On Wednesday, April 18, 2018 10:06 AM, Archit Taneja wrote: > On Tuesday 17 April 2018 05:51 PM, Daniel Mack wrote: > Thanks for debugging this so thoroughly. > >> 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. > > If I understood right, the main cause of the flicker is that we always > end up re-locking/reconfiguring the DSI PLL every time we send a command > (since dsi_link_clk_enable() is called in msm_dsi_host_xfer_prepare())? > The re-configuration results in a glitch on the DSI clock, which is > probably unacceptable for DSI Video mode transfer, especially for panels > that don't have their own timing generators, which rely entirely on > DSI clock lanes for scanning out the pixel data. > > According to you, the reason why the reconfiguration happens is because > the DSI PLL was never set exactly to 56.25 Mhz in the first place, and > the core clock framework notices a difference in the requested rate and > the current rate (56.246 Mhz), and goes ahead to configure the PLL > again when it's not needed. And this was averted in the downstream > patch you mentioned as a side affect? Yes, exactly. >> The same problem applies to other clocks too. dsi0vco_clk for example >> will be 449970703 rather than the requested 450000000 etc. >> > > One easy way to get around this would be to not try to set the clock > rates every time we try to send a command. We just enable/disable them. Yes, that could work as well, but how about rounding the rates in the callback that exists for that purpose? We're off by a fraction of a permille only, after all. Thanks, Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
