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Date: Mon, 13 Jun 2022 18:16:34 +0000 From: "Vijaya Krishna Nivarthi (Temp) (QUIC)" <quic_vnivarth@...cinc.com> To: Doug Anderson <dianders@...omium.org>, "Vijaya Krishna Nivarthi (Temp) (QUIC)" <quic_vnivarth@...cinc.com> CC: Andy Gross <agross@...nel.org>, "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, linux-arm-msm <linux-arm-msm@...r.kernel.org>, "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, "Mukesh Savaliya (QUIC)" <quic_msavaliy@...cinc.com>, Matthias Kaehlcke <mka@...omium.org>, Stephen Boyd <swboyd@...omium.org> Subject: RE: [PATCH] tty: serial: qcom-geni-serial: minor fixes to get_clk_div_rate() Hi, > > > > > > ser_clk = 0; > > > maxdiv = CLK_DIV_MSK >> CLK_DIV_SHFT; > > > div = 1; > > > while (div < maxdiv) { > > > > > > div <= maxdiv ? > > Ah, sure. Thank you. > > > > > mult = (unsigned long long)div * desired_clk; > > > if (mult != (unsigned long)mult) > > > break; > > > > > > two_percent = mult / 50; > > > > > > /* > > > * Loop requesting (freq - 2%) and possibly (freq). > > > * > > > * We'll keep track of the lowest freq inexact match we found > > > * but always try to find a perfect match. NOTE: this algorithm > > > * could miss a slightly better freq if there's more than one > > > * freq between (freq - 2%) and (freq) but (freq) can't be made > > > * exactly, but that's OK. > > > * > > > * This absolutely relies on the fact that the Qualcomm clock > > > * driver always rounds up. > > > */ > > > test_freq = mult - two_percent; > > > while (test_freq <= mult) { > > > freq = clk_round_rate(clk, test_freq); > > > > > > /* > > > * A dead-on freq is an insta-win. This implicitly > > > * handles when "freq == mult" > > > */ > > > if (!(freq % desired_clk)) { > > > *clk_div = freq / desired_clk; > > > return freq; > > > } > > > > > > /* > > > * Only time clock framework doesn't round up is if > > > * we're past the max clock rate. We're done searching > > > * if that's the case. > > > */ > > > if (freq < test_freq) > > > return ser_clk; > > > > > > /* Save the first (lowest freq) within 2% */ > > > if (!ser_clk && freq <= mult + two_percent) { > > > ser_clk = freq; > > > *clk_div = div; > > > } > > > > My last concern is with search happening only within 2% tolerance. > > Do we fail otherwise? > > > > This real case has best tolerance of 1.9% and seems close. > > > > [ 17.963672] 20220530 desired_clk-51200000 > > [ 21.193550] 20220530 returning ser_clk-52174000, div-1, diff-974000 > > > > Perhaps we can fallback on 1st clock rate? > > I don't feel super comfortable just blindly falling back on the 1st clock rate. It > could be wildly (more than 5%) wrong, can't it? > > IMO: > * If you're not comfortable with 2%, you could always pick 3% or 4%. > As I said, my random web search seemed to indicate that up to 5% was > perhaps OK. > * It's probably overkill, but you could abstract the whole search out and try > searching once for 2% and then try 4%? > Ok, I will implement a function that searches within an input tolerance. And have a conditional 2nd call to same with higher tolerance of 5%. This would mean that we will still run through 2 loops in some cases, but that’s ok. Thank you. > > -Doug
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