[PATCH v4 1/4] clk: fractional-divider: Export approximation algorithm to the CCF users

Chris Morgan macromorgan at hotmail.com
Tue Sep 7 19:17:47 PDT 2021

On Tue, Sep 07, 2021 at 09:06:10PM +0300, Andy Shevchenko wrote:
> On Tue, Sep 07, 2021 at 08:54:04PM +0300, Andy Shevchenko wrote:
> > On Tue, Sep 07, 2021 at 10:44:00AM -0500, Chris Morgan wrote:
> > > From: Chris Morgan <macromorgan at hotmail.com>
> > > 
> > > Unfortunately, I can confirm this breaks the DSI panel on the Rockchip
> > > PX30 (and possibly other SoCs). Tested on my Odroid Go Advance. When
> > > I revert 4e7cf74fa3b2 "clk: fractional-divider: Export approximation
> > > algorithm to the CCF users" and 928f9e268611 "clk: fractional-divider:
> > > Hide clk_fractional_divider_ops from wide audience" the panel begins
> > > working again as expected on the master branch.
> > > 
> > > It looks like an assumption is made in the vop_crtc_mode_fixup()
> > > function in the rockchip_drm_vop.c that gets broken with this change.
> > > Specifically, the function says in the comments "When DRM gives us a
> > > mode, we should add 999 Hz to it.". I believe this is no longer true
> > > after this clk change, and when I remove the + 999 from the function
> > > the DSI panel works again. Note that I do not know the implications
> > > of removing this 999 aside from that it fixes the DSI panel on my
> > > PX30 after this change, so I don't know if it's a positive change
> > > or not.
> > > 
> > > Thank you.
> > 
> > Thank you for the report!
> > 
> > I'll check this. Perhaps Heiko can help with testing as well on his side.
> On the first glance the mentioned patch may not be the culprit because it does
> not change the functional behaviour (if I'm not mistaken). What really changes
> it is the additional flag that removes the left-shift of the rate in the
> calculations.

I noticed the behavior on the 5.14 kernel was to set the numerator at
an ungodly 7649082492112076800 and the denominator at 1 (no, that's not
a typo). I think it tried to write 65535 to the register though, but it
would go through this a few times and eventually settle on 1:1 as the
fractional ratio (which I assume is all good, because that would work).

Contrast this to the 5.15 behavior where it would try to set the ratio
to 17001:17000, which would cause the DSI screen to fail to initalize.

After tracing through the code I figured out that the VOP was trying to
add 999 to the clock and set it at 17000999. 17000000/17000999 gives us
0, and subtracting 1 from that gives us a -1. The fls_long function 
would then return 64, and if we subtract 16 (the value of fd->mwidth
for my board) it would tell us to shift the 17000999 48 bits to the
left, which matches the ungodly large number.

With the changes in 5.15 if I remove the + 999 from the VOP driver the
clock then gets set at 17000000, since the parent is at 17000000 that
gives us a 1:1 where everything works and everything is fine.

Long story short I think this is a bug that's existed all along, and
this change simply exposed it in a manner where it stopped working
despite the bug being present. Unfortunately I neither know enough
about the hardware to be confident in this fix beyond my specific
board, nor do I have enough hardware to test it on anything except
a Rockchip rk3326 with a DSI panel.

> To me sounds like you found a proper solution to the issue and that +999 is
> a hack against the (blindly?) copied part of the algorithm used in fractional
> divider. Have you read the top comment in clk-fractional-divider.c? It should
> explain how it works after my series.

No, but I probably should read the docs more. I just stumbled on this
series doing a bisect when the DSI panel stopped working.

> In any case I'm not going to come to any conclusions right now and also want
> to hear from people who have better understanding of this hardware.

Yeah, I want to see what Heiko says after some more research, or anyone
who has more familiarity with clocks/DRM than I do or who has more
hardware to test on than I do.

I intended to send a message informing you that "hey, this breaks
upstream", but I think it turns out it's more a matter of "hey,
this makes a broken upstream break instead of limp along".

Thank you.

> -- 
> With Best Regards,
> Andy Shevchenko

More information about the Linux-rockchip mailing list