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

[Eduardo Habkost <ehabkost@redhat.com>]

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.

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

-- 
Eduardo