Re: [PATCH master] dma: correctly honour dma-noncoherent device tree property

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, Mar 22, 2024 at 03:24:24PM +0100, Ahmad Fatoum wrote:
> Most barebox platforms are either completely cache-coherent with respect to
> DMA or completely non-coherent. To avoid having to walk the device tree for
> non-existent dma-coherent and dma-noncohrent properties, barebox thus
> only does this when CONFIG_OF_DMA_COHERENCY is selected.
> 
> CONFIG_OF_DMA_COHERENCY is to be selected by platforms that have mixed
> coherency for DMA masters. Forgetting to select this option can be
> annoying to debug, which is why devinfo will inform the user of default
> DMA coherency assignments:
> 
>   DMA Coherent: false (default)
> 
> In order to allow devinfo to differentiate implicit default and explicit
> device tree DMA coherency configuration, the struct device::dma_coherent
> member is not of boolean type, but Instead an enumeration that besides
> coherent and non-coherent also encodes a DEV_DMA_COHERENCE_DEFAULT state.
> 
> In practice though, a boolean was saved in this dma_coherent member,
> leading to any dma-noncoherent property being ignored and the default
> coherency setting being taken for the node.
> 
> By fixing the type confusion, we restore working Ethernet on the
> StarFive JH7100, which is the only SoC we currently support that
> requires /soc/dma-noncohrent to be set for proper operation of its DMA
> masters.
> 
> Fixes: fbdea8fd54fe ("of: populate new device_d::dma_coherent attribute")
> Reported-by: Antony Pavlov <antonynpavlov@gmail.com>
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
>  drivers/of/platform.c | 34 +++++++++++++++++++++++-----------
>  1 file changed, 23 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> index ec1482b27797..e2c252b1ffee 100644
> --- a/drivers/of/platform.c
> +++ b/drivers/of/platform.c
> @@ -90,6 +90,21 @@ static struct device_node *of_get_next_dma_parent(const struct device_node *np)
>  	return args.np;
>  }
>  
> +static enum dev_dma_coherence of_dma_get_coherence(struct device_node *node)
> +{
> +	if (IS_ENABLED(CONFIG_OF_DMA_COHERENCY)) {
> +		while (node) {
> +			if (of_property_read_bool(node, "dma-coherent"))
> +				return DEV_DMA_COHERENT;
> +			if (of_property_read_bool(node, "dma-noncoherent"))
> +				return DEV_DMA_NON_COHERENT;
> +			node = of_get_next_dma_parent(node);
> +		}
> +	}
> +
> +	return DEV_DMA_COHERENCE_DEFAULT;
> +}
> +
>  /**
>   * of_dma_is_coherent - Check if device is coherent
>   * @np:	device node
> @@ -101,17 +116,14 @@ static struct device_node *of_get_next_dma_parent(const struct device_node *np)
>   */
>  bool of_dma_is_coherent(struct device_node *node)
>  {
> -	if (IS_ENABLED(CONFIG_OF_DMA_COHERENCY)) {
> -		while (node) {
> -			if (of_property_read_bool(node, "dma-coherent"))
> -				return true;
> -			if (of_property_read_bool(node, "dma-noncoherent"))
> -				return false;
> -			node = of_get_next_dma_parent(node);
> -		}
> +	switch (of_dma_get_coherence(node)) {
> +	case DEV_DMA_COHERENT:
> +		return true;
> +	case DEV_DMA_NON_COHERENT:
> +		return false;
> +	case DEV_DMA_COHERENCE_DEFAULT:
> +		return IS_ENABLED(CONFIG_ARCH_DMA_DEFAULT_COHERENT);
>  	}
> -
> -	return IS_ENABLED(CONFIG_ARCH_DMA_DEFAULT_COHERENT);
>  }

This brings us:

drivers/of/platform.c: In function 'of_dma_is_coherent':
drivers/of/platform.c:127:1: warning: control reaches end of non-void function [-Wreturn-type]

what shall we return in the default case? also
IS_ENABLED(CONFIG_ARCH_DMA_DEFAULT_COHERENT)?

Sascha

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