dma/edma.c
> +++ b/drivers/dma/edma.c
> @@ -891,6 +891,14 @@ static u32 edma_residue(struct edma_desc *edesc)
>  	pos = edma_get_position(edesc->echan->slot[0], dst);
>  
>  	/*
> +	 * "pos" may represent a transfer request that is still being
> +	 * processed by the EDMACC or EDMATC. Wait until all transfer
> +	 * requests on the active slot are finished before proceeding.
> +	 */
> +	while (edma_is_actv(edesc->echan->slot[0]))
> +		cpu_relax();

What happens if ACTV does not switch to 0? I know the chances are low, but are
we going to spin forever here?

> +
> +	/*
>  	 * Cyclic is simple. Just subtract pset[0].addr from pos.
>  	 *
>  	 * We never update edesc->residue in the cyclic case, so we
> diff --git a/include/linux/platform_data/edma.h b/include/linux/platform_data/edma.h
> index bdb2710..20c50e2 100644
> --- a/include/linux/platform_data/edma.h
> +++ b/include/linux/platform_data/edma.h
> @@ -130,6 +130,7 @@ void edma_set_src(unsigned slot, dma_addr_t src_port,
>  				enum address_mode mode, enum fifo_width);
>  void edma_set_dest(unsigned slot, dma_addr_t dest_port,
>  				 enum address_mode mode, enum fifo_width);
> +bool edma_is_actv(unsigned slot);
>  dma_addr_t edma_get_position(unsigned slot, bool dst);
>  void edma_set_src_index(unsigned slot, s16 src_bidx, s16 src_cidx);
>  void edma_set_dest_index(unsigned slot, s16 dest_bidx, s16 dest_cidx);
> 


-- 
Péter
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