Re: [PATCH v2] media: mali-c55: fix integer overflow in scaler factor calculation

[David CARLIER]

Hi Jacopo,

  On Sat, May 30, 2026 at 10:55:59AM +0200, Jacopo Mondi wrote:
  > Have you hit this issue ?

  Not on hardware, I found it by code analysis. The sink format is clamped to
  8192 and crop is clamped against the sink, so crop->width can reach
  4096+, where (crop << 20) overflows 32 bits before landing in the u64.
  I don't have a >=4096 source to reproduce on, but it's provable from the
  operand widths and the clamp. UHD (3840) is just under; 4096 gives a
  zero increment, wider values a garbage one.

  > Could we maybe first do the crop/scale division and then do the Q4.20
  > conversion ? We could maybe save the below do_div() [...]

  I don't think we can - dividing first loses the fraction the Q4.20
  factor is there to keep. E.g. crop=4096, scale=1920:

    correct:      4096 * 2^20 / 1920 = 2236962  (~2.133)
    divide-first: (4096 / 1920) << 20 = 2097152  (2.0)   -> ~6.7% off

  So the multiply has to come first, and that pushes the numerator up to
  8192 * 2^20 = 2^33, which needs a 64-bit divide either way. BIT_ULL()
  just does the existing multiply in 64-bit. Happy to switch do_div() to
  div_u64() if you prefer, but that's orthogonal.

Cheers !