[PATCH v2 2/3] drm/msm/dsi: Set PHY usescase (and mode) before registering DSI host

[Dmitry Baryshkov]

On Mon, Feb 10, 2025 at 01:54:29PM +0100, Marijn Suijten wrote:
> On 2025-02-10 01:11:59, Dmitry Baryshkov wrote:
> > On Sun, Feb 09, 2025 at 10:42:53PM +0100, Marijn Suijten wrote:
> > > Ordering issues here cause an uninitialized (default STANDALONE)
> > > usecase to be programmed (which appears to be a MUX) in some cases
> > > when msm_dsi_host_register() is called, leading to the slave PLL in
> > > bonded-DSI mode to source from a clock parent (dsi1vco) that is off.
> > > 
> > > This should seemingly not be a problem as the actual dispcc clocks from
> > > DSI1 that are muxed in the clock tree of DSI0 are way further down, this
> > > bit still seems to have an effect on them somehow and causes the right
> > > side of the panel controlled by DSI1 to not function.
> > > 
> > > In an ideal world this code is refactored to no longer have such
> > > error-prone calls "across subsystems", and instead model the "PLL src"
> > > register field as a regular mux so that changing the clock parents
> > > programmatically or in DTS via `assigned-clock-parents` has the
> > > desired effect.
> > > But for the avid reader, the clocks that we *are* muxing into DSI0's
> > > tree are way further down, so if this bit turns out to be a simple mux
> > > between dsiXvco and out_div, that shouldn't have any effect as this
> > > whole tree is off anyway.
> > > 
> > > Signed-off-by: Marijn Suijten <marijn.suijten at somainline.org>
> > > ---
> > >  drivers/gpu/drm/msm/dsi/dsi_manager.c | 30 +++++++++++++++++++-----------
> > >  1 file changed, 19 insertions(+), 11 deletions(-)
> > 
> > 
> > Fixes: 57bf43389337 ("drm/msm/dsi: Pass down use case to PHY")
> 
> I'm not exactly confident about that.  Abhinav pointed out in
> https://gitlab.freedesktop.org/drm/msm/-/issues/41#note_2375646 that
> msm_dsi_host_register() was not supposed to be enabling the PHY, which I
> provided a counter-stacktrace for to show that is indeed the case.
> 
> Either this was always a problem that's only become visible now (and it's an
> issue with that patch), or a different change causes msm_dsi_host_register()
> to enable the PHY and program the usecase too early?

As currently usecase is being programmed after the DSI host being
registered, there might be a race condition between panel driver probe
_and_ usecase programming.

> 
> What do you think?
> 
> - Marijn
> 
> > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> > 
> > > 
> > > diff --git a/drivers/gpu/drm/msm/dsi/dsi_manager.c b/drivers/gpu/drm/msm/dsi/dsi_manager.c
> > > index a210b7c9e5ca281a46fbdb226e25832719a684ea..b93205c034e4acc73d536deeddce6ebd694b4a80 100644
> > > --- a/drivers/gpu/drm/msm/dsi/dsi_manager.c
> > > +++ b/drivers/gpu/drm/msm/dsi/dsi_manager.c
> > > @@ -74,17 +74,33 @@ static int dsi_mgr_setup_components(int id)
> > >  	int ret;
> > >  
> > >  	if (!IS_BONDED_DSI()) {
> > > +		/* Set the usecase before calling msm_dsi_host_register(), which would
> > > +		 * already program the PLL source mux based on a default usecase.
> > > +		 */
> > > +		msm_dsi_phy_set_usecase(msm_dsi->phy, MSM_DSI_PHY_STANDALONE);
> > > +		msm_dsi_host_set_phy_mode(msm_dsi->host, msm_dsi->phy);
> > > +
> > >  		ret = msm_dsi_host_register(msm_dsi->host);
> > >  		if (ret)
> > >  			return ret;
> > > -
> > > -		msm_dsi_phy_set_usecase(msm_dsi->phy, MSM_DSI_PHY_STANDALONE);
> > > -		msm_dsi_host_set_phy_mode(msm_dsi->host, msm_dsi->phy);
> > >  	} else if (other_dsi) {
> > >  		struct msm_dsi *master_link_dsi = IS_MASTER_DSI_LINK(id) ?
> > >  							msm_dsi : other_dsi;
> > >  		struct msm_dsi *slave_link_dsi = IS_MASTER_DSI_LINK(id) ?
> > >  							other_dsi : msm_dsi;
> > > +
> > > +		/* PLL0 is to drive both DSI link clocks in bonded DSI mode.
> > > +		 *
> > > +		/* Set the usecase before calling msm_dsi_host_register(), which would
> > > +		 * already program the PLL source mux based on a default usecase.
> > > +		 */
> > > +		msm_dsi_phy_set_usecase(clk_master_dsi->phy,
> > > +					MSM_DSI_PHY_MASTER);
> > > +		msm_dsi_phy_set_usecase(clk_slave_dsi->phy,
> > > +					MSM_DSI_PHY_SLAVE);
> > > +		msm_dsi_host_set_phy_mode(msm_dsi->host, msm_dsi->phy);
> > > +		msm_dsi_host_set_phy_mode(other_dsi->host, other_dsi->phy);
> > > +
> > >  		/* Register slave host first, so that slave DSI device
> > >  		 * has a chance to probe, and do not block the master
> > >  		 * DSI device's probe.
> > > @@ -98,14 +114,6 @@ static int dsi_mgr_setup_components(int id)
> > >  		ret = msm_dsi_host_register(master_link_dsi->host);
> > >  		if (ret)
> > >  			return ret;
> > > -
> > > -		/* PLL0 is to drive both 2 DSI link clocks in bonded DSI mode. */
> > > -		msm_dsi_phy_set_usecase(clk_master_dsi->phy,
> > > -					MSM_DSI_PHY_MASTER);
> > > -		msm_dsi_phy_set_usecase(clk_slave_dsi->phy,
> > > -					MSM_DSI_PHY_SLAVE);
> > > -		msm_dsi_host_set_phy_mode(msm_dsi->host, msm_dsi->phy);
> > > -		msm_dsi_host_set_phy_mode(other_dsi->host, other_dsi->phy);
> > >  	}
> > >  
> > >  	return 0;
> > > 
> > > -- 
> > > 2.48.1
> > > 
> > 
> > -- 
> > With best wishes
> > Dmitry

-- 
With best wishes
Dmitry