[U-Boot] [PATCH v6 5/8] x86: slimbootloader: Set TSC information for tsc_timer

[Park, Aiden]

Hi Andy,

> -----Original Message-----
> From: Andy Shevchenko [mailto:andy.shevchenko at gmail.com]
> Sent: Saturday, July 27, 2019 6:06 AM
> To: Park, Aiden <aiden.park at intel.com>
> Cc: Bin Meng <bmeng.cn at gmail.com>; U-Boot Mailing List <u-
> boot at lists.denx.de>; Simon Glass <sjg at chromium.org>
> Subject: Re: [PATCH v6 5/8] x86: slimbootloader: Set TSC information for
> tsc_timer
> 
> On Fri, Jul 26, 2019 at 3:50 PM Park, Aiden <aiden.park at intel.com> wrote:
> > > On Fri, Jul 26, 2019 at 10:00 AM Park, Aiden <aiden.park at intel.com> wrote:
> > > >
> > > > Slim Bootloader already calibrated TSC and provides it to U-Boot.
> > > > Therefore, U-Boot does not have to re-calibrate TSC.
> > > > Configuring tsc_base and clock_rate makes x86 tsc_timer driver
> > > > bypass TSC calibration and use the provided TSC frequency.
> > > > - Get TSC frequency from performance info hob
> > > > - Set tsc_base and clock_rate for tsc_timer driver
> > >
> > > I'm still not convinced to have this. As kernel followed by U-Boot
> > > we thrust hardware more than something else.
> > > One more layer in between is usually an additional point to be error-prone.
> > >
> > > So, if something we may get directly from hardware, I consider
> > > better to get it from there.
> > >
> > > Of course, data structures can be left for sake of self-documentation.
> > >
> > Basically, Providing known information to a payload is Slim Bootloader
> > architecture concept. Slim Bootloader does initialize chipset and
> > hardware as much as it can, and provides useful information to a
> > payload in HOBs to minimize hardware re-initialization in a payload,
> > make it generic and make it light-weight.
> >
> > TSC frequency is specific to CPU or chipset, and the way to get exact
> > TSC frequency also varies depending on CPU.
> > ex) checking cpuid, platform info MSR, perf MSR for LFM/HFM/Turbo Mode,
> or FSB.
> > All these consideration has already done in Slim Bootloader on each CPUs.
> >
> > I think that U-Boot tsc_timer also considers this case, and that's why
> > tsc_timer skips calibration in its probing time if tsc_base and
> > clock_rate are already configured.
> >
> > Kernel uses many timers like TSC, APIC timer, 8254 timer, HPET and so on.
> > I also understand kernel trusts hardware and re-calibrate them because
> > all these timers may not be properly configured in boot firmware stage.
> > But, HOB information including TSC must be trusted between Slim
> > Bootloader and its payload(U-Boot) in Slim Bootloader architecture.
> 
> Okay, I didn't remember if this is described in documentation.
> If it's not, please, add a section and put there somelike you expleined to me here.
> 
Thanks for your understanding. Let me add more description in documentation.

> --
> With Best Regards,
> Andy Shevchenko

Best Regards,
Aiden