From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hans de Goede Subject: Re: [PATCH v2 03/15] pwm: lpss: Add range limit check for the base_unit register value Date: Mon, 8 Jun 2020 16:19:08 +0200 Message-ID: References: <20200607181840.13536-1-hdegoede@redhat.com> <20200607181840.13536-4-hdegoede@redhat.com> <20200608035023.GZ2428291@smile.fi.intel.com> <90769dc0-3174-195b-34e0-ef4bb9d9b982@redhat.com> <20200608125156.GL2428291@smile.fi.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20200608125156.GL2428291@smile.fi.intel.com> Content-Language: en-US Sender: linux-acpi-owner@vger.kernel.org To: Andy Shevchenko Cc: Thierry Reding , =?UTF-8?Q?Uwe_Kleine-K=c3=b6nig?= , Jani Nikula , Joonas Lahtinen , =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= , "Rafael J . Wysocki" , Len Brown , linux-pwm@vger.kernel.org, intel-gfx , dri-devel@lists.freedesktop.org, Mika Westerberg , linux-acpi@vger.kernel.org List-Id: linux-pwm@vger.kernel.org Hi, On 6/8/20 2:51 PM, Andy Shevchenko wrote: > On Mon, Jun 08, 2020 at 01:07:12PM +0200, Hans de Goede wrote: >> On 6/8/20 5:50 AM, Andy Shevchenko wrote: >>> On Sun, Jun 07, 2020 at 08:18:28PM +0200, Hans de Goede wrote: >>>> When the user requests a high enough period ns value, then the >>>> calculations in pwm_lpss_prepare() might result in a base_unit value of 0. >>>> >>>> But according to the data-sheet the way the PWM controller works is that >>>> each input clock-cycle the base_unit gets added to a N bit counter and >>>> that counter overflowing determines the PWM output frequency. Adding 0 >>>> to the counter is a no-op. The data-sheet even explicitly states that >>>> writing 0 to the base_unit bits will result in the PWM outputting a >>>> continuous 0 signal. >>> >>> So, and why it's a problem? >> >> Lets sya the user requests a PWM output frequency of 100Hz on Cherry Trail >> which has a 19200000 Hz clock this will result in 100 * 65536 / 19200000 = >> 0.3 -> 0 as base-unit value. So instead of getting 100 Hz the user will >> now get a pin which is always outputting low. >> >> OTOH if we clamp to 1 as lowest value, the user will get 192000000 / 65536 >> = 292 Hz as output frequency which is as close to the requested value as >> we can get while actually still working as a PWM controller. > > So, we should basically divide and round up, no? Yes, that will work for the low limit of base_unit but it will make all the other requested period values less accurate. > At least for 0 we will get 0. We're dealing with frequency here, but the API is dealing with period, so to get 0 HZ the API user would have to request a period of > 1s e.g. request 2s / 0.5 Hz but then the user is still not really requesting 0Hz (that would correspond with a period of infinity which integers cannot represent. >>>> base_unit values > (base_unit_range / 256), or iow base_unit values using >>>> the 8 most significant bits, cause loss of resolution of the duty-cycle. >>>> E.g. assuming a base_unit_range of 65536 steps, then a base_unit value of >>>> 768 (256 * 3), limits the duty-cycle resolution to 65536 / 768 = 85 steps. >>>> Clamp the max base_unit value to base_unit_range / 32 to ensure a >>>> duty-cycle resolution of at least 32 steps. This limits the maximum >>>> output frequency to 600 KHz / 780 KHz depending on the base clock. >>> >>> This part I don't understand. Why we limiting base unit? I seems like a >>> deliberate regression. >> >> The way the PWM controller works is that the base-unit gets added to >> say a 16 bit (on CHT) counter each input clock and then the highest 8 >> bits of that counter get compared to the value programmed into the >> ON_TIME_DIV bits. >> >> Lets say we do not clamp and allow any value and lets say the user >> selects an output frequency of half the input clock, so base-unit >> value is 32768, then the counter will only have 2 values: >> 0 and 32768 after that it will wrap around again. So any on time-div >> value < 128 will result in the output being always high and any >> value > 128 will result in the output being high/low 50% of the time >> and a value of 255 will make the output always low. >> >> So in essence we now only have 3 duty cycle levels, which seems like >> a bad idea to me / not what a pwm controller is supposed to do. > > It's exactly what is written in the documentation. I can't buy base unit clamp. > Though, I can buy, perhaps, on time divisor granularity, i.e. > 1/ 0% - 25%-1 (0%) > 2/ 25% - 50% - 75% (50%) > 3/ 75%+1 - 100% (100%) > And so on till we got a maximum resolution (8 bits). Note that the PWM API does not expose the granularity to the API user, which is why I went with just putting a minimum on it of 32 steps. Anyways I don't have a strong opinion on this, so I'm fine with not clamping the base-unit to preserve granularity. We should still clamp it to avoid overflow if the user us requesting a really high frequency though! The old code had: base_unit &= base_unit_range; Which means that if the user requests a too high value, then we first overflow base_unit and then truncate it to fit leading to a random frequency. So if we forget my minimal granularity argument, then at a minimum we need to replace the above line with: base_unit = clamp_t(unsigned long long, base_unit, 1, base_unit_range - 1); And since we need the clamp anyways we can then keep the current round-closest behavior. Regards, Hans