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

Leizhen (ThunderTown)
2021-01-29T07:24:49Z

On 2021/1/28 22:24, Arnd Bergmann wrote:
> On Sat, Jan 16, 2021 at 4:27 AM Zhen Lei <thunder.leizhen@huawei.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.

Right, this readl() can be replaced with readl_relaxed(). Thanks.

I'll check and correct the readl() and writel() in other places.

>
>> +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 don't think there's any essential difference between the two! I don't know
if the compiler or tool will do anything extra. I checked the git log of the
l2x0 driver and it used raw_spin_lock_irqsave() at the beginning. Maybe
there's a description in 2.6. Since you mentioned this potential risk, I'll
change it to raw_spin_lock_irqsave.

include/linux/spinlock.h：
static __always_inline raw_spinlock_t *spinlock_check(spinlock_t *lock)
{
return &lock->rlock;
}

#define spin_lock_irqsave(lock, flags) \
do { \
raw_spin_lock_irqsave(spinlock_check(lock), flags); \
} while (0)

>
> 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.

I have added "depends on ARCH_KUNPENG50X" for this driver. But it's OK to
return 0.

>
>> 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.

Okay, I'll move it.

>
> Arnd
>
> .
>