[RFC] ARM64: simplify dma_get_ops

[Arnd Bergmann]

On Monday 16 November 2015 18:39:41 Catalin Marinas wrote:
> On Mon, Nov 16, 2015 at 05:25:48PM +0100, Arnd Bergmann wrote:
> > Including linux/acpi.h from asm/dma-mapping.h causes tons of compile-time
> > warnings, e.g.
> > 
> >  drivers/isdn/mISDN/dsp_ecdis.h:43:0: warning: "FALSE" redefined
> >  drivers/isdn/mISDN/dsp_ecdis.h:44:0: warning: "TRUE" redefined
> >  drivers/net/fddi/skfp/h/targetos.h:62:0: warning: "TRUE" redefined
> >  drivers/net/fddi/skfp/h/targetos.h:63:0: warning: "FALSE" redefined
> > 
> > However, it looks like the dependency should not even there as
> > I do not see why __generic_dma_ops() cares about whether we have
> > an ACPI based system or not.
> > 
> > The current behavior is to fall back to the global dma_ops when
> > a device has not set its own dma_ops, but only for DT based systems.
> > This seems dangerous, as a random device might have different
> > requirements regarding IOMMU or coherency, so we should really
> > never have that fallback and just forbid DMA when we have not
> > initialized DMA for a device.
> 
> I think this is from the days when we didn't have an
> arch_setup_dma_ops() to be called from the DT code.

Makes sense.

> I did placed a WARN_ON() to see who's getting the dummy_dma_ops and it
> triggered this call trace:
> 
> WARNING: at /work/Linux/linux-2.6-aarch64/arch/arm64/include/asm/dma-mapping.h:40
> Modules linked in:
> 
> CPU: 3 PID: 1 Comm: swapper/0 Tainted: G        W       4.4.0-rc1+ #601
> Hardware name: Juno (DT)
> task: ffffffc9768a0000 ti: ffffffc9768a8000 task.ti: ffffffc9768a8000
> PC is at ohci_platform_probe+0x2b8/0x518
> LR is at ohci_platform_probe+0x2c/0x518
> Call trace:
> [<ffffffc0004920e0>] ohci_platform_probe+0x2b8/0x518
> [<ffffffc0003e842c>] platform_drv_probe+0x54/0xb8
> [<ffffffc0003e68d4>] driver_probe_device+0x1ec/0x2f0
> [<ffffffc0003e6a74>] __driver_attach+0x9c/0xa0
> [<ffffffc0003e4a10>] bus_for_each_dev+0x60/0xa0
> [<ffffffc0003e61b8>] driver_attach+0x20/0x28
> [<ffffffc0003e5dd8>] bus_add_driver+0x1d0/0x238
> [<ffffffc0003e7218>] driver_register+0x60/0xf8
> [<ffffffc0003e8328>] __platform_driver_register+0x40/0x48
> [<ffffffc00086f720>] ohci_platform_init+0x50/0x60
> [<ffffffc000082938>] do_one_initcall+0x90/0x1a0
> [<ffffffc00084caac>] kernel_init_freeable+0x154/0x1f8
> [<ffffffc0005e9c40>] kernel_init+0x10/0xe0
> [<ffffffc000085c50>] ret_from_fork+0x10/0x40
> ohci-platform: probe of 7ffb0000.ohci failed with error -5
> 
> I need to check whether we break anything by no longer returning
> swiotlb_dma_ops.

Interesting. Note that this driver calls 'dma_coerce_mask_and_coherent()',
which can be regarded a bug by itself, because it overrides the
platform specific DMA mask.

> > This removes the global dma_ops variable and the special-casing
> > for ACPI, and just returns the dma ops that got set for the
> > device, or the dummy_dma_ops if none were present.
> > 
> > The original code has apparently been copied from arm32 where we
> > rely on it for ISA devices things like the floppy controller, but
> > we should have no such devices on ARM64.
> 
> IIRC, this was required for DT before we had the arch_set_dma_ops()
> (though I may be wrong, I haven't checked the logs). The dummy ops were
> introduced with ACPI to avoid any default, I think this was related to
> the _CCA property.

Right.

> > @@ -985,7 +977,7 @@ void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> >  			struct iommu_ops *iommu, bool coherent)
> >  {
> >  	if (!acpi_disabled && !dev->archdata.dma_ops)
> > -		dev->archdata.dma_ops = dma_ops;
> > +		dev->archdata.dma_ops = &swiotlb_dma_ops;
> 
> Why do we still keep the !acpi_disabled check here? If I remove it, the
> WARN_ON() above disappears.

Ah, good. That must be my mistake then. This looks much better. On a
related note, we should also urgently fix the arch_setup_dma_ops() function
to no longer ignore the base and size arguments. For dma_mase, we can simply
WARN_ON(dma_base != 0), so we can implement support for that whenever we need
it, but for the size we need to prevent drivers from calling dma_set_mask()
with an argument larger than the size we pass in here, unless the size is
also larger than max_pfn.

	Arnd