Re: [PATCH] drm/xe/mmio: Fix xe_mmio_wait32() to honor delay/sleep maximums

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Medium] The custom helper `__div_round_up64` silently truncates 64-bit divisors to 32 bits, creating a mathematically unsound helper.
- [Low] Inverted architecture check for 64-bit division causes a `__divdi3` link error on 32-bit builds, and needlessly uses a custom fallback on 64-bit builds.
--

commit 2d4751e6d3870228fa27b4fdbbb10070a48ae9ce
Author: Alan Previn <alan.previn.teres.alexis@intel.com>

drm/xe/mmio: Fix xe_mmio_wait32() to honor delay/sleep maximums

This commit fixes xe_mmio_wait32() to honor delay and sleep maximums by
checking for overflow in udelay and usleep_range. It also adds a custom
64-bit division helper and caps the in-loop wait time to prevent
intolerably large delays.

> diff --git a/drivers/gpu/drm/xe/xe_mmio.c b/drivers/gpu/drm/xe/xe_mmio.c
> index 7fa18dfcb5a28..1ae382f009de5 100644
> --- a/drivers/gpu/drm/xe/xe_mmio.c
> +++ b/drivers/gpu/drm/xe/xe_mmio.c
[ ... ]
> +static inline s64 __div_round_up64(s64 dividend, s64 divisor)
> +{
> +	u64 abs_dividend = abs(dividend);
> +	u64 abs_divisor = abs(divisor);
> +	u64 result = abs_dividend + abs_divisor - 1;
> +
> +	do_div(result, abs_divisor);

[Severity: Medium]
Does passing a 64-bit divisor to do_div() cause silent truncation here?

The do_div() macro is strictly designed for 32-bit divisors and will
silently truncate the 64-bit abs_divisor parameter. While the current caller
in __xe_mmio_wait32() only passes USEC_PER_MSEC (which fits in 32 bits),
creating a generic helper that accepts an s64 divisor is structurally
unsafe.

Could this use DIV64_U64_ROUND_UP() from linux/math64.h instead?

> +	/* dont check for negative values as local caller only uses positive numbers */
> +	return (s64)result;
> +}
[ ... ]
> @@ -349,11 +375,25 @@ static int __xe_mmio_wait32(struct xe_mmio *mmio, struct xe_reg reg, u32 mask, u
[ ... ]
> -		if (atomic)
> -			udelay(wait);
> -		else
> -			usleep_range(wait, wait << 1);
> -		wait <<= 1;
> +#define __XE_MMIO_WAIT_MAX_INLOOP_100MS (100 * USEC_PER_MSEC)
> +		if (atomic) {
> +			if (wait <= MAX_UDELAY_MS * USEC_PER_MSEC)
> +				udelay(wait);
> +			else if (BITS_PER_LONG == 32)
> +				mdelay(DIV_ROUND_UP(wait, USEC_PER_MSEC));

[Severity: Low]
Does this architecture check invert the intended 64-bit division logic?

When BITS_PER_LONG == 32, this branch executes the native 64-bit division
DIV_ROUND_UP(wait, USEC_PER_MSEC). This causes a __divdi3 unresolved symbol
link error on 32-bit builds, which appears to be the exact build break this
helper was intended to avoid.

> +			else
> +				mdelay(__div_round_up64(wait, USEC_PER_MSEC));

Consequently, 64-bit builds will take this else branch and needlessly use
the custom division helper instead of the native division macro.

> +		} else {
> +			usleep_range(wait, wait + (wait >> 2)); /* range till wait + 25% */
> +		}
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260908185609.359565-2-alan.previn.teres.alexis@intel.com?part=1