[RFC] ARC: allow to use IOC and non-IOC DMA devices simultaneously

The ARC HS processor provides an IOC port (I/O coherency bus
interface) that allows external devices such as DMA devices
to access memory through the cache hierarchy, providing
coherency between I/O transactions and the complete memory
hierarchy.

Some recent SoC with ARC HS (like HSDK) allow to select bus
port (IOC or non-IOC port) for connecting DMA devices in runtime.

With this patch we can use both HW-coherent and regular DMA
peripherals simultaneously.

For example we can connect USB and SDIO controllers through IOC port
(so we don't need to need to maintain cache coherency for these
devices manualy. All cache sync ops will be nop)
And we can connect Ethernet directly to RAM port (so we had to
maintain cache coherency manualy. Cache sync ops will be real
flush/invalidate operations)

Cache ops are set per-device and depends on "dma-coherent" device
tree property:
"dma_noncoherent_ops" are used if no "dma-coherent" property is
present (or IOC is disabled)
"dma_direct_ops" are used if "dma-coherent" property is present.


NOTE 1:
It works perfectly fine only if we don't have ZONE_HIGHMEM used
as IOC doesn't cover all physical memory. As for today It configured
to cover 1GiB starting from 0x8z (which is ZONE_NORMAL memory for
us). Transactions outside this region are sent on the non-coherent
I/O bus interface.
We can't configure IOC to cover all physical memory as it has several
limitations relating to aperture size and start address.

And if we get DMA buffer from ZONE_HIGHMEM memory we need to
do real flush/invalidate operations on that buffer, which is obviously
not done by "dma_direct_ops".

So I am not sure about "dma_direct_ops" using - probably we need to
create our special cache ops like "arc_ioc_ops" which will handle
ZONE_HIGHMEM case.

(BTW: current ARC dma_noncoherent_ops implementation also has same
problem if IOC and HIGHMEM are enabled.)


NOTE 2:
In this RFC only hsdk.dts changes are shown to reduce patch size.
AXS103 device tree changes are not shown.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
---
 arch/arc/Kconfig                   |  1 +
 arch/arc/boot/dts/hsdk.dts         |  4 ++++
 arch/arc/include/asm/dma-mapping.h | 14 ++++++++++++++
 arch/arc/mm/Makefile               |  2 +-
 arch/arc/mm/cache.c                | 15 +--------------
 arch/arc/mm/dma-mapping.c          | 20 ++++++++++++++++++++
 arch/arc/mm/dma.c                  | 14 +-------------
 7 files changed, 42 insertions(+), 28 deletions(-)
 create mode 100644 arch/arc/include/asm/dma-mapping.h
 create mode 100644 arch/arc/mm/dma-mapping.c

> +#ifndef ASM_ARC_DMA_MAPPING_H
> +#define ASM_ARC_DMA_MAPPING_H
> +
> +#define arch_setup_dma_ops arch_setup_dma_ops
> +
> +#include <asm-generic/dma-mapping.h>
> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +			const struct iommu_ops *iommu, bool coherent);

Can you keep the #define for arch_setup_dma_ops right next to the
prototype?

> index 000000000000..9d0d310bbf5a
> --- /dev/null
> +++ b/arch/arc/mm/dma-mapping.c
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// (C) 2018 Synopsys, Inc. (www.synopsys.com)
> +
> +#include <linux/dma-mapping.h>
> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +			const struct iommu_ops *iommu, bool coherent)
> +{
> +	const struct dma_map_ops *dma_ops = &dma_noncoherent_ops;
> +
> +	/*
> +	 * IOC hardware snoops all DMA traffic keeping the caches consistent
> +	 * with memory - eliding need for any explicit cache maintenance of
> +	 * DMA buffers - so we can use dma_direct cache ops.
> +	 */
> +	if (is_isa_arcv2() && ioc_enable && coherent)
> +		dma_ops = &dma_direct_ops;
> +
> +	set_dma_ops(dev, dma_ops);
> +}


Why not just write the routines as:

void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
			const struct iommu_ops *iommu, bool coherent)
{
	if (is_isa_arcv2() && ioc_enable && coherent)
		set_dma_ops(dev, &dma_direct_ops);
	else
		set_dma_ops(dev, &dma_noncoherent_ops);
}

Also a new file just for this routine seems a little wasteful.  Can't
you just squeeze it into setup.c or some other file?

