Re: [PATCH] hw/nvram: Avoid unnecessary Xilinx eFuse backstore write

[Francisco Iglesias <frasse.iglesias@gmail.com>]

On [2023 Apr 26] Wed 14:16:07, Tong Ho wrote:
> Add a check in the bit-set operation to write the backstore
> only if the affected bit is 0 before.
> 
> With this in place, there will be no need for callers to
> do the checking in order to avoid unnecessary writes.
> 
> Signed-off-by: Tong Ho <tong.ho@amd.com>

Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>

> ---
>  hw/nvram/xlnx-efuse.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/nvram/xlnx-efuse.c b/hw/nvram/xlnx-efuse.c
> index fdfffaab99..655c40b8d1 100644
> --- a/hw/nvram/xlnx-efuse.c
> +++ b/hw/nvram/xlnx-efuse.c
> @@ -143,6 +143,8 @@ static bool efuse_ro_bits_find(XlnxEFuse *s, uint32_t k)
>  
>  bool xlnx_efuse_set_bit(XlnxEFuse *s, unsigned int bit)
>  {
> +    uint32_t set, *row;
> +
>      if (efuse_ro_bits_find(s, bit)) {
>          g_autofree char *path = object_get_canonical_path(OBJECT(s));
>  
> @@ -152,8 +154,13 @@ bool xlnx_efuse_set_bit(XlnxEFuse *s, unsigned int bit)
>          return false;
>      }
>  
> -    s->fuse32[bit / 32] |= 1 << (bit % 32);
> -    efuse_bdrv_sync(s, bit);
> +    /* Avoid back-end write unless there is a real update */
> +    row = &s->fuse32[bit / 32];
> +    set = 1 << (bit % 32);
> +    if (!(set & *row)) {
> +        *row |= set;
> +        efuse_bdrv_sync(s, bit);
> +    }
>      return true;
>  }
>  
> -- 
> 2.25.1
>