Re: drm/rockchip: vop2: ACLK_VOP pinned at 500 MHz starves DP 4K120 on RK3588

[Chaoyi Chen <chaoyi.chen@rock-chips.com>]

Hello Cristian, Igor,

On 8/10/2026 11:36 PM, Cristian Ciocaltea wrote:
> Hi Igor,
> 
> On 8/8/26 1:42 PM, Igor Paunovic wrote:
>> Hello,
>>
>> On RK3588 the VOP2 AXI clock is pinned to 500 MHz by device tree and
>> nothing in mainline ever raises it.  For DisplayPort at 3840x2160@120
>> that is not enough bandwidth, and the failure is not graceful: the
>> video port floods POST_BUF_EMPTY interrupts and the picture is
>> corrupted.
>>
>> I mentioned this in passing when I sent a Tested-by for the dw-dp v11
>> series:
>> https://lore.kernel.org/all/20260808094138.7205-1-royalnet026@gmail.com/
>>
>> The pin is explicit.  In arch/arm64/boot/dts/rockchip/rk3588-base.dtsi
>> the cru node lists <&cru ACLK_VOP> in assigned-clocks with 500000000 in
>> the matching position of assigned-clock-rates.  It is a deliberate
>> assignment rather than a boot default, and it is identical in mainline
>> and in the Collabora rockchip-devel branch.
>>
>> The rate request does propagate.  ACLK_VOP is registered as a gate with
>> flags 0 in clk-rk3588.c, but drivers/clk/rockchip/clk.c adds
>> CLK_SET_RATE_PARENT for every branch_gate, so a request on aclk_vop
>> travels up through aclk_vop_sub_src (a mux carrying the same flag) to
>> aclk_vop_root, which is a settable composite.
>>
>> What I measured, on an Orange Pi 5 Plus driving a DP monitor over USB-C
>> Alt Mode at 3840x2160@120 (YCbCr 4:2:0, so dclk sits at 594 MHz), on
>> drm-misc-next of 2026-08-07 (dc2f9f7fed1a) plus the dw-dp v11 and usbdp
>> v13 series.  I built with CLOCK_ALLOW_WRITE_DEBUGFS and changed the
>> rate by writing aclk_vop_root's clk_rate in clock debugfs, one level
>> up, which has the same effect.  The mode was unchanged across all three
>> phases, so dclk stayed at 594 MHz and the AXI rate was the only thing
>> that moved:
>>
>>   500 MHz:  vop2_isr reported ~594000 suppressed callbacks per 5 s,
>>             roughly 119k interrupts per second, all POST_BUF_EMPTY on
>>             the DP video port; picture unusable
>>   750 MHz:  no POST_BUF_EMPTY logged at all for 42 s, the whole phase;
>>             the picture became correct the moment the write landed
>>   500 MHz:  ~607000 suppressed callbacks per 5 s; broken again
>>
>> Both transitions are immediate, and the timestamps line up with the
>> phases of my test script.
>>
>> Everything I can drive up to 2560x1440@144 (about 586 MHz pixel clock)
>> stays clean at 500 MHz; 3840x2160@120 (1188 MHz) does not.  That
>> comparison comes from a mode sweep on my rockchip-devel based daily
>> kernel rather than from the run above, and my monitor offers nothing in
>> between, so I cannot bisect the threshold.
>>
>> In mainline rockchip_drm_vop2.c there is no clk_set_rate() on the VOP
>> aclk at all; the only clk_set_rate() in the file is on vp->dclk.  To
>> reproduce: enable dp0 on an RK3588 board - it is disabled in the
>> mainline DTS, so this needs a board DT change - drive a DP monitor at
>> 3840x2160@120, and watch dmesg or the vop interrupt count in
>> /proc/interrupts.
>>
>> The only code I know of that raises the rate is a commit in the
>> Collabora rockchip-devel branch, "drm/rockchip: vop2: Scale ACLK rate
>> up for RK3588 FRL display modes" by Cristian Ciocaltea.  It defines a
>> 750 MHz rate and calls clk_set_rate() from vop2_crtc_atomic_enable()
>> and _disable(), gated on vcstate->frl_enabled with a refcount over the
>> video ports.  I could not find it on lore, but I may well have missed
>> it.
> 
> I haven't submitted that yet; it's part of a work-in-progress FRL enablement
> patchset that's currently blocked on the HDMI 2.0 support series [1], still
> under review upstream.
> 
>> If that is the intended shape of the fix, DisplayPort still would not
>> benefit: frl_enabled is set only by dw_hdmi_qp-rockchip.c, and neither
>> dw-dp.c nor dw_dp-rockchip.c mentions FRL at all, so the condition is
>> HDMI specific while the bandwidth problem is not.  It also explains
>> what I had been seeing since I enabled dp0 on this board last week:
>> 4K120 over USB-C only worked while an HDMI output happened to be
>> running in FRL mode.
>>
>> Raising the DT rate to 750 MHz unconditionally would be simpler, but it
>> would cost power on boards that never drive such a mode, which is
>> presumably the reason for making it dynamic.
> 
> Yes.
> 
>> Cristian, Heiko - hence my question: should the ACLK rate be derived
>> from the bandwidth the enabled video ports actually need, rather than
>> from an HDMI-specific link flag?  A per-video-port requirement combined
>> with max() across the active ports would cover DP and HDMI under one
>> rule and would not need a refcount.
> 
> Unfortunately, deriving ACLK purely from bandwidth wouldn't suffice as a general
> solution for HDMI. FRL and TMDS overlap for certain display modes, and hardware
> may impose its own limits on max TMDS character rate and/or supported min/max
> FRL bandwidth - so the same mode can map to different ACLK requirements
> depending on which link type is used. 
> 
> Later, we might want to support user-prioritization of FRL over TMDS, even for
> modes that TMDS could otherwise handle cleanly. Since FRL uses fixed rates
> regardless of the active display mode, ACLK can't be reliably inferred from the
> mode alone for HDMI.
> 
> DP may still follow a bandwidth-derived max() rule, though I haven't been
> personally involved in the DP use-case. I recall Alexey (added to CC) mentioning
> additional issues with PLL rates on RK3576, I think [2] is the most recent one.

In my opinion, POST BUF EMPTY has nothing to do with the display
interface actually being used, but rather relates to the plane you
are actually using.

For example, when using DP at 4K@120, we can perfectly well use a
lower ACLK with a smaller 1080P plane without encountering the POST
BUF EMPTY issue. Similarly, when using 4K@60, if multiple 4K ARGB
planes are displayed simultaneously, we may encounter the same issue 
as well.

> 
> Cheers,
> Cristian
> 
> [1] https://lore.kernel.org/all/20260731-dw-hdmi-qp-scramb-v10-0-294364b2cf15@collabora.com/
> [2] https://lore.kernel.org/all/CAKTNdwFPhVrjb4iVpN19f0Nw+MA_BxXLOLs4V50fuyf+_rbEPg@mail.gmail.com
> 
> 

-- 
Best, 
Chaoyi