On 06/15/2018 05:58 AM, Eugeniy Paltsev wrote:
> The ARC HS processor provides an IOC port (I/O coherency bus
> interface) that allows external devices such as DMA devices
> to access memory through the cache hierarchy, providing
> coherency between I/O transactions and the complete memory
> hierarchy.

This is really nice: having this a per device behaviour has been desirable rather
than the current blunt system-wide behaviour.

However the patch doesn't seem to change the routing off non-coherent traffic -
everything would still go thru it - per the current default setting of
CREG_AXI_M_*_SLV[0-1] registers. Ideally you would want to disable that as well,
an addon patch is fine.

>
> Some recent SoC with ARC HS (like HSDK) allow to select bus
> port (IOC or non-IOC port) for connecting DMA devices in runtime.
>
> With this patch we can use both HW-coherent and regular DMA
> peripherals simultaneously.
>
> For example we can connect USB and SDIO controllers through IOC port
> (so we don't need to need to maintain cache coherency for these
> devices manualy. All cache sync ops will be nop)
> And we can connect Ethernet directly to RAM port (so we had to
> maintain cache coherency manualy. Cache sync ops will be real
> flush/invalidate operations)
>
> Cache ops are set per-device and depends on "dma-coherent" device
> tree property:
> "dma_noncoherent_ops" are used if no "dma-coherent" property is
> present (or IOC is disabled)
> "dma_direct_ops" are used if "dma-coherent" property is present.

I agree with Christoph that creating a new file for this seems excessive.

> NOTE 1:
> It works perfectly fine only if we don't have ZONE_HIGHMEM used
> as IOC doesn't cover all physical memory. As for today It configured
> to cover 1GiB starting from 0x8z (which is ZONE_NORMAL memory for
> us). Transactions outside this region are sent on the non-coherent
> I/O bus interface.
> We can't configure IOC to cover all physical memory as it has several
> limitations relating to aperture size and start address.
>
> And if we get DMA buffer from ZONE_HIGHMEM memory we need to
> do real flush/invalidate operations on that buffer, which is obviously
> not done by "dma_direct_ops".
>
> So I am not sure about "dma_direct_ops" using - probably we need to
> create our special cache ops like "arc_ioc_ops" which will handle
> ZONE_HIGHMEM case.
>
> (BTW: current ARC dma_noncoherent_ops implementation also has same
> problem if IOC and HIGHMEM are enabled.)

Can we highlight this fact, add error prints somewhere ?

> NOTE 2:
> In this RFC only hsdk.dts changes are shown to reduce patch size.
> AXS103 device tree changes are not shown.
>
> Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
> ---
>  arch/arc/Kconfig                   |  1 +
>  arch/arc/boot/dts/hsdk.dts         |  4 ++++
>  arch/arc/include/asm/dma-mapping.h | 14 ++++++++++++++
>  arch/arc/mm/Makefile               |  2 +-
>  arch/arc/mm/cache.c                | 15 +--------------
>  arch/arc/mm/dma-mapping.c          | 20 ++++++++++++++++++++
>  arch/arc/mm/dma.c                  | 14 +-------------
>  7 files changed, 42 insertions(+), 28 deletions(-)
>  create mode 100644 arch/arc/include/asm/dma-mapping.h
>  create mode 100644 arch/arc/mm/dma-mapping.c
>
> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> index e81bcd271be7..0a2fcd2a8c32 100644
> --- a/arch/arc/Kconfig
> +++ b/arch/arc/Kconfig
> @@ -17,6 +17,7 @@ config ARC
>  	select CLONE_BACKWARDS
>  	select COMMON_CLK
>  	select DMA_NONCOHERENT_OPS
> +	select DMA_DIRECT_OPS
>  	select DMA_NONCOHERENT_MMAP
>  	select GENERIC_ATOMIC64 if !ISA_ARCV2 || !(ARC_HAS_LL64 && ARC_HAS_LLSC)
>  	select GENERIC_CLOCKEVENTS
> diff --git a/arch/arc/boot/dts/hsdk.dts b/arch/arc/boot/dts/hsdk.dts
> index 006aa3de5348..ebb686c21393 100644
> --- a/arch/arc/boot/dts/hsdk.dts
> +++ b/arch/arc/boot/dts/hsdk.dts
> @@ -176,6 +176,7 @@
>  			phy-handle = <&phy0>;
>  			resets = <&cgu_rst HSDK_ETH_RESET>;
>  			reset-names = "stmmaceth";
> +			dma-coherent;
>  
>  			mdio {
>  				#address-cells = <1>;
> @@ -194,12 +195,14 @@
>  			compatible = "snps,hsdk-v1.0-ohci", "generic-ohci";
>  			reg = <0x60000 0x100>;
>  			interrupts = <15>;
> +			dma-coherent;
>  		};
>  
>  		ehci@40000 {
>  			compatible = "snps,hsdk-v1.0-ehci", "generic-ehci";
>  			reg = <0x40000 0x100>;
>  			interrupts = <15>;
> +			dma-coherent;
>  		};
>  
>  		mmc@a000 {
> @@ -212,6 +215,7 @@
>  			clock-names = "biu", "ciu";
>  			interrupts = <12>;
>  			bus-width = <4>;
> +			dma-coherent;
>  		};
>  	};
>  
> diff --git a/arch/arc/include/asm/dma-mapping.h b/arch/arc/include/asm/dma-mapping.h
> new file mode 100644
> index 000000000000..640a851bd331
> --- /dev/null
> +++ b/arch/arc/include/asm/dma-mapping.h
> @@ -0,0 +1,14 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// (C) 2018 Synopsys, Inc. (www.synopsys.com)
> +
> +#ifndef ASM_ARC_DMA_MAPPING_H
> +#define ASM_ARC_DMA_MAPPING_H
> +
> +#define arch_setup_dma_ops arch_setup_dma_ops
> +
> +#include <asm-generic/dma-mapping.h>
> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +			const struct iommu_ops *iommu, bool coherent);
> +
> +#endif
> diff --git a/arch/arc/mm/Makefile b/arch/arc/mm/Makefile
> index 3703a4969349..45683897c27b 100644
> --- a/arch/arc/mm/Makefile
> +++ b/arch/arc/mm/Makefile
> @@ -7,5 +7,5 @@
>  #
>  
>  obj-y	:= extable.o ioremap.o dma.o fault.o init.o
> -obj-y	+= tlb.o tlbex.o cache.o mmap.o
> +obj-y	+= tlb.o tlbex.o cache.o mmap.o dma-mapping.o
>  obj-$(CONFIG_HIGHMEM)	+= highmem.o
> diff --git a/arch/arc/mm/cache.c b/arch/arc/mm/cache.c
> index 9dbe645ee127..c5d1f2a2c4da 100644
> --- a/arch/arc/mm/cache.c
> +++ b/arch/arc/mm/cache.c
> @@ -896,15 +896,6 @@ static void __dma_cache_wback_slc(phys_addr_t start, unsigned long sz)
>  	slc_op(start, sz, OP_FLUSH);
>  }
>  
> -/*
> - * DMA ops for systems with IOC
> - * IOC hardware snoops all DMA traffic keeping the caches consistent with
> - * memory - eliding need for any explicit cache maintenance of DMA buffers
> - */
> -static void __dma_cache_wback_inv_ioc(phys_addr_t start, unsigned long sz) {}
> -static void __dma_cache_inv_ioc(phys_addr_t start, unsigned long sz) {}
> -static void __dma_cache_wback_ioc(phys_addr_t start, unsigned long sz) {}
> -
>  /*
>   * Exported DMA API
>   */
> @@ -1253,11 +1244,7 @@ void __init arc_cache_init_master(void)
>  	if (is_isa_arcv2() && ioc_enable)
>  		arc_ioc_setup();
>  
> -	if (is_isa_arcv2() && ioc_enable) {
> -		__dma_cache_wback_inv = __dma_cache_wback_inv_ioc;
> -		__dma_cache_inv = __dma_cache_inv_ioc;
> -		__dma_cache_wback = __dma_cache_wback_ioc;
> -	} else if (is_isa_arcv2() && l2_line_sz && slc_enable) {

Maybe also tweak the boot printing in setup.c to indicate that we now do per
peripheral ioc !

