linux-kernel - Re: [PATCH 13/20] riscv/mmiowb: Hook up mmwiob() implementation to asm-generic code

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Date:   Fri, 01 Mar 2019 13:13:26 -0800 (PST)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     Will Deacon <will.deacon@....com>
CC:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        Will Deacon <will.deacon@....com>, paulmck@...ux.ibm.com,
        benh@...nel.crashing.org, mpe@...erman.id.au,
        Arnd Bergmann <arnd@...db.de>, peterz@...radead.org,
        andrea.parri@...rulasolutions.com,
        Daniel Lustig <dlustig@...dia.com>, dhowells@...hat.com,
        stern@...land.harvard.edu,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        macro@...ux-mips.org, paul.burton@...s.com, mingo@...nel.org,
        ysato@...rs.sourceforge.jp, dalias@...c.org, tony.luck@...el.com
Subject:     Re: [PATCH 13/20] riscv/mmiowb: Hook up mmwiob() implementation to asm-generic code

On Fri, 01 Mar 2019 06:03:41 PST (-0800), Will Deacon wrote:
> In a bid to kill off explicit mmiowb() usage in driver code, hook up
> the asm-generic mmiowb() tracking code for riscv, so that an mmiowb()
> is automatically issued from spin_unlock() if an I/O write was performed
> in the critical section.
>
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>  arch/riscv/Kconfig              |  1 +
>  arch/riscv/include/asm/Kbuild   |  1 -
>  arch/riscv/include/asm/io.h     | 15 ++-------------
>  arch/riscv/include/asm/mmiowb.h | 14 ++++++++++++++
>  4 files changed, 17 insertions(+), 14 deletions(-)
>  create mode 100644 arch/riscv/include/asm/mmiowb.h
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 515fc3cc9687..08f4415203c5 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -49,6 +49,7 @@ config RISCV
>  	select RISCV_TIMER
>  	select GENERIC_IRQ_MULTI_HANDLER
>  	select ARCH_HAS_PTE_SPECIAL
> +	select ARCH_HAS_MMIOWB
>
>  config MMU
>  	def_bool y
> diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
> index 221cd2ec78a4..cccd12cf27d4 100644
> --- a/arch/riscv/include/asm/Kbuild
> +++ b/arch/riscv/include/asm/Kbuild
> @@ -21,7 +21,6 @@ generic-y += kvm_para.h
>  generic-y += local.h
>  generic-y += local64.h
>  generic-y += mm-arch-hooks.h
> -generic-y += mmiowb.h
>  generic-y += mutex.h
>  generic-y += percpu.h
>  generic-y += preempt.h
> diff --git a/arch/riscv/include/asm/io.h b/arch/riscv/include/asm/io.h
> index 1d9c1376dc64..744fd92e77bc 100644
> --- a/arch/riscv/include/asm/io.h
> +++ b/arch/riscv/include/asm/io.h
> @@ -20,6 +20,7 @@
>  #define _ASM_RISCV_IO_H
>
>  #include <linux/types.h>
> +#include <asm/mmiowb.h>
>
>  extern void __iomem *ioremap(phys_addr_t offset, unsigned long size);
>
> @@ -100,18 +101,6 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
>  #endif
>
>  /*
> - * FIXME: I'm flip-flopping on whether or not we should keep this or enforce
> - * the ordering with I/O on spinlocks like PowerPC does.  The worry is that
> - * drivers won't get this correct, but I also don't want to introduce a fence
> - * into the lock code that otherwise only uses AMOs (and is essentially defined
> - * by the ISA to be correct).   For now I'm leaving this here: "o,w" is
> - * sufficient to ensure that all writes to the device have completed before the
> - * write to the spinlock is allowed to commit.  I surmised this from reading
> - * "ACQUIRES VS I/O ACCESSES" in memory-barriers.txt.
> - */
> -#define mmiowb()	__asm__ __volatile__ ("fence o,w" : : : "memory");
> -
> -/*
>   * Unordered I/O memory access primitives.  These are even more relaxed than
>   * the relaxed versions, as they don't even order accesses between successive
>   * operations to the I/O regions.
> @@ -165,7 +154,7 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
>  #define __io_br()	do {} while (0)
>  #define __io_ar(v)	__asm__ __volatile__ ("fence i,r" : : : "memory");
>  #define __io_bw()	__asm__ __volatile__ ("fence w,o" : : : "memory");
> -#define __io_aw()	do {} while (0)
> +#define __io_aw()	mmiowb_set_pending()
>
>  #define readb(c)	({ u8  __v; __io_br(); __v = readb_cpu(c); __io_ar(__v); __v; })
>  #define readw(c)	({ u16 __v; __io_br(); __v = readw_cpu(c); __io_ar(__v); __v; })
> diff --git a/arch/riscv/include/asm/mmiowb.h b/arch/riscv/include/asm/mmiowb.h
> new file mode 100644
> index 000000000000..5d7e3a2b4e3b
> --- /dev/null
> +++ b/arch/riscv/include/asm/mmiowb.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _ASM_RISCV_MMIOWB_H
> +#define _ASM_RISCV_MMIOWB_H
> +
> +/*
> + * "o,w" is sufficient to ensure that all writes to the device have completed
> + * before the write to the spinlock is allowed to commit.
> + */
> +#define mmiowb()	__asm__ __volatile__ ("fence o,w" : : : "memory");
> +
> +#include <asm-generic/mmiowb.h>
> +
> +#endif	/* ASM_RISCV_MMIOWB_H */

Reviewed-by: Palmer Dabbelt <palmer@...ive.com>

Thanks for doing this, that comment was one of the more headache-incuding 
FIXMEs in our port.  I think it's better to keep __io_aw next to the others: 
even if it's the same as the generic implementation, it's easier to reason 
about this way.