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

[Philippe Mathieu-Daudé]

On 10/5/20 9:22 PM, Eduardo Habkost wrote:
> On Mon, Oct 05, 2020 at 08:29:24PM +0200, Philippe Mathieu-Daudé wrote:
>> 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.
> 
> The interchangeable usage of "clock source" and "clock input" is
> what confuses me here.  CPUState.clock seems to be a clock input,
> which may or may not be connected to a clock source.
> 
> You seem to imply that "clock source" and "clock input" are
> synonymous, but that's not what I understand from the clock API
> documentation.

Hmm the concepts are different.

>From a clock generator point of view, the "output" is
the point where the signal leaves the block, to be
eventually consumed by a sink. The generator doesn't
know what the sinks are made of:

+-----------+   +----------> Sink1
|           |   |
|           +---+ ClkOut1
|           |   |
|  ClkGen   |   +----------> Sink2
|           |
|           +--> ClkOut2
|           |
+-----------+

>From a device point of view, the "input" is where
the external signal (generated by a source) is
connected. The device doesn't know what is the
source made of:

                     +-----------+
                     |           |
                     |           |
(Source)  ClkIn ----->  Device   |
                     |           |
                     |           |
                     +-----------+

So input/output refer to interface to connect the
clock signal.

Source/sink refer to the other object having a
relation with this signal.

> 
>>
>> 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);  
>>>>>>>   
>>>>>>
>>>>>
>>>>
>>>
>>
>