> +	if (is_isa_arcv2() && l2_line_sz && slc_enable) {
>  		__dma_cache_wback_inv = __dma_cache_wback_inv_slc;
>  		__dma_cache_inv = __dma_cache_inv_slc;
>  		__dma_cache_wback = __dma_cache_wback_slc;
> diff --git a/arch/arc/mm/dma-mapping.c b/arch/arc/mm/dma-mapping.c
> new file mode 100644
> index 000000000000..9d0d310bbf5a
> --- /dev/null
> +++ b/arch/arc/mm/dma-mapping.c
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// (C) 2018 Synopsys, Inc. (www.synopsys.com)
> +
> +#include <linux/dma-mapping.h>
> +
> +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> +			const struct iommu_ops *iommu, bool coherent)
> +{
> +	const struct dma_map_ops *dma_ops = &dma_noncoherent_ops;
> +
> +	/*
> +	 * IOC hardware snoops all DMA traffic keeping the caches consistent
> +	 * with memory - eliding need for any explicit cache maintenance of
> +	 * DMA buffers - so we can use dma_direct cache ops.
> +	 */
> +	if (is_isa_arcv2() && ioc_enable && coherent)
> +		dma_ops = &dma_direct_ops;
> +
> +	set_dma_ops(dev, dma_ops);

Add a debug printk here maybe ?

> +}
> diff --git a/arch/arc/mm/dma.c b/arch/arc/mm/dma.c
> index 8c1071840979..4fd130e786c7 100644
> --- a/arch/arc/mm/dma.c
> +++ b/arch/arc/mm/dma.c
> @@ -33,19 +33,7 @@ void *arch_dma_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
>  	if (!page)
>  		return NULL;
>  
> -	/*
> -	 * IOC relies on all data (even coherent DMA data) being in cache
> -	 * Thus allocate normal cached memory
> -	 *
> -	 * The gains with IOC are two pronged:
> -	 *   -For streaming data, elides need for cache maintenance, saving
> -	 *    cycles in flush code, and bus bandwidth as all the lines of a
> -	 *    buffer need to be flushed out to memory
> -	 *   -For coherent data, Read/Write to buffers terminate early in cache
> -	 *   (vs. always going to memory - thus are faster)
> -	 */
> -	if ((is_isa_arcv2() && ioc_enable) ||
> -	    (attrs & DMA_ATTR_NON_CONSISTENT))
> +	if (attrs & DMA_ATTR_NON_CONSISTENT)
>  		need_coh = 0;
>  
>  	/*

Hi Vineet, Christoph,
thanks for responding.

On Mon, 2018-06-18 at 15:53 -0700, Vineet Gupta wrote:
> On 06/15/2018 05:58 AM, Eugeniy Paltsev wrote:
> > The ARC HS processor provides an IOC port (I/O coherency bus
> > interface) that allows external devices such as DMA devices
> > to access memory through the cache hierarchy, providing
> > coherency between I/O transactions and the complete memory
> > hierarchy.
> 
> This is really nice: having this a per device behaviour has been desirable rather
> than the current blunt system-wide behaviour.
> 
> However the patch doesn't seem to change the routing off non-coherent traffic -
> everything would still go thru it - per the current default setting of
> CREG_AXI_M_*_SLV[0-1] registers. Ideally you would want to disable that as well,
> an addon patch is fine.

Yep, in this patch I only implement infrastructure to be able to choose
coherent/non-coherent cache ops per device. As for today all
*used* (supported and enabled in the mainline kernel) DMA peripherals on AXS103
and HSDK boards are routed via IOC port. So in this patch I'm going to simply enable
io-coherency via device tree for every DMA peripheral on AXS103 and HSDK.

