Re: PROBLEM: Asus C201 video mode problems on HDMI hotplug (regression)

Nick Bowler
2017-12-05T03:22:47Z

On 2017-12-04 21:34 +0200, Laurent Pinchart wrote:
> On Monday, 4 December 2017 21:30:01 EET Nick Bowler wrote:
> > On 2017-12-04 21:06 +0200, Laurent Pinchart wrote:
> > > As you reported that the PLL lock failure message is not printed, the
> > > failure can only come from either the extra delay introduced by the
> > > above loop, or from reading the HDMI_PHY_STAT0 register.
> > >
> > > How many iterations of the for loop execute before the condition
> > > becomes true?
> >
> > Judging from the log posted elsethread (where I added extra printouts),
> > it seems to consistently become true on the second iteration.
> >
> > I will try to rule out read side effects by replacing the polling loop
> > with an unconditional delay.
>
> You're reading my mind :-)

I did this test by applying the following patch on 4.15-rc1, and the
problem remains. So it appears the delay is responsible somehow.

Cheers,
Nick

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index bf14214fa464..4aec4d5c130e 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -1101,8 +1101,6 @@ static void dw_hdmi_phy_power_off(struct dw_hdmi *hdmi)
static int dw_hdmi_phy_power_on(struct dw_hdmi *hdmi)
{
const struct dw_hdmi_phy_data *phy = hdmi->phy.data;
- unsigned int i;
- u8 val;

if (phy->gen == 1) {
dw_hdmi_phy_enable_powerdown(hdmi, false);
@@ -1116,21 +1114,7 @@ static int dw_hdmi_phy_power_on(struct dw_hdmi *hdmi)
dw_hdmi_phy_gen2_txpwron(hdmi, 1);
dw_hdmi_phy_gen2_pddq(hdmi, 0);

- /* Wait for PHY PLL lock */
- for (i = 0; i < 5; ++i) {
- val = hdmi_readb(hdmi, HDMI_PHY_STAT0) & HDMI_PHY_TX_PHY_LOCK;
- if (val)
- break;
-
- usleep_range(1000, 2000);
- }
-
- if (!val) {
- dev_err(hdmi->dev, "PHY PLL failed to lock\n");
- return -ETIMEDOUT;
- }
-
- dev_dbg(hdmi->dev, "PHY PLL locked %u iterations\n", i);
+ usleep_range(1000, 2000);
return 0;
}