Re: [PATCH v2] lspci: Show Slot Power Limit values above EFh

[Matthew Wilcox <willy@infradead.org>]

On Mon, Nov 01, 2021 at 03:47:40PM +0100, Pali Rohár wrote:
> PCI Express Base Specification rev. 3.0 has the following definition for
> the Slot Power Limit Value:
> 
> =======================================================================
> When the Slot Power Limit Scale field equals 00b (1.0x) and Slot Power
> Limit Value exceeds EFh, the following alternative encodings are used:
>   F0h = 250 W Slot Power Limit
>   F1h = 275 W Slot Power Limit
>   F2h = 300 W Slot Power Limit
>   F3h to FFh = Reserved for Slot Power Limit values above 300 W
> =======================================================================
> 
> Replace function power_limit() by show_power_limit() which also prints
> power limit value. Show reserved value as string ">300W" and omit usage of
> floating point variables as it is not needed.

I don't understand why you want to avoid the use of floating point here?

> +++ b/ls-caps.c
> @@ -656,10 +656,27 @@ static int exp_downstream_port(int type)
>  	 type == PCI_EXP_TYPE_PCIE_BRIDGE;	/* PCI/PCI-X to PCIe Bridge */
>  }
>  
> -static float power_limit(int value, int scale)
> +static void show_power_limit(int value, int scale)
>  {
> -  static const float scales[4] = { 1.0, 0.1, 0.01, 0.001 };
> -  return value * scales[scale];
> +  static const int scales[4] = { 1000, 100, 10, 1 };
> +  static const int scale0_values[3] = { 250, 275, 300 };
> +  if (scale == 0 && value >= 0xF0) {
> +    /* F3h to FFh = Reserved for Slot Power Limit values above 300 W */
> +    if (value >= 0xF3) {
> +      printf(">300W");
> +      return;
> +    }
> +    value = scale0_values[value - 0xF0];
> +  }
> +  value *= scales[scale];
> +  printf("%d", value / 1000);
> +  if (value % 10)
> +    printf(".%03d", value % 1000);
> +  else if (value % 100)
> +    printf(".%02d", (value / 10) % 100);
> +  else if (value % 1000)
> +    printf(".%d", (value / 100) % 10);
> +  printf("W");

Wouldn't this be clearer if written as:

static void show_power_limit(int value, int scale)
{
  static const float scales[4] = { 1.0, 0.1, 0.01, 0.001 };
  static const int scale0_values[3] = { 250, 275, 300 };

  if (scale == 0 && value >= 0xF0) {
    /* F3h to FFh = Reserved for Slot Power Limit values above 300 W */
    if (value >= 0xF3) {
      printf(">300W");
      return;
    }
    value = scale0_values[value - 0xF0];
  }
  printf("%.3fW", value * scales[scale]);
}