Then I going to modify apertures configuration in separate patch.
It will be something like it is done in u-boot:
https://elixir.bootlin.com/u-boot/v2018.07-rc2/source/board/synopsys/hsdk/hsdk.c#L441

> > 
> > Some recent SoC with ARC HS (like HSDK) allow to select bus
> > port (IOC or non-IOC port) for connecting DMA devices in runtime.
> > 
> > With this patch we can use both HW-coherent and regular DMA
> > peripherals simultaneously.
> > 
> > For example we can connect USB and SDIO controllers through IOC port
> > (so we don't need to need to maintain cache coherency for these
> > devices manualy. All cache sync ops will be nop)
> > And we can connect Ethernet directly to RAM port (so we had to
> > maintain cache coherency manualy. Cache sync ops will be real
> > flush/invalidate operations)
> > 
> > Cache ops are set per-device and depends on "dma-coherent" device
> > tree property:
> > "dma_noncoherent_ops" are used if no "dma-coherent" property is
> > present (or IOC is disabled)
> > "dma_direct_ops" are used if "dma-coherent" property is present.
> 
> I agree with Christoph that creating a new file for this seems excessive.
Ok, I'll move it to arch/arc/mm/dma.c

> > NOTE 1:
> > It works perfectly fine only if we don't have ZONE_HIGHMEM used
> > as IOC doesn't cover all physical memory. As for today It configured
> > to cover 1GiB starting from 0x8z (which is ZONE_NORMAL memory for
> > us). Transactions outside this region are sent on the non-coherent
> > I/O bus interface.
> > We can't configure IOC to cover all physical memory as it has several
> > limitations relating to aperture size and start address.
> > 
> > And if we get DMA buffer from ZONE_HIGHMEM memory we need to
> > do real flush/invalidate operations on that buffer, which is obviously
> > not done by "dma_direct_ops".
> > 
> > So I am not sure about "dma_direct_ops" using - probably we need to
> > create our special cache ops like "arc_ioc_ops" which will handle
> > ZONE_HIGHMEM case.

Any ideas about ZONE_HIGHMEM? Should I implement separate cache ops
like "arc_ioc_ops" where I'll check and process HIGHMEM pages special way?

> > (BTW: current ARC dma_noncoherent_ops implementation also has same
> > problem if IOC and HIGHMEM are enabled.)
> 
> Can we highlight this fact, add error prints somewhere ?
The easiest solution is to panic if both IOC and ZONE_HIGHMEM enabled.

