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

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Date:   Tue,  7 Sep 2021 10:44:00 -0500
From:   Chris Morgan <macroalpha82@...il.com>
To:     andriy.shevchenko@...ux.intel.com
Cc:     abel.vesa@....com, festevam@...il.com, heiko@...ech.de,
        kernel@...gutronix.de, lee.jones@...aro.org, lenb@...nel.org,
        linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-clk@...r.kernel.org, linux-imx@....com,
        linux-kernel@...r.kernel.org, linux-rockchip@...ts.infradead.org,
        mturquette@...libre.com, rafael.j.wysocki@...el.com,
        rjw@...ysocki.net, s.hauer@...gutronix.de, sboyd@...nel.org,
        shawnguo@...nel.org, zhangqing@...k-chips.com,
        Chris Morgan <macromorgan@...mail.com>
Subject: Re: [PATCH v4 1/4] clk: fractional-divider: Export approximation algorithm to the CCF users

From: Chris Morgan <macromorgan@...mail.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.