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

[Grant Likely <grant.likely@linaro.org>]

On Wed, 30 Apr 2014 10:19:15 -0400, Santosh Shilimkar <santosh.shilimkar@ti.com> wrote:
> 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@ti.com> 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@linuxfoundation.org>
> >> Cc: Russell King <linux@arm.linux.org.uk>
> >> Cc: Arnd Bergmann <arnd@arndb.de>
> >> Cc: Olof Johansson <olof@lixom.net>
> >> Cc: Grant Likely <grant.likely@linaro.org>
> >> Cc: Rob Herring <robh+dt@kernel.org>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: Linus Walleij <linus.walleij@linaro.org>
> >> Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
> >> Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
> >> ---
> >>  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.

Okay. If anyone ever does implement multiple ranges then this code will
need to be revisited.

> > 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.

okay.

g.