Re: [PATCH v4 3/6] bswap: Add the ability to store to an unaligned 24 bit field

[Philippe Mathieu-Daudé]

On 3/3/23 16:09, Jonathan Cameron wrote:
> From: Ira Weiny <ira.weiny@intel.com>
>
> CXL has 24 bit unaligned fields which need to be stored to.  CXL is
> specified as little endian.
>
> Define st24_le_p() and the supporting functions to store such a field
> from a 32 bit host native value.
>
> The use of b, w, l, q as the size specifier is limiting.  So "24" was
> used for the size part of the function name.
>
> Reviewed-by: Fan Ni <fan.ni@samsung.com>
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>
> ---
> v8:
>    - Picked up tag from Fan Ni.
> ---
>   include/qemu/bswap.h | 23 +++++++++++++++++++++++
>   1 file changed, 23 insertions(+)
>
> diff --git a/include/qemu/bswap.h b/include/qemu/bswap.h
> index 15a78c0db5..ee71cbeaaa 100644
> --- a/include/qemu/bswap.h
> +++ b/include/qemu/bswap.h
> @@ -8,11 +8,23 @@
>   #undef  bswap64
>   #define bswap64(_x) __builtin_bswap64(_x)
>   
> +static inline uint32_t bswap24(uint32_t x)
> +{

Could it be safer to add:

       assert(x & 0xff000000U == 0);

...

> +    return (((x & 0x000000ffU) << 16) |
> +            ((x & 0x0000ff00U) <<  0) |
> +            ((x & 0x00ff0000U) >> 16));
> +}
> +
>   static inline void bswap16s(uint16_t *s)
>   {
>       *s = __builtin_bswap16(*s);
>   }
>   
> +static inline void bswap24s(uint32_t *s)
> +{
> +    *s = bswap24(*s);

... and here use:

       *s = bswap24(*s & 0x00ffffffU);

?

> +}
> +
>   static inline void bswap32s(uint32_t *s)
>   {
>       *s = __builtin_bswap32(*s);
> @@ -176,6 +188,7 @@ CPU_CONVERT(le, 64, uint64_t)
>    * size is:
>    *   b: 8 bits
>    *   w: 16 bits
> + *   24: 24 bits

Following the pattern, shouldn't this be 's' for sēsquiword?

Regardless you need to update the doc in docs/devel/loads-stores.rst.

>    *   l: 32 bits
>    *   q: 64 bits
>    *
> @@ -248,6 +261,11 @@ static inline void stw_he_p(void *ptr, uint16_t v)
>       __builtin_memcpy(ptr, &v, sizeof(v));
>   }
>   
> +static inline void st24_he_p(void *ptr, uint32_t v)
> +{
> +    __builtin_memcpy(ptr, &v, 3);
> +}
> +
>   static inline int ldl_he_p(const void *ptr)
>   {
>       int32_t r;
> @@ -297,6 +315,11 @@ static inline void stw_le_p(void *ptr, uint16_t v)
>       stw_he_p(ptr, le_bswap(v, 16));
>   }
>   
> +static inline void st24_le_p(void *ptr, uint32_t v)
> +{
> +    st24_he_p(ptr, le_bswap(v, 24));
> +}