Re: [PATCH] hw/mips/loongson3_virt: Implement node counter timer

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Hi Jiaxun,

On 12/5/24 14:20, Jiaxun Yang wrote:
> Node counter is a timer presents on Loongson-3 chips, which runs
> as fast as CPU clock. It's being mapped into a MMIO location.
> 
> Emulate this for loongson3_virt machine, in hope that kernel can
> use it as a better clock source.
> 
> Hardware's behavior on 32-bit read/write is also emulated in case
> legacy kernel is trying to use it with hi/lo splitted read.
> 
> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
> ---
>   hw/mips/loongson3_bootp.h |  1 +
>   hw/mips/loongson3_virt.c  | 38 ++++++++++++++++++++++++++++++++++----
>   2 files changed, 35 insertions(+), 4 deletions(-)


> +static uint64_t loongson3_nodecnt_read(void *opaque,
> +                                        hwaddr addr, unsigned size)
> +{
> +    LoongsonMachineState *s = opaque;
> +    int64_t now_ns = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +    uint64_t ticks = clock_ns_to_ticks(s->cpuclk, now_ns);
> +
> +    if (addr == 0x4) {
> +        return ticks >> 32;

Does that imply .endianness = DEVICE_BIG_ENDIAN?

It could be simpler to let the core MMIO code do the hi/lo
bits management using ".impl.min_access_size = 8".

> +    }
> +
> +    return ticks;
> +}
> +
> +static const MemoryRegionOps loongson3_nodecnt_ops = {
> +    .read  = loongson3_nodecnt_read,
> +    .endianness = DEVICE_NATIVE_ENDIAN,
> +    .valid.min_access_size = 4,
> +    .valid.max_access_size = 8,
> +    .impl.min_access_size = 4,
> +    .impl.max_access_size = 8,
> +};