Re: [PATCH 01/16] hw/core/cpu: Let CPU object have a clock source

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

On 10/5/20 7:44 PM, Eduardo Habkost wrote:
> On Mon, Oct 05, 2020 at 06:40:09PM +0200, Igor Mammedov wrote:
>> On Wed, 30 Sep 2020 12:16:53 +0200
>> Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>>
>>> +arm/ppc/riscv folks
>>>
>>> On 9/30/20 9:43 AM, Igor Mammedov wrote:
>>>> On Mon, 28 Sep 2020 19:15:24 +0200
>>>> Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>>>>   
>>>>> Let CPUState have a clock source (named 'clk') and CPUClass
> 
> The language here confuses me: is this a clock source inside the
> CPU, or just a clock input that can be connected to a clock
> source somewhere?

2nd description, "somewhere". I'll reword.

> 
> See also comment below[1].
> 
>>>>> have a clock_update() callback. The clock can be optionally
>>>>> set Using qdev_connect_clock_in() from the Clock API.
>>>>> If the clock changes, the optional clock_update() will be
>>>>> called.  
> 
> What does "clock change" means?  Is this just about the
> frequency, or something else?

A frequency changes -- which can be because a parent (in the
clock tree) changed its source using a MUX.

> 
> (By reading the Clock API documentation, it looks like it only
> represents the clock frequency, but I'm not sure)
> 
>>>>
>>>> the sole user of it is mips cpu, so question is why
>>>> you are making it part of generic CPUm instead of
>>>> MIPSCPUClass/MIPSCPU?  
>>>
>>> This is a feature of the CPU, regardless its architecture.
>>>
>>> I expect the other archs to start using it soon.
>>
>> if there aren't any plans to actually to do that,
>> I'd keep it to MIPS class and generalize later when there is demand.

No problem.

> 
> I normally don't mind if a feature is generic from the beginning.
> But in this case I'm inclined to agree with Igor.  Unless we
> expect to see arch-independent code to use CPUState.clock soon
> (do we?), having CPUState.clock existing but unused by most
> architectures would be misleading.
> 
> Also, at least on x86 there are so many different clock sources,
> that I'm not sure it would be a desirable to have a generic clock
> input named "clk".

Well X86 is the arch I'm less confident with. Anyhow if it has
multiple clock sources, I'd expect a Clock MUX block to select
an unique clock to feed the CPU.

> 
>>  
>>>
>>>>   
>>>>>
>>>>> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
>>>>> ---
>>>>>  include/hw/core/cpu.h |  5 +++++
>>>>>  hw/core/cpu.c         | 12 ++++++++++++
>>>>>  2 files changed, 17 insertions(+)
>>>>>
>>>>> diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
>>>>> index 6c34798c8b3..6989d90c193 100644
>>>>> --- a/include/hw/core/cpu.h
>>>>> +++ b/include/hw/core/cpu.h
>>>>> @@ -31,6 +31,7 @@
>>>>>  #include "qemu/thread.h"
>>>>>  #include "qemu/plugin.h"
>>>>>  #include "qom/object.h"
>>>>> +#include "hw/clock.h"
>>>>>  
>>>>>  typedef int (*WriteCoreDumpFunction)(const void *buf, size_t size,
>>>>>                                       void *opaque);
>>>>> @@ -155,6 +156,7 @@ struct TranslationBlock;
>>>>>   * @disas_set_info: Setup architecture specific components of disassembly info
>>>>>   * @adjust_watchpoint_address: Perform a target-specific adjustment to an
>>>>>   * address before attempting to match it against watchpoints.
>>>>> + * @clock_update: Callback for input clock changes
>>>>>   *
>>>>>   * Represents a CPU family or model.
>>>>>   */
>>>>> @@ -176,6 +178,7 @@ struct CPUClass {
>>>>>                                    unsigned size, MMUAccessType access_type,
>>>>>                                    int mmu_idx, MemTxAttrs attrs,
>>>>>                                    MemTxResult response, uintptr_t retaddr);
>>>>> +    void (*clock_update)(CPUState *cpu);
>>>>>      bool (*virtio_is_big_endian)(CPUState *cpu);
>>>>>      int (*memory_rw_debug)(CPUState *cpu, vaddr addr,
>>>>>                             uint8_t *buf, int len, bool is_write);
>>>>> @@ -316,6 +319,7 @@ struct qemu_work_item;
>>>>>   *   QOM parent.
>>>>>   * @nr_cores: Number of cores within this CPU package.
>>>>>   * @nr_threads: Number of threads within this CPU.
>>>>> + * @clock: this CPU source clock (an output clock of another device)
> 
> [1]
> 
> What does "source clock" means?  Is this the same as "clock input"?

Yes, for clocks it is common to use source/sink instead of input/output.
I'll try to reword.

> 
> 
>>>>>   * @running: #true if CPU is currently running (lockless).
>>>>>   * @has_waiter: #true if a CPU is currently waiting for the cpu_exec_end;
>>>>>   * valid under cpu_list_lock.
>>>>> @@ -400,6 +404,7 @@ struct CPUState {
>>>>>      int num_ases;
>>>>>      AddressSpace *as;
>>>>>      MemoryRegion *memory;
>>>>> +    Clock *clock;
>>>>>  
>>>>>      void *env_ptr; /* CPUArchState */
>>>>>      IcountDecr *icount_decr_ptr;
>>>>> diff --git a/hw/core/cpu.c b/hw/core/cpu.c
>>>>> index c55c09f734c..37fcff3ec64 100644
>>>>> --- a/hw/core/cpu.c
>>>>> +++ b/hw/core/cpu.c
>>>>> @@ -30,6 +30,7 @@
>>>>>  #include "qemu/qemu-print.h"
>>>>>  #include "sysemu/tcg.h"
>>>>>  #include "hw/boards.h"
>>>>> +#include "hw/qdev-clock.h"
>>>>>  #include "hw/qdev-properties.h"
>>>>>  #include "trace/trace-root.h"
>>>>>  #include "qemu/plugin.h"
>>>>> @@ -247,6 +248,16 @@ void cpu_reset(CPUState *cpu)
>>>>>      trace_guest_cpu_reset(cpu);
>>>>>  }
>>>>>  
>>>>> +static void cpu_clk_update(void *opaque)
>>>>> +{
>>>>> +    CPUState *cpu = opaque;
>>>>> +    CPUClass *cc = CPU_GET_CLASS(cpu);
>>>>> +
>>>>> +    if (cc->clock_update) {
>>>>> +        cc->clock_update(cpu);
>>>>> +    }
>>>>> +}
>>>>> +
>>>>>  static void cpu_common_reset(DeviceState *dev)
>>>>>  {
>>>>>      CPUState *cpu = CPU(dev);
>>>>> @@ -367,6 +378,7 @@ static void cpu_common_initfn(Object *obj)
>>>>>      /* the default value is changed by qemu_init_vcpu() for softmmu */
>>>>>      cpu->nr_cores = 1;
>>>>>      cpu->nr_threads = 1;
>>>>> +    cpu->clock = qdev_init_clock_in(DEVICE(obj), "clk", cpu_clk_update, cpu);
>>>>>  
>>>>>      qemu_mutex_init(&cpu->work_mutex);
>>>>>      QSIMPLEQ_INIT(&cpu->work_list);  
>>>>   
>>>
>>
>