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Date: Sun, 5 Dec 2021 12:43:31 +0800 (GMT+08:00)
From: 隋景峰 <suijingfeng@...ngson.cn>
To: "Jiaxun Yang" <jiaxun.yang@...goat.com>
Cc: "Huacai Chen" <chenhuacai@...nel.org>,
"Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
"linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH] mips/loongson64: using __fast_iob implement
__wbflush() instead of sync
Hi Jiaxun,
I'm not toward to loongson cpu's barriers, but the semantic of wbflush().
And it is not necessary the safest way:
before applying this patch, wbflush() will be expanded to double __sync(),
if __sync() can empty the uncached store buffer, single one is enough.
After apply this patch, wbflush() will be expanded to a __sync() followed by a __fast_iob();
this is more safe than your's version which blindly copy code from loongson2ef/common/setup.c.
Note, Uncached Accelerated cache mode is only implemented after ls3a2000.
See Mips Run Say:
Most write queues can be emptied out by performing an uncached store to
any location and then performing an operation that reads the same data back.
Put a sync instruction between the write and the read, and that should be effective
on any system compliant with MIPS32/64.
A write queue certainly can't permit the read to overtake the write, it would return stale data.
This is effective, but not necessarily efficient; you can minimize the overhead
by loading from the fastest memory available. Perhaps your system offers
something system-specific but faster.
Again, wbflush is mean to empty data gathered in the uncache store buffers within the CPU.
__wbflush() is only mean to be used internally, wbflush() should be used outside of wbflush.h.
by separate __wbflush out of setup.c, the code is more modularity. it allow you to deselect
CPU_HAS_WB easily and say what will happen.
Actually nothing strange is happen, because uncached accelerated cache mode does not
get used in upstream kernel.
> -----Original Messages-----
> From: "Jiaxun Yang" <jiaxun.yang@...goat.com>
> Sent Time: 2021-12-04 20:32:37 (Saturday)
> To: suijingfeng <suijingfeng@...ngson.cn>, "Huacai Chen" <chenhuacai@...nel.org>, "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>
> Cc: "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>, linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] mips/loongson64: using __fast_iob implement __wbflush() instead of sync
>
> Hi Jingfeng,
> I'd suggest you not to mess with barriers on Loongson.
> It's a hell.
>
> Also Loongson had changed semantics of sync/synci many times.
> They got redefined and swapped. So the present way is just the safest way.
>
> Thanks.
>
> - Jiaxun
>
> 在2021年12月4日十二月 下午12:00，suijingfeng写道：
> > 1) loongson's cpu(ls3a3000, ls3a4000, ls3a5000) have uncache store buffers
> > which is for uncache accleration.
> >
> > Uncached Accelerated is the name under which the R10000 introduced
> > a cache mode that uses the CPU's write buffer to combine writes
> > but that otherwise is uncached.
> >
> > wbflush is mean to empty data gathered in the uncache store buffers
> > within the CPU.
> >
> > 2) The SYNC instruction in R10000
> >
> > A SYNC instruction is not prevented from graduating if the uncached
> > buffer contains any uncached accelerated stores[1].
> >
> > 3) wbflush() implementation of IDT CPU.
> >
> > IDT CPUs enforce strict write priority (all pending writes retired
> > to memory before main memory is read). Thus, implementing wbflush()
> > is as simple as implementing an uncached load.
> >
> > for loongson's cpu, __wbflush should also be implemented with
> > __fast_iob not sync.
