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charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Maxime, On 10/10/22 16:49, Maxime Ripard wrote: > Hi, > > On Sun, Oct 02, 2022 at 10:17:24AM +0800, Quanyang Wang wrote: >> On 10/1/22 18:40, Maxime Ripard wrote: >>> Hi >>> >>> On Fri, Sep 30, 2022 at 05:05:01PM -0700, Stephen Boyd wrote: >>>> +Maxime >>>> >>>> Quoting Quanyang Wang (2022-09-28 18:05:10) >>>>> Hi Laurent, >>>>> >>>>> I have sent a patch as below to fix this issue which set rate failed and >>>>> it's in linux-next repo now. >>>>> >>>>> https://lore.kernel.org/linux-arm-kernel/20220826142030.213805-1-quanyang.wang@windriver.com/T/ >>>>> >>> >>> It looks to me that the fundamental issue is that, in some situations, >>> the round_rate implementation can return a rate outside of the >>> boundaries enforced on a clock. >> >> In my limited view, the round_rate callbacks should return a rate within >> boundaries as output, > > I guess it would be s/should/must/, but yeah, we agree. > >> but can take a rate outside of boundaries as input. > > I'm not sure what that would change though? > >> Take Xilinx Zynqmp for instance, VPLL's rate range is 1.5GHz~3GHz. A >> consumer dp_video_ref wants a 200MHz rate, its request walks upward through >> multiplexers and dividers then reaches to VPLL, VPLL receives this 200MHz >> request and call zynqmp_pll_round_rate to "round" this out-of-range rate >> 200MHz to 1600MHz via multiplying by 8. zynqmp_pll_round_rate returns >> 1600MHz and clk subsystem will call determine callbacks to configure >> dividers correctly to make sure that dp_video_ref can get an exact rate >> 200MHz. > > Sounds good to me indeed. > >> But the commit 948fb0969eae8 ("clk: Always clamp the rounded rate") adds >> >> req->rate = clamp(req->rate, req->min_rate, req->max_rate); >> >> before >> >> rate = core->ops->round_rate(core->hw, req->rate,&req->best_parent_rate); >> >> This results that .round_rate callbacks lose functionality since they have >> no chance to pick up a precise rate but only a boundary rate. >> Still for Xilinx Zynqmp, the 200MHz rate request to PLL will be set to >> 1500MHz by clamp function and then zynqmp_pll_round_rate does nothing, > > I'm a bit confused now. > > If I understand your clock topology, you have a PLL, and then a divider > of 8, and want the final clock to be running at 200MHz? > > If so, the divider should call its parent round/determine_rate with 200 > * 8 MHz = 1600MHz, which is is still inside the boundaries of 1.5-3.0GHz > and won't be affected? > > Why should the child be affected by the parent boundaries, or the other > way aroundSorry, I didn't explain the problem clearly. As below is the vpll clk topology in /sys/kernel/debug/clk/clk_summary when reverted "clk: Always clamp the rounded rate". clk_name MHz pss_ref_clk 33333333 vpll_post_src 33333333 vpll_pre_src 33333333 vpll_int 1599999984 vpll_half 799999992 vpll_int_mux 799999992 vpll 799999992 dp_video_ref_mux 799999992 dp_video_ref_div1 99999999 dp_video_ref_div2 99999999 dp_video_ref 99999999 When call clk_set_rate(dp_video_ref, 100MHz), there is a clk_calc_new_rates request passed from bottom (dp_video_ref) to top (vpll_int), every clk will calculate its clk_rate and its best_parent_rate. vpll_half will calculate its clk rate is 100MHz and its parent clk vpll_int should be 200MHz since vpll_half is a half divider. But vpll_int ranges from 1.5GHz~3GHz, so it call zynqmp_pll_round_rate to calculate a new rate 1.6GHz for itself via 200MHz * 8 because 8 is the smallest integer: zynqmp_pll_round_rate: if (200MHz < 1.5GHz) { mult = DIV_ROUND_UP(1.5GHz, 200MHz); rate = rate * mult; The question is that implementation of round_rate callbacks for some clk drivers have abilities to round a out-of-range rate req to a inside-range rate: clk_core_determine_round_nolock: rate(1.6GHz) = core->ops->round_rate("vpll_int", req->rate(200MHz), &req->best_parent_rate); But the commit 948fb096 ("clk: Always clamp the rounded rate") breaks this like: clk_core_determine_round_nolock: req->rate (1.5GHz) = clamp(req->rate (200MHz), req->min_rate (1.5GHz), req->max_rate (3GHz)); ...... rate(1.5GHz) = core->ops->round_rate("vpll_int", req->rate(1.5GHz), &req->best_parent_rate); Thanks, Quanyang > Maxime _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel