GPU/DRM issue with DSI commands on msm 8916

[Daniel Mack]

(cc Stephen)

Hi Archit,

On Monday, April 16, 2018 07:06 PM, Daniel Mack wrote:
> On Monday, April 09, 2018 03:08 PM, Archit Taneja wrote:
>>>> You could comment out the pm_runtime_put_sync() calls in
>>>> drivers/gpu/drm/msm/dsi/dsi_host.c, in case some registers got
>>>> reset during put_sync and weren't restored correctly after get_sync().
>>>
>>> That was my first suspicion too, but unfortunately, that's not the culprit.
>>> I think it would be good to comment out the put_sync() calls in
>> drivers/gpu/drm/msm/mdp/mdp5/mdp5_mdss.c and 
>> drivers/gpu/drm/msm/msm_drv.c too, since there is a parent-child 
>> hierarchy between DSI
>> and the top level MDSS block. Commenting out the put_syncs() just
>> in put_sync() might still result in the MDSS GDSC to switch off.
> 
> I spent some more time debugging this today and it turns out that
> calling into dsi_link_clk_enable() from msm_dsi_host_xfer_prepare() is
> already causing the drop-outs, even when no command buffers, DMA
> transfers etc. are involved. I then drilled further down and it showed
> that at least
> 
>   clk_set_rate(msm_host->byte_clk, msm_host->byte_clk_rate);
> 
> in dsi_link_clk_enable_6g() one of the culprits. If I don't touch the
> clocks anymore after the initialization is done, everything is fine.

Okay, I finally found the bits between the two trees that make the
difference. It's a downstream patch that is included in the Linaro 4.9
branch but that's missing upstream and in their 4.14 branch:


https://git.linaro.org/landing-teams/working/qualcomm/kernel.git/commit/?h=release/qcomlt-4.9&id=4ce2d6108d

What this patch does as a side-effect is that is sets the clock's actual
rate to the requested rate (core->rate = core->new_rate), and that fixes
the problem I'm seeing.

It shows an underlying problem in the msm driver's clock components
though, because without this patch, the clocks will be effectively
slightly off from what was requested. For instance, when
gcc_mdss_byte0_clk is configured to 56250000 Hz by the msm drm driver,
the clk core will let the DSI PLL recalculate its actual rate which is
56246337 Hz. Hence, clk_set_rate() will always end up fiddling with the
knobs of that clock provider. That's what happens every time DSI
commands are sent, and that causes the image to flicker.

The same problem applies to other clocks too. dsi0vco_clk for example
will be 449970703 rather than the requested 450000000 etc.

I guess a way to fix this properly would be to use
msm_dsi_pll_helper_clk_round_rate() to actually round the rates, but I'm
not sure.


Thanks,
Daniel