linux-kernel - Re: [PATCH] arm64/pci: fix dma coherency inheritance for PCI devices

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Date:	Fri, 19 Sep 2014 12:32:20 -0400
From:	Jon Masters <jcm@...hat.com>
To:	Will Deacon <will.deacon@....com>, Mark Salter <msalter@...hat.com>
CC:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64/pci: fix dma coherency inheritance for PCI devices

On 09/19/2014 12:28 PM, Will Deacon wrote:
> On Fri, Sep 19, 2014 at 04:51:26PM +0100, Mark Salter wrote:
>> The default dma_ops for devices on arm64 systems are noncoherent in
>> nature and rely upon special operations and bounce buffers to
>> perform a device DMA operation to/from memory. Some drivers rely
>> upon coherent operations involving suitably capable hardware. In
>> this case, a "dma-coherent" property will exist on the corresponding
>> Device Tree node for the bridge device, or one of its ancestors.
>> This patch adds support for applying a DMA coherent dma_ops for
>> PCI devices in the case of such a property.
>>
>> Signed-off-by: Jon Masters <jcm@...hat.com>
>> [added search for device with of_node]
>> Signed-off-by: Mark Salter <msalter@...hat.com>
>> ---
>>  arch/arm64/mm/dma-mapping.c | 21 +++++++++++++++++++--
>>  1 file changed, 19 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
>> index 4164c5a..0e26bd7 100644
>> --- a/arch/arm64/mm/dma-mapping.c
>> +++ b/arch/arm64/mm/dma-mapping.c
>> @@ -23,10 +23,12 @@
>>  #include <linux/dma-mapping.h>
>>  #include <linux/dma-contiguous.h>
>>  #include <linux/of.h>
>> +#include <linux/of_address.h>
>>  #include <linux/platform_device.h>
>>  #include <linux/vmalloc.h>
>>  #include <linux/swiotlb.h>
>>  #include <linux/amba/bus.h>
>> +#include <linux/pci.h>
>>  
>>  #include <asm/cacheflush.h>
>>  
>> @@ -316,8 +318,18 @@ static int dma_bus_notifier(struct notifier_block *nb,
>>  	if (event != BUS_NOTIFY_ADD_DEVICE)
>>  		return NOTIFY_DONE;
>>  
>> -	if (of_property_read_bool(dev->of_node, "dma-coherent"))
>> -		set_dma_ops(dev, &coherent_swiotlb_dma_ops);
>> +	/*
>> +	 * Some devices won't have an of_node but a bus controller/bridge will.
>> +	 * Search up the device chain until we find an of_node to check.
>> +	 */
>> +	while (dev) {
>> +		if (dev->of_node) {
>> +			if (of_dma_is_coherent(dev->of_node))
>> +				set_dma_ops(_dev, &coherent_swiotlb_dma_ops);
>> +			break;
>> +		}
>> +		dev = dev->parent;
>> +	}
> 
> Hmm, how does this interact with of_dma_configure? I don't think the
> dma-coherent property is inherited there, so I'm uneasy about doing this for
> buses other than PCI (i.e. the platform bus).
> 
> In other words, we should move to using of_dma_configure for the platform
> and amba buses, and have a separate notifier for the PCI bus. Is it likely
> that people will have a mixture of coherent and non-coherent devices behind
> the same host controller?

Right. My original internal patch had a separate method for the PCI bus
rather than rolling these back into one...but I suggested to Mark that I
thought Catalin and you wanted this rolled back into one. If you'd
rather there be two, well that's really easy as we know that works.

Jon.

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