Re: [PATCH v5 02/10] phy: rockchip: samsung-hdptx: Prevent divide-by-zero when computing clk rate

[Vinod Koul <vkoul@kernel.org>]

On 07-08-26, 17:19, Cristian Ciocaltea wrote:
> Hi Mani,
> 
> On 8/7/26 3:56 PM, Manivannan Sadhasivam wrote:
> > On Thu, Jul 23, 2026 at 10:41:45PM +0300, Cristian Ciocaltea wrote:
> >> Calculating 'sdm' fraction in rk_hdptx_phy_clk_calc_rate_from_pll_cfg()
> >> could trigger a divide-by-zero, as it uses div_u64() with a denominator
> >> read directly from hardware: the values ropll_hw.sdm_deno,
> >> ropll_hw.sdc_deno, ropll_hw.sdc_n, and ropll_hw.sdc_num are populated
> >> from PLL registers which, in theory, could be left by the bootloader
> >> uninitialized/misconfigured.
> >>
> >> Provide the necessary sanitization to avoid trusting the hardware state.
> >>
> >> Reported-by: Sashiko <sashiko-bot@kernel.org>
> >> Closes: https://sashiko.dev/#/patchset/20260611-hdptx-clk-fixes-v3-0-67b1b0c00e16@collabora.com?part=1
> >> Fixes: 3481fc04d969 ("phy: rockchip: samsung-hdptx: Compute clk rate from PLL config")
> >> Tested-by: Diederik de Haas <diederik@cknow-tech.com>  # NanoPC-T6 LTS
> >> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
> >> Reviewed-by: Andy Yan <andy.yan@rock-chips.com>
> >> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
> >> ---
> >>  drivers/phy/rockchip/phy-rockchip-samsung-hdptx.c | 11 ++++++++---
> >>  1 file changed, 8 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/phy/rockchip/phy-rockchip-samsung-hdptx.c b/drivers/phy/rockchip/phy-rockchip-samsung-hdptx.c
> >> index 710603afff86..44e99343f249 100644
> >> --- a/drivers/phy/rockchip/phy-rockchip-samsung-hdptx.c
> >> +++ b/drivers/phy/rockchip/phy-rockchip-samsung-hdptx.c
> >> @@ -2270,10 +2270,15 @@ static u64 rk_hdptx_phy_clk_calc_rate_from_pll_cfg(struct rk_hdptx_phy *hdptx)
> >>  
> >>  	fout = PLL_REF_CLK * ropll_hw.pms_mdiv;
> >>  	if (ropll_hw.sdm_en) {
> >> +		val = 16U * ropll_hw.sdm_deno *
> >> +		      (ropll_hw.sdc_deno * ropll_hw.sdc_n - ropll_hw.sdc_num);
> >> +		if (!val) {
> >> +			dev_dbg(hdptx->dev, "Invalid ROPLL hw state: deno == 0\n");
> > 
> > If the hardware state is invalid, why can't this be a hard failure?
> 
> This is an internal helper called from the .recalc_rate clk_op, which tries to
> compute the current rate by reading back the hardware state. 
> 
> Since .recalc_rate returns unsigned long, we cannot propagate errors to the
> caller.  However, per the clk_ops documentation, the driver is expected to
> return 0 if it cannot figure out the rate.
> 
> Moreover, this condition isn't really fatal, as it only means the PLL
> configuration currently stored in hardware can't be translated into a rate.  The
> next .set_rate should program a valid configuration and help with the recovery.

This is a good explanation which may not be obvious to everyone. I would
think that it would be good idea to add a comment for this here

-- 
~Vinod

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