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[88.21.103.182]) by smtp.gmail.com with ESMTPSA id f1sm11787323wrp.93.2019.12.05.01.59.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Dec 2019 01:59:58 -0800 (PST) Subject: Re: [PATCH v6 0/9] Clock framework API To: Damien Hedde , Peter Maydell , "Dr. David Alan Gilbert" References: <20190904125531.27545-1-damien.hedde@greensocs.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <279a0fd5-1ea5-b3c7-27bb-b1d22db5e359@redhat.com> Date: Thu, 5 Dec 2019 10:59:57 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-MC-Unique: D7DyDscuNLeP_9j9oYWuLA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.81 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Daniel P. Berrange" , Eduardo Habkost , Alistair Francis , Mark Burton , QEMU Developers , =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= , qemu-arm , Paolo Bonzini , "Edgar E. Iglesias" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 12/5/19 10:36 AM, Damien Hedde wrote: > On 12/4/19 9:34 PM, Philippe Mathieu-Daud=C3=A9 wrote: >> On 12/4/19 5:40 PM, Damien Hedde wrote: >>> On 12/2/19 5:15 PM, Peter Maydell wrote: >>>> >>>> The one topic I think we could do with discussing is whether >>>> a simple uint64_t giving the frequency of the clock in Hz is >>>> the right representation. In particular in your patch 9 the >>>> board has a clock frequency that's not a nice integer number >>>> of Hz. I think Philippe also mentioned on irc some board where >>>> the UART clock ends up at a weird frequency. Since the >>>> representation of the frequency is baked into the migration >>>> format it's going to be easier to get it right first rather >>>> than trying to change it later. >> >> Important precision for Damien, IIUC we can not migrate float/double typ= es. >> >>>> So what should the representation be? Some random thoughts: >>>> >>>> 1) ptimer internally uses a 'period plus fraction' representation: >>>> =C2=A0 int64_t period is the integer part of the period in nanosecond= s, >>>> =C2=A0 uint32_t period_frac is the fractional part of the period >>>> (if you like you can think of this as "96-bit integer >>>> period measured in units of one-2^32nd of a nanosecond"). >>>> However its only public interfaces for setting the frequency >>>> are (a) set the frequency in Hz (uint32_t) or (b) set >>>> the period in nanoseconds (int64_t); the period_frac part >>>> is used to handle frequencies which don't work out to >>>> a nice whole number of nanoseconds per cycle. >> >> This is very clear, thanks Peter! >> >> The period+period_frac split allow us to migrate the 96 bits: >> >> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 VMSTATE_UINT32(period_frac, = ptimer_state), >> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 VMSTATE_INT64(period, ptimer= _state), >> >>>> 2) I hear that SystemC uses "value plus a time unit", with >>>> the smallest unit being a picosecond. (I think SystemC >>>> also lets you specify the duty cycle, but we definitely >>>> don't want to get into that!) >>> >>> The "value" is internally stored in a 64bits unsigned integer. >>> >>>> >>>> 3) QEMUTimers are basically just nanosecond timers >> >> Similarly to SystemC, the QEMUTimers macro use a 'scale' unit, of: >> >> #define SCALE_MS 1000000 >> #define SCALE_US 1000 >> #define SCALE_NS 1 >> >>>> >>>> 4) The MAME emulator seems to work with periods of >>>> 96-bit attoseconds (represented internally by a >>>> 32-bit count of seconds plus a 64-bit count of >>>> attoseconds). One attosecond is 1e-18 seconds. >>>> >>>> Does anybody else have experience with other modelling >>>> or emulator technology and how it represents clocks ? >>> >>> 5) In linux, a clock rate is an "unsigned long" representing Hz. >>> >>>> >>>> I feel we should at least be able to represent clocks >>>> with the same accuracy that ptimer has. >>> >>> Then is a maybe a good idea to store the period and not the frequency i= n >>> clocks so that we don't loose anything when we switch from a clock to a >>> ptimer ? >> >> I think storing the period as an integer type is a good idea. >> >> However if we store the period in nanoseconds, we get at most 1GHz >> frequency. >> >> The attosecond granularity feels overkill. >> >> If we use a 96-bit integer to store picoseconds and use similar SCALE >> macros we get to 1THz. >> >> Regardless the unit chosen, as long it is integer, we can migrate it. >> If can migrate the period, we don't need to migrate the frequency. >> We can then use the float type in with the timer API to pass frequencies >> (which in the modeled hardware are ratios, likely not integers). >> >> So we could use set_freq(100e6 / 3), set_freq(40e6 / 5.5) directly. >> >>> Regarding the clock, I don't see any strong obstacle to switch >>> internally to a period based value. >>> The only things we have to choose is how to represent a disabled clock. >>> Since putting a "0" period to a ptimer will disable the timer in >>> ptimer_reload(). We can choose that (and it's a good value because we >>> can multiply or divide it, it stays the same). >>> >>> We could use the same representation as a ptimer. But if we don't keep = a >>> C number representation, then computation of frequencies/periods will b= e >>> complicated at best and error prone. >>> >>> =C2=A0From that point of view, if we could stick to a 64bits integer (= or >>> floating point number) it would be great. Can we use a sub nanosecond >>> unit that fit our needs ? >>> >>> I did some test with a unit of 2^-32 of nanoseconds on 64bits (is that >>> the unit of the ptimer fractional part ?) and if I'm not mistaken >>> + we have a frequency range from ~0.2Hz up to 10^18Hz >>> + the resolution is decreasing with the frequency (but at 100Mhz we hav= e >>> a ~2.3mHz resolution, at 1GHz it's ~0.23Hz and at 10GHz ~23Hz >>> resolution). We hit 1Hz resolution around 2GHz. >>> >>> So it sounds to me we have largely enough resolution to model clocks in >>> the range of frequencies we will have to handle. What do you think ? >> >> Back to your series, I wonder why you want to store the frequency in >> ClockIn. ClockIn shouldn't be aware at what frequency it is clocked. >> What matters is ClockOut, and each device exposing ClockOuts has a >> (migrated) state of the output frequencies (rather in fields, or encoded >> in registers). Once migrated, after the state is loaded back into the >> device, we call post_load(). Isn't it a good place to call >> clock_set_frequency(ClockOut[]) which will correctly set each ClockIn >> frequency. >> >> IOW I don't think ClockIn/ClockOut require to migrate a frequency field. >> >=20 > I agree it is more logical to store the frequency in clock out. But, > regarding migration constraints, we have no choice I think because a > device cannot rely on values that are migrated by another device for > restoring its state. (when I checked, I add the impression that > post_load()s are called on a per device migration basis not all at the > end of migration). Cc'ing David to clear that out. > So we could store the frequency in clock out and migrate things there. > But since we have no way to ensure all clock out states are migrated > before some device fetch a ClockIn: we'll have to say "don't fetch one > of your ClockIn frequency during migration and migrate the value > yourself if you need it", pretty much like gpios. >=20 > So we will probably migrate all ClockOut and almost all ClockIn. >=20 > It would nice if we had a way to ensure clocks are migrated before > devices try to use them. But I don't think this is possible. >=20 > -- > Damien >=20