[PATCH v2 2/2] arm64: dts: rockchip: Add Radxa ROCK 3C

[Folker Schwesinger]

Hi Jonas and Dragan,

On Thu May 2, 2024 at 3:48 PM CEST, Dragan Simic wrote:
> >>>> [...]
> >>>> +&sdhci {
> >>>> +	bus-width = <8>;
> >>>> +	max-frequency = <200000000>;
> >>>> +	mmc-hs200-1_8v;
> >>> 
> >>> Out of curiosity, does this board also support HS400? From a very 
> >>> brief
> >>> look this board seems quite similar to Rock 4 series boards, that do
> >>> support HS400.
> >> 
> >> In a word, no, unfortunately.  In more detail, the Rockchip RK3566
> >> and RK3568 SoCs support the eMMC modes up to and including HS200, but
> >> not the HS400 mode.
> > 
> > The datasheet for RK3568 only mention HS200 mode, however, HS400 mode 
> > is
> > working in U-Boot after adjusting tap number for transmit clock [1].
> > Linux may need similar adjustment to make HS400 mode work on RK3568.
> > 
> > RK3566 that this board use only support HS200, as specified in 
> > datasheet.
>
> Oh, that's very interesting, thanks for pointing it out!
>
> What makes me a bit worried about enabling the undocumented HS400 mode
> on the RK3568 is that neither the RK3568 Hardware Design Guide (HDG),
> version 1.2, [2] mentions HS400 support.  I mean, the RK3568 datasheet
> may be wrong about not mentioning HS400, but what are the chances for
> the RK3568 HDG to also be wrong there?
>
> In other words, maybe there are some hard-to-trigger hardware issues
> in the RK3568 that made the HS400 mode, although actually present in
> hardware, officially unsupported for the RK3568?  Maybe there's even
> some non-public errata from Rockchip, who knows.
>
> [1] 
> https://source.denx.de/u-boot/u-boot/-/commit/5c053f3adc69eaf83645b5d44c6adbb2d49a3fa7
> [2] https://dl.xkwy2018.com/downloads/RK3568/RK356X/Hardware/

Interesting indeed! Thanks to both of you for the info and for providing
links to the U-Boot commit and design guide. Much appreciated!