> > NOTE 2:
> > In this RFC only hsdk.dts changes are shown to reduce patch size.
> > AXS103 device tree changes are not shown.
> > 
> > Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
> > ---
> >  arch/arc/Kconfig                   |  1 +
> >  arch/arc/boot/dts/hsdk.dts         |  4 ++++
> >  arch/arc/include/asm/dma-mapping.h | 14 ++++++++++++++
> >  arch/arc/mm/Makefile               |  2 +-
> >  arch/arc/mm/cache.c                | 15 +--------------
> >  arch/arc/mm/dma-mapping.c          | 20 ++++++++++++++++++++
> >  arch/arc/mm/dma.c                  | 14 +-------------
> >  7 files changed, 42 insertions(+), 28 deletions(-)
> >  create mode 100644 arch/arc/include/asm/dma-mapping.h
> >  create mode 100644 arch/arc/mm/dma-mapping.c
> > 
> > diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> > index e81bcd271be7..0a2fcd2a8c32 100644
> > --- a/arch/arc/Kconfig
> > +++ b/arch/arc/Kconfig
> > @@ -17,6 +17,7 @@ config ARC
> >  	select CLONE_BACKWARDS
> >  	select COMMON_CLK
> >  	select DMA_NONCOHERENT_OPS
> > +	select DMA_DIRECT_OPS
> >  	select DMA_NONCOHERENT_MMAP
> >  	select GENERIC_ATOMIC64 if !ISA_ARCV2 || !(ARC_HAS_LL64 && ARC_HAS_LLSC)
> >  	select GENERIC_CLOCKEVENTS
> > diff --git a/arch/arc/boot/dts/hsdk.dts b/arch/arc/boot/dts/hsdk.dts
> > index 006aa3de5348..ebb686c21393 100644
> > --- a/arch/arc/boot/dts/hsdk.dts
> > +++ b/arch/arc/boot/dts/hsdk.dts
> > @@ -176,6 +176,7 @@
> >  			phy-handle = <&phy0>;
> >  			resets = <&cgu_rst HSDK_ETH_RESET>;
> >  			reset-names = "stmmaceth";
> > +			dma-coherent;
> >  
> >  			mdio {
> >  				#address-cells = <1>;
> > @@ -194,12 +195,14 @@
> >  			compatible = "snps,hsdk-v1.0-ohci", "generic-ohci";
> >  			reg = <0x60000 0x100>;
> >  			interrupts = <15>;
> > +			dma-coherent;
> >  		};
> >  
> >  		ehci@40000 {
> >  			compatible = "snps,hsdk-v1.0-ehci", "generic-ehci";
> >  			reg = <0x40000 0x100>;
> >  			interrupts = <15>;
> > +			dma-coherent;
> >  		};
> >  
> >  		mmc@a000 {
> > @@ -212,6 +215,7 @@
> >  			clock-names = "biu", "ciu";
> >  			interrupts = <12>;
> >  			bus-width = <4>;
> > +			dma-coherent;
> >  		};
> >  	};
> >  
> > diff --git a/arch/arc/include/asm/dma-mapping.h b/arch/arc/include/asm/dma-mapping.h
> > new file mode 100644
> > index 000000000000..640a851bd331
> > --- /dev/null
> > +++ b/arch/arc/include/asm/dma-mapping.h
> > @@ -0,0 +1,14 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +// (C) 2018 Synopsys, Inc. (www.synopsys.com)
> > +
> > +#ifndef ASM_ARC_DMA_MAPPING_H
> > +#define ASM_ARC_DMA_MAPPING_H
> > +
> > +#define arch_setup_dma_ops arch_setup_dma_ops
> > +
> > +#include <asm-generic/dma-mapping.h>
> > +
> > +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> > +			const struct iommu_ops *iommu, bool coherent);
> > +
> > +#endif
> > diff --git a/arch/arc/mm/Makefile b/arch/arc/mm/Makefile
> > index 3703a4969349..45683897c27b 100644
> > --- a/arch/arc/mm/Makefile
> > +++ b/arch/arc/mm/Makefile
> > @@ -7,5 +7,5 @@
> >  #
> >  
> >  obj-y	:= extable.o ioremap.o dma.o fault.o init.o
> > -obj-y	+= tlb.o tlbex.o cache.o mmap.o
> > +obj-y	+= tlb.o tlbex.o cache.o mmap.o dma-mapping.o
> >  obj-$(CONFIG_HIGHMEM)	+= highmem.o
> > diff --git a/arch/arc/mm/cache.c b/arch/arc/mm/cache.c
> > index 9dbe645ee127..c5d1f2a2c4da 100644
> > --- a/arch/arc/mm/cache.c
> > +++ b/arch/arc/mm/cache.c
> > @@ -896,15 +896,6 @@ static void __dma_cache_wback_slc(phys_addr_t start, unsigned long sz)
> >  	slc_op(start, sz, OP_FLUSH);
> >  }
> >  
> > -/*
> > - * DMA ops for systems with IOC
> > - * IOC hardware snoops all DMA traffic keeping the caches consistent with
> > - * memory - eliding need for any explicit cache maintenance of DMA buffers
> > - */
> > -static void __dma_cache_wback_inv_ioc(phys_addr_t start, unsigned long sz) {}
> > -static void __dma_cache_inv_ioc(phys_addr_t start, unsigned long sz) {}
> > -static void __dma_cache_wback_ioc(phys_addr_t start, unsigned long sz) {}
> > -
> >  /*
> >   * Exported DMA API
> >   */
> > @@ -1253,11 +1244,7 @@ void __init arc_cache_init_master(void)
> >  	if (is_isa_arcv2() && ioc_enable)
> >  		arc_ioc_setup();
> >  
> > -	if (is_isa_arcv2() && ioc_enable) {
> > -		__dma_cache_wback_inv = __dma_cache_wback_inv_ioc;
> > -		__dma_cache_inv = __dma_cache_inv_ioc;
> > -		__dma_cache_wback = __dma_cache_wback_ioc;
> > -	} else if (is_isa_arcv2() && l2_line_sz && slc_enable) {
> 
> Maybe also tweak the boot printing in setup.c to indicate that we now do per
> peripheral ioc !