> >
> >
> > Signed-off-by: suijingfeng <suijingfeng@...ngson.cn>
> > ---
> > arch/mips/loongson64/Makefile | 1 +
> > arch/mips/loongson64/setup.c | 17 -----------------
> > arch/mips/loongson64/smp.c | 6 +++---
> > arch/mips/loongson64/wbflush.c | 28 ++++++++++++++++++++++++++++
> > 4 files changed, 32 insertions(+), 20 deletions(-)
> > create mode 100644 arch/mips/loongson64/wbflush.c
> >
> > diff --git a/arch/mips/loongson64/Makefile b/arch/mips/loongson64/Makefile
> > index e806280bbb85..ad00d92c2871 100644
> > --- a/arch/mips/loongson64/Makefile
> > +++ b/arch/mips/loongson64/Makefile
> > @@ -12,3 +12,4 @@ obj-$(CONFIG_SUSPEND) += pm.o
> > obj-$(CONFIG_PCI_QUIRKS) += vbios_quirk.o
> > obj-$(CONFIG_CPU_LOONGSON3_CPUCFG_EMULATION) += cpucfg-emul.o
> > obj-$(CONFIG_SYSFS) += boardinfo.o
> > +obj-$(CONFIG_CPU_HAS_WB) += wbflush.o
> > diff --git a/arch/mips/loongson64/setup.c b/arch/mips/loongson64/setup.c
> > index 6fe3ffffcaa6..cb10d14da433 100644
> > --- a/arch/mips/loongson64/setup.c
> > +++ b/arch/mips/loongson64/setup.c
> > @@ -3,10 +3,7 @@
> > * Copyright (C) 2007 Lemote Inc. & Institute of Computing Technology
> > * Author: Fuxin Zhang, zhangfx@...ote.com
> > */
> > -#include <linux export.h="">
> > #include <linux init.h="">
> > -
> > -#include <asm wbflush.h="">
> > #include <asm bootinfo.h="">
> > #include <linux libfdt.h="">
> > #include <linux of_fdt.h="">
> > @@ -17,20 +14,6 @@
> >
> > void *loongson_fdt_blob;
> >
> > -static void wbflush_loongson(void)
> > -{
> > - asm(".set\tpush\n\t"
> > - ".set\tnoreorder\n\t"
> > - ".set mips3\n\t"
> > - "sync\n\t"
> > - "nop\n\t"
> > - ".set\tpop\n\t"
> > - ".set mips0\n\t");
> > -}
> > -
> > -void (*__wbflush)(void) = wbflush_loongson;
> > -EXPORT_SYMBOL(__wbflush);
> > -
> > void __init plat_mem_setup(void)
> > {
> > if (loongson_fdt_blob)
> > diff --git a/arch/mips/loongson64/smp.c b/arch/mips/loongson64/smp.c
> > index 660e1de4412a..0d9f249c95f9 100644
> > --- a/arch/mips/loongson64/smp.c
> > +++ b/arch/mips/loongson64/smp.c
> > @@ -42,13 +42,13 @@ static uint32_t core0_c0count[NR_CPUS];
> > #define loongson3_ipi_write32(action, addr) \
> > do { \
> > writel(action, addr); \
> > - __wbflush(); \
> > + wbflush(); \
> > } while (0)
> > /* write a 64bit value to ipi register */
> > #define loongson3_ipi_write64(action, addr) \
> > do { \
> > writeq(action, addr); \
> > - __wbflush(); \
> > + wbflush(); \
> > } while (0)
> >
> > static u32 (*ipi_read_clear)(int cpu);
> > @@ -418,7 +418,7 @@ static irqreturn_t loongson3_ipi_interrupt(int irq,
> > void *dev_id)
> > c0count = c0count ? c0count : 1;
> > for (i = 1; i < nr_cpu_ids; i++)
> > core0_c0count[i] = c0count;
> > - __wbflush(); /* Let others see the result ASAP */
> > + wbflush(); /* Let others see the result ASAP */
> > }
> >
> > return IRQ_HANDLED;
> > diff --git a/arch/mips/loongson64/wbflush.c b/arch/mips/loongson64/wbflush.c
> > new file mode 100644
> > index 000000000000..49f0e4c53196
> > --- /dev/null
> > +++ b/arch/mips/loongson64/wbflush.c
> > @@ -0,0 +1,28 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +/*
> > + * This file is subject to the terms and conditions of the GNU General Public
> > + * License. See the file "COPYING" in the main directory of this archive
> > + * for more details.
> > + *
> > + * Copyright (C) 2021 suijingfeng@...ngson.cn
> > + */
> > +#include <linux export.h="">
> > +#include <linux init.h="">
> > +#include <asm wbflush.h="">
> > +#include <asm barrier.h="">
> > +
> > +#ifdef CONFIG_CPU_HAS_WB
> > +
> > +/*
> > + * I/O ASIC systems use a standard writeback buffer that gets flushed
> > + * upon an uncached read.
> > + */
> > +static void wbflush_mips(void)
> > +{
> > + __fast_iob();
> > +}
> > +
> > +void (*__wbflush)(void) = wbflush_mips;
> > +EXPORT_SYMBOL(__wbflush);
> > +
> > +#endif
> > --
> > 2.25.1
>
> --
> - Jiaxun
</asm></asm></linux></linux></linux></linux></asm></asm></linux></linux></suijingfeng@...ngson.cn></linux-mips@...r.kernel.org></tsbogend@...ha.franken.de></chenhuacai@...nel.org></suijingfeng@...ngson.cn></jiaxun.yang@...goat.com>
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