dmaengine: avoid map_cnt overflow with CONFIG_DMA_ENGINE_RAID

[Vinod Koul <vinod.koul@intel.com>]

On Mon, Jan 08, 2018 at 10:50:50AM -0500, Zi Yan wrote:
> From: Zi Yan <zi.yan@cs.rutgers.edu>
> 
> When CONFIG_DMA_ENGINE_RAID is enabled, unmap pool size can reach to
> 256. But in struct dmaengine_unmap_data, map_cnt is only u8, wrapping
> to 0, if the unmap pool is maximally used. This triggers BUG() when
> struct dmaengine_unmap_data is freed. Use u16 to fix the problem.
> 
> Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
> ---
>  include/linux/dmaengine.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> index f838764993eb..861be5cab1df 100644
> --- a/include/linux/dmaengine.h
> +++ b/include/linux/dmaengine.h
> @@ -470,7 +470,11 @@ typedef void (*dma_async_tx_callback_result)(void *dma_async_param,
>  				const struct dmaengine_result *result);
>  
>  struct dmaengine_unmap_data {
> +#if IS_ENABLED(CONFIG_DMA_ENGINE_RAID)
> +	u16 map_cnt;
> +#else
>  	u8 map_cnt;
> +#endif
>  	u8 to_cnt;
>  	u8 from_cnt;
>  	u8 bidi_cnt;

Would that cause adverse performance, the data structure is not aligned
anymore. Dan was that a consideration while adding this?