Re: [PATCH] ARM: dts: Add missing CPU frequencies for Exynos5422/5800

[Krzysztof Kozlowski <krzk@kernel.org>]

On Thu, Dec 15, 2016 at 04:52:58PM -0800, Doug Anderson wrote:
> > [ I added Arjun to Cc:, maybe he can help in explaining this issue
> >   (unfortunately Inderpal's email is no longer working). ]
> >
> > Please also note that on Exynos5422/5800 SoCs the same ARM rail
> > voltage is used for 1.9 GHz & 2.0 GHz OPPs as for the 1.8 GHz one.
> > IOW if the problem exists it is already present in the mainline
> > kernel.
> 
> Interesting.  In the ChromeOS tree I see significantly higher voltages
> needed...  Note that one might naively look at
> <https://chromium.googlesource.com/chromiumos/third_party/kernel/+/chromeos-3.8/drivers/cpufreq/exynos5420-cpufreq.c#178>.
> 
> 1362500, /* L0  2100 */
> 1312500, /* L1  2000 */
> 
> ..but, amazingly enough those voltages aren't used at all.  Surprise!
> 
> I believe that the above numbers are actually not used and the ASV
> numbers are used instead.  See
> <https://chromium.googlesource.com/chromiumos/third_party/kernel/+/chromeos-3.8/arch/arm/mach-exynos/include/mach/asv-exynos542x.h#452>
> 
> { 2100000,
> 1350000, 1350000, 1350000, 1350000, 1350000,
> 1337500, 1325000, 1312500, 1300000, 1287500,
> 1275000, 1262500, 1250000, 1237500 },
> 
> I believe that interpretation there is: some bins of the CPU can run
> at 2.1 GHz just fine at 1.25 V but others need up to 1.35V.

That is definitely the case. One could just look at vendors ASV table
(for 1.9 GHz):
{ 1900000, 1300000, 1287500, 1262500, 1237500, 1225000, 1212500,
                    1200000, 1187500, 1175000, 1162500, 1150000,
		             1137500, 1125000, 1112500, 1112500},

The theoretical difference is up to 1.875V! From my experiments I saw
BIN1 chips which should be the same... but some working on 1.2V, some on
1.225V (@1.9 GHz). I didn't see any requiring higher voltages but that
does not mean that there aren't such...

> ...so if you're running at 2.1 GHz at 1.25V then perhaps you're just
> running on a CPU from a nice bin?

Would be nice to see a dump of PKG_ID and AUX_INFO chipid registers
along with name of tested board. Because the "Tested on XU3" is not
sufficient.

Best regards,
Krzysztof