Re: [PATCH] media: iris: avoid plain 64-bit division

[David Laight]

On Fri,  2 Oct 2026 14:44:22 +0200
Arnd Bergmann <arnd@xxxxxxxxxx> wrote:

> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> inst->fw_caps[BITRATE].value is a 64-bit integer, which on 32-bit
> architectures cannot be divided:
> 
> ERROR: modpost: drivers/media/platform/qcom/iris/qcom-iris.ko: symbol '__divdi3' undefined!
> 
> In particular, doing three divisions in a row is particularly slow,
> even on 64-bit CPUs.
> 
> Rework the calculation to replace it with a single division, thus
> avoiding the build failure.
> 
> Fixes: 5f21cd9a4ae4 ("media: iris: add support for multi_slice in iris encoder")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> Please check the math, as I'm not sure if this is the intended
> amount of rounding. Previously it was rounding down twice and up
> once, now it just rounds up during the combined division.

Can definitely change the result:
	DIV_ROUND_UP(7 / 3, 2) is 1
but
	DIV_ROUND_UP(7, 2 * 3) is 2
who knows what the value is supposed to be.

> 
> Ideally this would be replaced with a cast of the incoming value
> to u32, but doing this requires to prove that this does not overflow.

You only need to prove that in incoming value is less than 32G (Hz).
The '/ 8' is cheap(ish) and can be done first.

David

> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/media/platform/qcom/iris/iris_ctrls.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/platform/qcom/iris/iris_ctrls.c b/drivers/media/platform/qcom/iris/iris_ctrls.c
> index 8c4c47944f22..5294999fdf7b 100644
> --- a/drivers/media/platform/qcom/iris/iris_ctrls.c
> +++ b/drivers/media/platform/qcom/iris/iris_ctrls.c
> @@ -1588,8 +1588,9 @@ int iris_set_slice_count(struct iris_inst *inst, enum platform_inst_fw_cap_type
>  	} else if (slice_mode == V4L2_MPEG_VIDEO_MULTI_SLICE_MODE_MAX_BYTES) {
>  		hfi_value = inst->fw_caps[SLICE_MAX_BYTES].value;
>  		if (rc_type != HFI_RC_OFF) {
> -			max_avg_slicesize = DIV_ROUND_UP((inst->fw_caps[BITRATE].value / fps) / 8,
> -							 slice_caps->max_slices_per_frame);
> +			max_avg_slicesize = DIV_U64_ROUND_UP(inst->fw_caps[BITRATE].value,
> +							     fps * 8 *
> +							     slice_caps->max_slices_per_frame);
>  		} else {
>  			/*
>  			 * No bitrate target exists under RC_OFF, so approximate a