Re: [RFC PATCH 09/11] drivers: acpi: implement acpi_dma_configure

[Robin Murphy]

On 15/04/16 19:29, Timur Tabi wrote:
> On Thu, Apr 14, 2016 at 12:25 PM, Lorenzo Pieralisi
> <lorenzo.pieralisi@xxxxxxx> wrote:
> > +void acpi_dma_configure(struct device *dev, enum dev_dma_attr attr)
> > +{
> > +       struct iommu_ops *iommu;
> > +
> > +       iommu = iort_iommu_configure(dev);
> > +
> > +       /*
> > +        * Assume dma valid range starts at 0 and covers the whole
> > +        * coherent_dma_mask.
> > +        */
> > +       arch_setup_dma_ops(dev, 0, dev->coherent_dma_mask + 1, iommu,
> > +                          attr == DEV_DMA_COHERENT);
> > +}
>
> I have a network driver that is impacted by this code, so thank you
> for posting this. (See
> https://www.mail-archive.com/netdev@xxxxxxxxxxxxxxx/msg106249.html).
>
> One one SOC, the driver needs to set the mask to 32 bits.  On another
> SOC, it needs to set it to 64 bits.  On device tree, the driver will
> use dma-ranges.

That's the wrong way to look at it - the driver isn't _using_ 
dma-ranges, you're merely relying on the OF code setting the _default_ 
DMA mask differently based on the property. If your driver is in the 
minority of those which actually care about DMA masks, then it should be 
calling dma_set_mask_and_coherent() appropriately and not relying on the 
default.

> In your patches, where is coherent_dma_mask initialized?  I found this
> code in add_smmu_platform_device(), but I think this is setting the
> mask for the IOMMU driver, not the individual devices.

Yes, that's for the SMMU itself as a device (i.e. the page table walker) 
- as a handy example of "drivers which actually care about DMA masks", 
it specifically needs to avoid a too-small DMA mask because 
bounce-buffering the page tables tends to make things go horribly wrong.

Robin.

>  Either way, I
> don't understand where the correct value is going to be overridden.
>
> +       /*
> +        * Set default dma mask value for the table walker,
> +        * to be overridden on probing with correct value.
> +        */
> +       *pdev->dev.dma_mask = DMA_BIT_MASK(32);
> +       pdev->dev.coherent_dma_mask = *pdev->dev.dma_mask;