linux-kernel - Re: [PATCH v5 4/4] ARM: Add support for Hisilicon Kunpeng L3 cache controller

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Message-ID: <CAK8P3a1OqUn5A4F4hT4K=bzQwJuifVFZkvFoK6NMg+m9FjoKzw@mail.gmail.com>
Date:   Thu, 28 Jan 2021 15:24:20 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Russell King <rmk+kernel@....linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>,
        Wei Xu <xuwei5@...ilicon.com>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 4/4] ARM: Add support for Hisilicon Kunpeng L3 cache controller

On Sat, Jan 16, 2021 at 4:27 AM Zhen Lei <thunder.leizhen@...wei.com> wrote:
> diff --git a/arch/arm/mm/Makefile b/arch/arm/mm/Makefile
> +
> +static void l3cache_maint_common(u32 range, u32 op_type)
> +{
> +       u32 reg;
> +
> +       reg = readl(l3_ctrl_base + L3_MAINT_CTRL);
> +       reg &= ~(L3_MAINT_RANGE_MASK | L3_MAINT_TYPE_MASK);
> +       reg |= range | op_type;
> +       reg |= L3_MAINT_STATUS_START;
> +       writel(reg, l3_ctrl_base + L3_MAINT_CTRL);

Are there contents of L3_MAINT_CTRL that need to be preserved
across calls and can not be inferred? A 'readl()' is often expensive,
so it might be more efficient if you can avoid that.

> +static inline void l3cache_flush_all_nolock(void)
> +{
> +       l3cache_maint_common(L3_MAINT_RANGE_ALL, L3_MAINT_TYPE_FLUSH);
> +}
> +
> +static void l3cache_flush_all(void)
> +{
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&l3cache_lock, flags);
> +       l3cache_flush_all_nolock();
> +       spin_unlock_irqrestore(&l3cache_lock, flags);
> +}

I see that cache-l2x0 uses raw_spin_lock_irqsave() instead of
spin_lock_irqsave(), to avoid preemption in the middle of a cache
operation. This is probably a good idea here as well.

I also see that l2x0 uses readl_relaxed(), to avoid a deadlock
in l2x0_cache_sync(). This may also be beneficial for performance
reasons, so it might be helpful to compare performance
overhead. On the other hand, readl()/writel() are usually the
safe choice, as those avoid the need to argue over whether
the relaxed versions are safe in all corner cases.

> +static int __init l3cache_init(void)
> +{
> +       u32 reg;
> +       struct device_node *node;
> +
> +       node = of_find_matching_node(NULL, l3cache_ids);
> +       if (!node)
> +               return -ENODEV;

I think the initcall should return '0' to indicate success when running
a kernel with this driver built-in on a platform that does not have
this device.

> diff --git a/arch/arm/mm/cache-kunpeng-l3.h b/arch/arm/mm/cache-kunpeng-l3.h
> new file mode 100644
> index 000000000000000..9ef6a53e7d4db49
> --- /dev/null
> +++ b/arch/arm/mm/cache-kunpeng-l3.h
> @@ -0,0 +1,30 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __CACHE_KUNPENG_L3_H
> +#define __CACHE_KUNPENG_L3_H
> +
> +#define L3_CACHE_LINE_SHITF            6

I would suggest moving the contents of the header file into the .c file,
since there is only a single user of these macros.

          Arnd