Ok, good idea.

> > +	if (is_isa_arcv2() && l2_line_sz && slc_enable) {
> >  		__dma_cache_wback_inv = __dma_cache_wback_inv_slc;
> >  		__dma_cache_inv = __dma_cache_inv_slc;
> >  		__dma_cache_wback = __dma_cache_wback_slc;
> > diff --git a/arch/arc/mm/dma-mapping.c b/arch/arc/mm/dma-mapping.c
> > new file mode 100644
> > index 000000000000..9d0d310bbf5a
> > --- /dev/null
> > +++ b/arch/arc/mm/dma-mapping.c
> > @@ -0,0 +1,20 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +// (C) 2018 Synopsys, Inc. (www.synopsys.com)
> > +
> > +#include <linux/dma-mapping.h>
> > +
> > +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> > +			const struct iommu_ops *iommu, bool coherent)
> > +{
> > +	const struct dma_map_ops *dma_ops = &dma_noncoherent_ops;
> > +
> > +	/*
> > +	 * IOC hardware snoops all DMA traffic keeping the caches consistent
> > +	 * with memory - eliding need for any explicit cache maintenance of
> > +	 * DMA buffers - so we can use dma_direct cache ops.
> > +	 */
> > +	if (is_isa_arcv2() && ioc_enable && coherent)
> > +		dma_ops = &dma_direct_ops;
> > +
> > +	set_dma_ops(dev, dma_ops);
> 
> Add a debug printk here maybe ?

Ok.

> > +}
> > diff --git a/arch/arc/mm/dma.c b/arch/arc/mm/dma.c
> > index 8c1071840979..4fd130e786c7 100644
> > --- a/arch/arc/mm/dma.c
> > +++ b/arch/arc/mm/dma.c
> > @@ -33,19 +33,7 @@ void *arch_dma_alloc(struct device *dev, size_t size, dma_addr_t *dma_handle,
> >  	if (!page)
> >  		return NULL;
> >  
> > -	/*
> > -	 * IOC relies on all data (even coherent DMA data) being in cache
> > -	 * Thus allocate normal cached memory
> > -	 *
> > -	 * The gains with IOC are two pronged:
> > -	 *   -For streaming data, elides need for cache maintenance, saving
> > -	 *    cycles in flush code, and bus bandwidth as all the lines of a
> > -	 *    buffer need to be flushed out to memory
> > -	 *   -For coherent data, Read/Write to buffers terminate early in cache
> > -	 *   (vs. always going to memory - thus are faster)
> > -	 */
> > -	if ((is_isa_arcv2() && ioc_enable) ||
> > -	    (attrs & DMA_ATTR_NON_CONSISTENT))
> > +	if (attrs & DMA_ATTR_NON_CONSISTENT)
> >  		need_coh = 0;
> >  
> >  	/*
> 
>

Hi Eugeniy,

On Fri, 2018-06-22 at 16:30 +0200, Eugeniy Paltsev wrote:
> Hi Vineet, Christoph,
> thanks for responding.

[snip]

> > > (BTW: current ARC dma_noncoherent_ops implementation also has same
> > > problem if IOC and HIGHMEM are enabled.)
> > 
> > Can we highlight this fact, add error prints somewhere ?
> 
> The easiest solution is to panic if both IOC and ZONE_HIGHMEM enabled.

I don't think it's the best idea - I would expect to get PAE40 enabled
by default or at least to be used quite often on both AXS103 and HSDK
and so restricting usage of IOC on such configurations will be quite
confusing.

That said we need to figure out how to get IOC + HIGHMEM/PAE working
reliably if that's possible at all.

Christoph, do you think if this is doable?
The main concern here is: some random data buffer might be allocated
in any available memory [and in HIGHMEM/PAE if it exists essentially] and
when it comes to DMA exchange that involves that buffer what should we do?

The only thing I may think of could be a bounce-buffer in ZONE_NORMAL.
Is it used by anybody? Are there any better ideas? 

-Alexey