Re: [PATCH 1/1] include/qemu/bitops.h: Add deposit8 for uint8_t bit operation

[Jason Fan]

Re-send this to include the original mail-list.

Hi Peter, 

I am working on a i3c target model which requires bitops on the uint8_t registers.

deposit8 can help to check incorrect length or start input for 8 bit value. 

You are right that desposit32 should also work if we always pass the correct arguments, but since the implementation seems trivial enough, I just go ahead and create the patch.

Thank you.

On Tue, Aug 27, 2024 at 2:23 AM Peter Maydell <peter.maydell@linaro.org> wrote:

On Mon, 26 Aug 2024 at 22:01, Jason Fan <fanjason@google.com> wrote:
>
> Signed-off-by: Jason Fan <fanjason@google.com>
> ---
>  include/qemu/bitops.h | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
> index 2c0a2fe751..d01c4b42f2 100644
> --- a/include/qemu/bitops.h
> +++ b/include/qemu/bitops.h
> @@ -459,6 +459,32 @@ static inline int64_t sextract64(uint64_t value, int start, int length)
>      return ((int64_t)(value << (64 - length - start))) >> (64 - length);
>  }
>
> +/**
> + * deposit8:
> + * @value: initial value to insert bit field into
> + * @start: the lowest bit in the bit field (numbered from 0)
> + * @length: the length of the bit field
> + * @fieldval: the value to insert into the bit field
> + *
> + * Deposit @fieldval into the 8 bit @value at the bit field specified
> + * by the @start and @length parameters, and return the modified
> + * @value. Bits of @value outside the bit field are not modified.
> + * Bits of @fieldval above the least significant @length bits are
> + * ignored. The bit field must lie entirely within the 8 bit word.
> + * It is valid to request that all 8 bits are modified (ie @length
> + * 8 and @start 0).
> + *
> + * Returns: the modified @value.
> + */
> +static inline uint8_t deposit8(uint8_t value, int start, int length,
> +                               uint8_t fieldval)
> +{
> +    uint8_t mask = 0xFF;
> +    assert(start >= 0 && length > 0 && length <= 8 - start);
> +    mask = (mask >> (8 - length)) << start;
> +    return (value & ~mask) | ((fieldval << start) & mask);
> +}

What's the use case for this? Where would you need this
and not be able to use "deposit32" instead?

thanks
-- PMM