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Subject: Re: [PATCH] of: Add OF_DMA_DEFAULT_COHERENT & select it on powerpc
To: Michael Ellerman <mpe@ellerman.id.au>, robh+dt@kernel.org
References: <20200126115247.13402-1-mpe@ellerman.id.au>
From: Frank Rowand <frowand.list@gmail.com>
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Date: Mon, 27 Jan 2020 11:30:29 -0600
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Cc: devicetree@vger.kernel.org, ulf.hansson@linaro.org,
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+ Frank (me)

On 1/26/20 5:52 AM, Michael Ellerman wrote:
> There's an OF helper called of_dma_is_coherent(), which checks if a
> device has a "dma-coherent" property to see if the device is coherent
> for DMA.
> 
> But on some platforms devices are coherent by default, and on some
> platforms it's not possible to update existing device trees to add the
> "dma-coherent" property.
> 
> So add a Kconfig symbol to allow arch code to tell
> of_dma_is_coherent() that devices are coherent by default, regardless
> of the presence of the property.
> 
> Select that symbol on powerpc when NOT_COHERENT_CACHE is not set, ie.
> when the system has a coherent cache.
> 
> Fixes: 92ea637edea3 ("of: introduce of_dma_is_coherent() helper")
> Cc: stable@vger.kernel.org # v3.16+
> Reported-by: Christian Zigotzky <chzigotzky@xenosoft.de>
> Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/Kconfig | 1 +
>  drivers/of/Kconfig   | 4 ++++
>  drivers/of/address.c | 6 +++++-
>  3 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 1ec34e16ed65..19f5aa8ac9a3 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -238,6 +238,7 @@ config PPC
>  	select NEED_DMA_MAP_STATE		if PPC64 || NOT_COHERENT_CACHE
>  	select NEED_SG_DMA_LENGTH
>  	select OF
> +	select OF_DMA_DEFAULT_COHERENT		if !NOT_COHERENT_CACHE
>  	select OF_EARLY_FLATTREE
>  	select OLD_SIGACTION			if PPC32
>  	select OLD_SIGSUSPEND
> diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> index 37c2ccbefecd..d91618641be6 100644
> --- a/drivers/of/Kconfig
> +++ b/drivers/of/Kconfig
> @@ -103,4 +103,8 @@ config OF_OVERLAY
>  config OF_NUMA
>  	bool
>  
> +config OF_DMA_DEFAULT_COHERENT
> +	# arches should select this if DMA is coherent by default for OF devices
> +	bool
> +
>  endif # OF
> diff --git a/drivers/of/address.c b/drivers/of/address.c
> index 99c1b8058559..e8a39c3ec4d4 100644
> --- a/drivers/of/address.c
> +++ b/drivers/of/address.c
> @@ -995,12 +995,16 @@ int of_dma_get_range(struct device_node *np, u64 *dma_addr, u64 *paddr, u64 *siz
>   * @np:	device node
>   *
>   * It returns true if "dma-coherent" property was found
> - * for this device in DT.
> + * for this device in the DT, or if DMA is coherent by
> + * default for OF devices on the current platform.
>   */
>  bool of_dma_is_coherent(struct device_node *np)
>  {
>  	struct device_node *node = of_node_get(np);
>  
> +	if (IS_ENABLED(CONFIG_OF_DMA_DEFAULT_COHERENT))
> +		return true;
> +
>  	while (node) {
>  		if (of_property_read_bool(node, "dma-coherent")) {
>  			of_node_put(node);
>