Re: [PATCH v3 4/7] of: configure the platform device dma parameters

[Santosh Shilimkar]

Hi Grant,

On Tuesday 29 April 2014 10:41 AM, Grant Likely wrote:
> On Thu, 24 Apr 2014 11:30:04 -0400, Santosh Shilimkar <santosh.shilimkar@xxxxxx> wrote:
>> Retrieve DMA configuration from DT and setup platform device's DMA
>> parameters. The DMA configuration in DT has to be specified using
>> "dma-ranges" and "dma-coherent" properties if supported.
>>
>> We setup dma_pfn_offset using "dma-ranges" and dma_coherent_ops
>> using "dma-coherent" device tree properties.
>>
>> The set_arch_dma_coherent_ops macro has to be defined by arch if
>> it supports coherent dma_ops. Otherwise, set_arch_dma_coherent_ops() is
>> declared as nop.
>>
>> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
>> Cc: Arnd Bergmann <arnd@xxxxxxxx>
>> Cc: Olof Johansson <olof@xxxxxxxxx>
>> Cc: Grant Likely <grant.likely@xxxxxxxxxx>
>> Cc: Rob Herring <robh+dt@xxxxxxxxxx>
>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
>> Signed-off-by: Grygorii Strashko <grygorii.strashko@xxxxxx>
>> Signed-off-by: Santosh Shilimkar <santosh.shilimkar@xxxxxx>
>> ---
>>  drivers/of/platform.c       |   48 ++++++++++++++++++++++++++++++++++++++++---
>>  include/linux/dma-mapping.h |    7 +++++++
>>  2 files changed, 52 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
>> index 48de98f..270c0b9 100644
>> --- a/drivers/of/platform.c
>> +++ b/drivers/of/platform.c
>> @@ -187,6 +187,50 @@ struct platform_device *of_device_alloc(struct device_node *np,
>>  EXPORT_SYMBOL(of_device_alloc);
>>  
>>  /**
>> + * of_dma_configure - Setup DMA configuration
>> + * @dev:	Device to apply DMA configuration
>> + *
>> + * Try to get devices's DMA configuration from DT and update it
>> + * accordingly.
>> + *
>> + * In case if platform code need to use own special DMA configuration,it
>> + * can use Platform bus notifier and handle BUS_NOTIFY_ADD_DEVICE event
>> + * to fix up DMA configuration.
>> + */
>> +static void of_dma_configure(struct device *dev)
>> +{
>> +	u64 dma_addr, paddr, size;
>> +	int ret;
>> +
>> +	dev->coherent_dma_mask = DMA_BIT_MASK(32);
>> +	if (!dev->dma_mask)
>> +		dev->dma_mask = &dev->coherent_dma_mask;
>> +
>> +	/*
>> +	 * if dma-coherent property exist, call arch hook to setup
>> +	 * dma coherent operations.
>> +	 */
>> +	if (of_dma_is_coherent(dev->of_node)) {
>> +		set_arch_dma_coherent_ops(dev);
>> +		dev_dbg(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(dev->of_node, &dma_addr, &paddr, &size);
>> +	if ((ret == -ENODEV) || (ret < 0)) {
>> +		dev_dbg(dev, "no dma range information to setup\n");
>> +		return;
>> +	}
>> +
>> +	/* DMA ranges found. Calculate and set dma_pfn_offset */
>> +	dev->dma_pfn_offset = PFN_DOWN(paddr - dma_addr);
>> +	dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset);
> 
> I've got two concerns here. of_dma_get_range() retrieves only the first
> tuple from the dma-ranges property, but it is perfectly valid for
> dma-ranges to contain multiple tuples. How should we handle it if a
> device has multiple ranges it can DMA from?
> 

We've not found any cases in current Linux where more than one dma-ranges
would be used. Moreover, The MM (definitely for ARM) isn't supported such
cases at all (if i understand everything right).
- there are only one arm_dma_pfn_limit
- there is only one MM zone is used for ARM
- some arches like x86,mips can support 2 zones (per arch - not per device or bus)
  DMA & DMA32, but they configured once and forever per arch.

Example:
static void *loongson_dma_alloc_coherent(struct device *dev, size_t size,
		dma_addr_t *dma_handle, gfp_t gfp, struct dma_attrs *attrs)
{
...

	/* ignore region specifiers */
	gfp &= ~(__GFP_DMA | __GFP_DMA32 | __GFP_HIGHMEM);

#ifdef CONFIG_ISA
	if (dev == NULL)
		gfp |= __GFP_DMA;
	else
#endif
#ifdef CONFIG_ZONE_DMA
	if (dev->coherent_dma_mask < DMA_BIT_MASK(32))
		gfp |= __GFP_DMA;
	else
#endif
#ifdef CONFIG_ZONE_DMA32
	if (dev->coherent_dma_mask < DMA_BIT_MASK(40))
		gfp |= __GFP_DMA32;
	else
#endif
...
}

Any ways, it can be added later if we have an usecase for
that.

> Second, while the pfn offset is being determined, I don't see anything
> making use of either the base address or size. How is the device
> constrained to only getting DMA buffers from within that range? Is the
> driver expected to manage that directly?
> 
Drivers don't have to do anything special apart from setting
the correct mask. The pfn_offset case, we use DMA_ZONE which takes
care of masks already. Size is suppose to be used for dma_mask
setup which we discussed in previous threads.

Regards,
Santosh
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