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

[Philippe Mathieu-Daudé]

On 10/5/20 8:09 PM, Philippe Mathieu-Daudé wrote:
> 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.

Hard to reword when it looks clear to oneself...

@clock is the source, @cpu is the sink.
@clock clocks @cpu at some frequency.

One output from @clock is the @cpu.
The @cpu has an unique input: @clock.

Damien/Peter/Luc, do you have better description suggestions?

> 
>>
>>
>>>>>>   * @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);  
>>>>>   
>>>>
>>>
>>
>