linux-kernel - Re: [PATCH 4/4] drm/atmel-hlcdc: do not immediately disable planes, wait for next frame

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Message-ID: <20190110182904.5efd08f2@bbrezillon>
Date:   Thu, 10 Jan 2019 18:29:17 +0100
From:   Boris Brezillon <bbrezillon@...nel.org>
To:     Peter Rosin <peda@...ntia.se>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        David Airlie <airlied@...ux.ie>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 4/4] drm/atmel-hlcdc: do not immediately disable planes,
 wait for next frame

On Thu, 10 Jan 2019 15:10:48 +0000
Peter Rosin <peda@...ntia.se> wrote:

> The A2Q and UPDATE bits have no effect in the channel disable registers.
> However, since they are present, assume that the intention is to disable
> planes, not immediately as indicated by the RST bit, but on the next
> frame shift since that is what A2Q and UPDATE means in the channel enable
> registers.
> 
> Disabling the plane on the next frame shift is done with the EN bit,
> so use that.

It's been a long time, but I think I had a good reason for forcing a
reset. IIRC, when you don't do that and the CRTC is disabled before the
plane, the EN bit stays around, and next time you queue a plane update,
you'll start with an invalid buf pointer.

> 
> Signed-off-by: Peter Rosin <peda@...ntia.se>
> ---
>  drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c
> index 05519e8c6586..f2f570642f84 100644
> --- a/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c
> +++ b/drivers/gpu/drm/atmel-hlcdc/atmel_hlcdc_plane.c
> @@ -728,9 +728,7 @@ static void atmel_hlcdc_plane_atomic_disable(struct drm_plane *p,
>  
>  	/* Disable the layer */
>  	atmel_hlcdc_layer_write_reg(&plane->layer, ATMEL_HLCDC_LAYER_CHDR,
> -				    ATMEL_HLCDC_LAYER_RST |
> -				    ATMEL_HLCDC_LAYER_A2Q |
> -				    ATMEL_HLCDC_LAYER_UPDATE);
> +				    ATMEL_HLCDC_LAYER_EN);
>  
>  	/* Clear all pending interrupts */
>  	atmel_hlcdc_layer_read_reg(&plane->layer, ATMEL_HLCDC_LAYER_ISR);