Re: [PATCH] dmaengine: loongson: loongson2-apb: fix broken bus width validation in ls2x_dmac_detect_burst()

[Frank Li <Frank.li@nxp.com>]

On Wed, Mar 18, 2026 at 04:48:03PM +0000, David Carlier wrote:
> The bus width validation check in ls2x_dmac_detect_burst() compares raw
> enum dma_slave_buswidth values (e.g. 4, 8) directly against
> LDMA_SLAVE_BUSWIDTHS, which is a BIT()-encoded bitmask
> (BIT(4) | BIT(8) = 0x110). Since 4 & 0x110 == 0 and 8 & 0x110 == 0,
> the condition is always false for valid bus widths, making the
> validation dead code.
>
> Additionally, the logic was inverted: it rejected configurations where
> both widths matched valid values, rather than rejecting when neither
> width is supported.
>
> Fix by wrapping the enum values with BIT() before masking (matching the
> pattern used in sun6i-dma.c) and inverting the logic to reject when
> neither width is supported by the hardware.
>
> Fixes: 71e7d3cb6e55 ("dmaengine: ls2x-apb: New driver for the Loongson LS2X APB DMA controller")
> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
>  drivers/dma/loongson/loongson2-apb-dma.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma/loongson/loongson2-apb-dma.c b/drivers/dma/loongson/loongson2-apb-dma.c
> index aceb069e71fc..102c01f993ef 100644
> --- a/drivers/dma/loongson/loongson2-apb-dma.c
> +++ b/drivers/dma/loongson/loongson2-apb-dma.c
> @@ -220,8 +220,8 @@ static size_t ls2x_dmac_detect_burst(struct ls2x_dma_chan *lchan)
>  	u32 maxburst, buswidth;
>
>  	/* Reject definitely invalid configurations */
> -	if ((lchan->sconfig.src_addr_width & LDMA_SLAVE_BUSWIDTHS) &&
> -	    (lchan->sconfig.dst_addr_width & LDMA_SLAVE_BUSWIDTHS))
> +	if (!(BIT(lchan->sconfig.src_addr_width) & LDMA_SLAVE_BUSWIDTHS) &&
> +	    !(BIT(lchan->sconfig.dst_addr_width) & LDMA_SLAVE_BUSWIDTHS))

src_addr_width is enum dma_slave_buswidth, which allow
DMA_SLAVE_BUSWIDTH_128_BYTES = 128,

BIT(128) will overflow.

Frank

>  		return 0;
>
>  	if (lchan->sconfig.direction == DMA_MEM_TO_DEV) {
> --
> 2.53.0
>