Subject: [RFC PATCH 1/2] common: dma-mapping: introduce dma_get_parent_cfg() helper

From: [email-masked] (Murali Karicheri)
Date: Wed, 17 Dec 2014 18:24:43 -0500

On 12/17/2014 04:56 PM, Arnd Bergmann wrote:
> On Wednesday 17 December 2014 13:02:23 Murali Karicheri wrote:
>> Now, in Kernel, parent device's DMA parameters has to be applied to
>> the child as is - to enable DMA support for the device. Usually this
>> is happened in places where parent device manually instantiates child
>> device such as in drivers/pci/probe.c (pci_device_add() for example).
>>
>> Now DMA configuration is represented in device data structure not only
>> by DMA mask and DMA params, it also includes dma_pfn_offset at least.
>> Hence introduce common dma_get_parent_cfg() helper to apply dma
>> configuration from parent to child, and use __weak to allow arch to
>> override it if needed.
>>
>> Signed-off-by: Murali Karicheri<[email-masked]>
>> ---
>>   drivers/base/dma-mapping.c  |   18 ++++++++++++++++++
>>   include/linux/dma-mapping.h |    3 +++
>>   2 files changed, 21 insertions(+)
>>
>> diff --git a/drivers/base/dma-mapping.c b/drivers/base/dma-mapping.c
>> index 9e8bbdd..5322426 100644
>> --- a/drivers/base/dma-mapping.c
>> +++ b/drivers/base/dma-mapping.c
>> @@ -339,3 +339,21 @@ void dma_common_free_remap(void *cpu_addr, size_t size, unsigned long vm_flags)
>>   	vunmap(cpu_addr);
>>   }
>>   #endif
>> +
>> +int __weak dma_get_parent_cfg(struct device *dev, struct device *parent)
>> +{
>> +	struct device *temp = parent;
>> +
>> +	if (!temp)
>> +		temp = dev->parent;
>> +
>> +	if (temp&&  is_device_dma_capable(temp)) {
>> +		dev->dma_mask	= temp->dma_mask;
>> +		dev->coherent_dma_mask = temp->coherent_dma_mask;
>
> As discussed, setting the pointers like this is always wrong,
> so don't do it.
>
> What's wrong with using arch_setup_dma_ops() from PCI as suggested
> previously?
>
> 	Arnd

Arnd,

Thanks for the review.

I had originally written a code based on that line as below. But 
dma-ranges property is also used by ppc and other architectures in the 
pci device DT node. So I wasn't sure how this code impact PCI driver 
functionality on those platforms. Hence used a simpler change as all 
that is needed for keystone is to get the dma_pfn_offset rightly set in 
the pci slave device.

Initially I had a function implemented as below for this in of_pci.c.

+ * of_pci_dma_configure - Setup DMA configuration for a pci device
+ * @dev:       pci device to apply DMA configuration
+ *
+ * Try to get devices's DMA configuration from DT and update it
+ * accordingly. This is a similar to of_dma_configure() for platform
+ * devices.
+ *
+ */
+void of_pci_dma_configure(struct pci_dev *dev)
+{
+       struct device *host_bridge, *parent;
+       struct pci_bus *bus = dev->bus;
+       u64 dma_addr, paddr, size;
+       int ret;
+
+       while (!pci_is_root_bus(bus))
+               bus = bus->parent;
+       host_bridge = bus->bridge;
+
+       parent = host_bridge->parent;
+       if (parent->of_node) {
+               /*
+                * if dma-coherent property exist, call arch hook to setup
+                * dma coherent operations.
+                */
+               if (of_dma_is_coherent(parent->of_node)) {
+                       set_arch_dma_coherent_ops(&dev->dev);
+                       dev_info(&dev->dev, "device is dma coherent\n");
+               }
+
+               /*
+                * if dma-ranges property doesn't exist - just return else
+                * setup the dma offset
+                */
+               ret = of_dma_get_range(parent->of_node, &dma_addr, 
&paddr, &size);
+               if (ret < 0) {
+                       dev_info(&dev->dev, "no dma range information to 
setup\n");
+                       printk("no dma range information to setup\n");
+                       return;
+               }
+
+               /* DMA ranges found. Calculate and set dma_pfn_offset */
+               dev->dev.dma_pfn_offset = PFN_DOWN(paddr - dma_addr);
+               dev_info(&dev->dev, "dma_pfn_offset(%#08lx)\n", 
dev->dev.dma_pfn_offset);
+       }
+}
+EXPORT_SYMBOL_GPL(of_pci_dma_configure);

and then called from pci/probe.c as

@@ -1527,6 +1528,9 @@ void pci_device_add(struct pci_dev *dev, struct 
pci_bus *bus)
         dev->dev.dma_parms = &dev->dma_parms;
         dev->dev.coherent_dma_mask = 0xffffffffull;

+       /* Get the DMA configuration from root bridge's parent if present */
+       of_pci_dma_configure(dev);

If you think this is a better way to implement this, I can work on a 
patch set using this approach. Let me know.

-- 
Murali Karicheri
Linux Kernel, Texas Instruments