linux-kernel - Re: [PATCH v4 6/9] drm: rcar-du: crtc: Enable and disable CMMs

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Message-ID: <20190918232336.GB11474@pendragon.ideasonboard.com>
Date:   Thu, 19 Sep 2019 02:23:36 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
Cc:     Jacopo Mondi <jacopo@...ndi.org>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>, geert@...ux-m68k.org,
        horms@...ge.net.au, uli+renesas@...nd.eu,
        VenkataRajesh.Kalakodima@...bosch.com, airlied@...ux.ie,
        daniel@...ll.ch, koji.matsuoka.xm@...esas.com, muroya@....co.jp,
        Harsha.ManjulaMallikarjun@...bosch.com,
        linux-renesas-soc@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/9] drm: rcar-du: crtc: Enable and disable CMMs

Hello,

On Thu, Sep 12, 2019 at 10:19:30AM +0100, Kieran Bingham wrote:
> On 12/09/2019 09:07, Jacopo Mondi wrote:
> > On Wed, Sep 11, 2019 at 07:40:27PM +0100, Kieran Bingham wrote:
> >> On 06/09/2019 14:54, Jacopo Mondi wrote:
> >>> Enable/disable the CMM associated with a CRTC at CRTC start and stop
> >>> time and enable the CMM unit through the Display Extensional Functions
> >>> register at group setup time.
> >>>
> >>> Reviewed-by: Ulrich Hecht <uli+renesas@...nd.eu>
> >>> Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> >>> Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
> >>> ---
> >>>  drivers/gpu/drm/rcar-du/rcar_du_crtc.c  | 7 +++++++
> >>>  drivers/gpu/drm/rcar-du/rcar_du_group.c | 8 ++++++++
> >>>  drivers/gpu/drm/rcar-du/rcar_du_regs.h  | 5 +++++
> >>>  3 files changed, 20 insertions(+)
> >>>
> >>> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
> >>> index 23f1d6cc1719..3dac605c3a67 100644
> >>> --- a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
> >>> +++ b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c
> >>> @@ -21,6 +21,7 @@
> >>>  #include <drm/drm_plane_helper.h>
> >>>  #include <drm/drm_vblank.h>
> >>>
> >>> +#include "rcar_cmm.h"
> >>>  #include "rcar_du_crtc.h"
> >>>  #include "rcar_du_drv.h"
> >>>  #include "rcar_du_encoder.h"
> >>> @@ -619,6 +620,9 @@ static void rcar_du_crtc_stop(struct rcar_du_crtc *rcrtc)
> >>>  	if (rcar_du_has(rcrtc->dev, RCAR_DU_FEATURE_VSP1_SOURCE))
> >>>  		rcar_du_vsp_disable(rcrtc);
> >>>
> >>> +	if (rcrtc->cmm)
> >>> +		rcar_cmm_disable(rcrtc->cmm);
> >>> +
> >>>  	/*
> >>>  	 * Select switch sync mode. This stops display operation and configures
> >>>  	 * the HSYNC and VSYNC signals as inputs.
> >>> @@ -686,6 +690,9 @@ static void rcar_du_crtc_atomic_enable(struct drm_crtc *crtc,
> >>>  	}
> >>>
> >>>  	rcar_du_crtc_start(rcrtc);
> >>> +
> >>> +	if (rcrtc->cmm)
> >>> +		rcar_cmm_enable(rcrtc->cmm);
> >>>  }
> >>>
> >>>  static void rcar_du_crtc_atomic_disable(struct drm_crtc *crtc,
> >>> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_group.c b/drivers/gpu/drm/rcar-du/rcar_du_group.c
> >>> index 9eee47969e77..25d0fc125d7a 100644
> >>> --- a/drivers/gpu/drm/rcar-du/rcar_du_group.c
> >>> +++ b/drivers/gpu/drm/rcar-du/rcar_du_group.c
> >>> @@ -147,6 +147,14 @@ static void rcar_du_group_setup(struct rcar_du_group *rgrp)
> >>>
> >>>  	rcar_du_group_setup_pins(rgrp);
> >>>
> >>> +	if (rcar_du_has(rcdu, RCAR_DU_FEATURE_CMM)) {
> >>> +		u32 defr7 = DEFR7_CODE
> >>> +			  | (rgrp->cmms_mask & BIT(1) ? DEFR7_CMME1 : 0)
> >>> +			  | (rgrp->cmms_mask & BIT(0) ? DEFR7_CMME0 : 0);
> >>> +
> >>> +		rcar_du_group_write(rgrp, DEFR7, defr7);
> >>> +	}
> >>> +
> >>
> >> What's the effect here on platforms with a CMM, but with
> >> CONFIG_DRM_RCAR_CMM unset?
> >>
> >> Will this incorrectly configure the DU ?
> >>
> >> Will it stall the display if the DU tries to interact with another
> >> module which is not enabled?
> > 
> > I recall I tested that (that's why I had to add stubs for CMM
> > functions, as I had linkage errors otherwise) and thing seems to be
> > fine as the CMM configuration/enblement resolve to an empty function.
> 
> Yes, I see the stubs to allow for linkage, but it's the hardware I'm
> concerned about. If it passes the tests and doesn't break then that's
> probably ok ... but I'm really weary that we're enabling a hardware
> pipeline with a disabled component in the middle.
> 
> > Would you prefer to have this guarded by an #if IS_ENABLED() ?
> 
> I don't think we need a compile time conditional, but I'd say it
> probably needs to be more about whether the CMM has actually probed or not
> 
> Aha, and I see that in rcar_du_cmm_init() we already do a
> call to rcar_cmm_init(), which if fails will leave rcdu->cmms[i] as
> NULL. So that's catered for, which results in the rgrp->cmms_mask being
> correctly representative of whether there is a CMM connected or not.

