[PATCH libdrm] xf86drm.c: Use integer logarithm.

[Pekka Paalanen]

On Mon, 26 Oct 2020 16:11:20 +0300
Paul Gofman <pgofman at codeweavers.com> wrote:

> log() is affected by FP control word and can provide inaccurate result.
> 
> Fixes Killer Instinct under Wine not being able to find AMD vulkan
> device.
> 
> Signed-off-by: Paul Gofman <pgofman at codeweavers.com>
> ---
>     With the rounding mode the application sets (unsigned int)log2(4) is 1.
>     The log2_int() implemetation is copied from radeon/radeon_surface.c.
> 
>  xf86drm.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/xf86drm.c b/xf86drm.c
> index 50a6f092..dbb7c14b 100644
> --- a/xf86drm.c
> +++ b/xf86drm.c
> @@ -124,6 +124,22 @@ static drmServerInfoPtr drm_server_info;
>  static bool drmNodeIsDRM(int maj, int min);
>  static char *drmGetMinorNameForFD(int fd, int type);
>  
> +static unsigned log2_int(unsigned x)
> +{
> +    unsigned l;
> +
> +    if (x < 2) {
> +        return 0;
> +    }
> +    for (l = 2; ; l++) {
> +        if ((unsigned)(1 << l) > x) {

Hi,

wouldn't this loop fail to end when x >= 0x80000000?

Sure, such value probably cannot occur where this is currently used,
but it seems like a landmine for the next developer to step on.


Thanks,
pq

> +            return l - 1;
> +        }
> +    }
> +    return 0;
> +}
> +
> +
>  drm_public void drmSetServerInfo(drmServerInfoPtr info)
>  {
>      drm_server_info = info;
> @@ -4001,7 +4017,7 @@ static void drmFoldDuplicatedDevices(drmDevicePtr local_devices[], int count)
>          for (j = i + 1; j < count; j++) {
>              if (drmDevicesEqual(local_devices[i], local_devices[j])) {
>                  local_devices[i]->available_nodes |= local_devices[j]->available_nodes;
> -                node_type = log2(local_devices[j]->available_nodes);
> +                node_type = log2_int(local_devices[j]->available_nodes);
>                  memcpy(local_devices[i]->nodes[node_type],
>                         local_devices[j]->nodes[node_type], drmGetMaxNodeName());
>                  drmFreeDevice(&local_devices[j]);

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