Re: [PATCH v3 1/3] ARM: uniphier: add outer cache support

[Masahiro Yamada]

Hi Russell,


2015-09-22 4:38 GMT+09:00 Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>:
> On Fri, Sep 18, 2015 at 01:37:32PM +0900, Masahiro Yamada wrote:
>> +/**
>> + * __uniphier_cache_maint_common - run a queue operation for a particular level
>> + *
>> + * @data: cache controller specific data
>> + * @start: start address of range operation (don't care for "all" operation)
>> + * @size: data size of range operation (don't care for "all" operation)
>> + * @operation: flags to specify the desired cache operation
>> + */
>> +static void __uniphier_cache_maint_common(struct uniphier_cache_data *data,
>> +                                       unsigned long start,
>> +                                       unsigned long size,
>> +                                       u32 operation)
>> +{
>> +     unsigned long flags;
>> +
>> +     /*
>> +      * The IRQ must be disable during this sequence because the accessor
>> +      * holds the access right of the operation queue registers.  The IRQ
>> +      * should be restored after releasing the register access right.
>> +      */
>> +     local_irq_save(flags);
>> +
>> +     /* clear the complete notification flag */
>> +     writel_relaxed(UNIPHIER_SSCOLPQS_EF, data->op_base + UNIPHIER_SSCOLPQS);
>> +
>> +     /*
>> +      * We do not need a spin lock here because the hardware guarantees
>> +      * this sequence is atomic, i.e. the write access is arbitrated
>> +      * and only the winner's write accesses take effect.
>> +      * After register settings, we need to check the UNIPHIER_SSCOPPQSEF to
>> +      * see if we won the arbitration or not.
>> +      * If the command was not successfully set, just try again.
>> +      */
>> +     do {
>> +             /* set cache operation */
>> +             writel_relaxed(UNIPHIER_SSCOQM_CE | operation,
>> +                            data->op_base + UNIPHIER_SSCOQM);
>> +
>> +             /* set address range if needed */
>> +             if (likely(UNIPHIER_SSCOQM_S_IS_RANGE(operation))) {
>> +                     writel_relaxed(start, data->op_base + UNIPHIER_SSCOQAD);
>> +                     writel_relaxed(size, data->op_base + UNIPHIER_SSCOQSZ);
>> +             }
>> +
>> +             /* set target ways if needed */
>> +             if (unlikely(UNIPHIER_SSCOQM_TID_IS_WAY(operation)))
>> +                     writel_relaxed(data->way_locked_mask,
>> +                                    data->op_base + UNIPHIER_SSCOQWN);
>> +     } while (unlikely(readl_relaxed(data->op_base + UNIPHIER_SSCOPPQSEF) &
>> +                       (UNIPHIER_SSCOPPQSEF_FE | UNIPHIER_SSCOPPQSEF_OE)));
>> +
>> +     /* wait until the operation is completed */
>> +     while (likely(readl_relaxed(data->op_base + UNIPHIER_SSCOLPQS) !=
>> +                   UNIPHIER_SSCOLPQS_EF))
>> +             cpu_relax();
>> +
>> +     local_irq_restore(flags);
>
> I'm concerned about this.  We've had caches like this (ARM L220) which
> require only one operation to be performed at a time.  In a SMP system,
> that requires a spinlock to prevent one CPU triggering a L2 maintanence
> operation while another CPU tries to operate on the L2 cache.
>
> From the overall series diffstat, I see that you are adding SMP support
> too.  So I have to ask the obvious question: if you need to disable
> local IRQs around the L2 cache operations, what happens if two CPUs
> both try to perform a L2 cache operation concurrently?


This cache controller is able to accept operations from multiple CPUs
at the same time.

Let's assume the following scenario:

CPU0 issues some operation.
Before the cache controller finishes the operation,
CPU1 issues another operation;  this is OK.
The operation is stored in the queue of the cache controller
until the operation under way is completed.
When the operation from CPU0 is finished, the controller starts
the operation from CPU1.

If the queue is full (this unlikely happens though),
the CPU can know by checking UNIPHIER_SSCOPPQSEF register.
This is checked by the code:

unlikely(readl_relaxed(data->op_base + UNIPHIER_SSCOPPQSEF) &
                       (UNIPHIER_SSCOPPQSEF_FE | UNIPHIER_SSCOPPQSEF_OE))


The status register (UNIPHIER_SSCOLPQS) has each instance for each CPU.
That means, CPU0 can know if the operation issued by itself is finished or not.
Likewise for CPU1, CPU2, ...

To sum up, the cache controller can nicely handles cache operations in SMP.



-- 
Best Regards
Masahiro Yamada
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