Doesn't this result in probe failure ?

>  ... so I think that means the ...
>  "if (rcar_du_has(rcdu, RCAR_DU_FEATURE_CMM))" is somewhat redundant:
> 
> 
> This could be:
> 
>   if (rgrp->cmms_mask) {
> 	u32 defr7 = DEFR7_CODE
> 		  | (rgrp->cmms_mask & BIT(1) ? DEFR7_CMME1 : 0)
> 		  | (rgrp->cmms_mask & BIT(0) ? DEFR7_CMME0 : 0);
> 
>   rcar_du_group_write(rgrp, DEFR7, defr7);
> 
> Or in fact, if we don't mind writing 0 to DEFR7 when there is no CMM
> (which is safe by the looks of things as DEFR7 is available on all
> platforms), then we can even remove the outer conditional, and leave
> this all up to the ternary operators to write the correct value to the
> defr7.
> 
> Phew ... net result - your current code *is* safe with the
> CONFIG_DRM_RCAR_CMM option disabled. I'll leave it up to you if you want
> to simplify the code here and remove the RCAR_DU_FEATURE_CMM.
> 
> As this RCAR_DU_FEATURE_CMM flag is only checked here, removing it would
> however simplify all of the rcar_du_device_info structures.
> 
> So - with or without the _FEATURE_CMM" simplification, this patch looks
> functional and safe so:
> 
> 
> Reviewed-by: Kieran Bingham <kieran.bingham+renesas@...asonboard.com>
> 
> >>>  	if (rcdu->info->gen >= 2) {
> >>>  		rcar_du_group_setup_defr8(rgrp);
> >>>  		rcar_du_group_setup_didsr(rgrp);
> >>> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_regs.h b/drivers/gpu/drm/rcar-du/rcar_du_regs.h
> >>> index bc87f080b170..fb9964949368 100644
> >>> --- a/drivers/gpu/drm/rcar-du/rcar_du_regs.h
> >>> +++ b/drivers/gpu/drm/rcar-du/rcar_du_regs.h
> >>> @@ -197,6 +197,11 @@
> >>>  #define DEFR6_MLOS1		(1 << 2)
> >>>  #define DEFR6_DEFAULT		(DEFR6_CODE | DEFR6_TCNE1)
> >>>
> >>> +#define DEFR7			0x000ec
> >>> +#define DEFR7_CODE		(0x7779 << 16)
> >>> +#define DEFR7_CMME1		BIT(6)
> >>> +#define DEFR7_CMME0		BIT(4)
> >>> +
> >>>  /* -----------------------------------------------------------------------------
> >>>   * R8A7790-only Control Registers
> >>>   */

-- 
Regards,

Laurent Pinchart