linux-kernel - Re: [PATCH] drm/sun4i: tcon: Initialize regmap after enabling bus clocks

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Message-ID: <20161117190501.hjpwposqzidwhwdc@lukather>
Date:   Thu, 17 Nov 2016 20:05:01 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     David Airlie <airlied@...ux.ie>, linux-sunxi@...glegroups.com,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/sun4i: tcon: Initialize regmap after enabling bus
 clocks

On Wed, Nov 16, 2016 at 05:37:32PM +0800, Chen-Yu Tsai wrote:
> If we attempt to read/write the TCON registers before the bus clock
> is enabled, those accesses get ignored.
> 
> In practice this almost never occurs because U-boot had already enabled
> the bus clock as part of its firmware provided framebuffer (simplefb).
> 
> Fixes: 9026e0d122ac ("drm: Add Allwinner A10 Display Engine support")
> Signed-off-by: Chen-Yu Tsai <wens@...e.org>
> ---
> 
> I was looking around the DRM driver and noticed this sequence was off.
> 
> ---
>  drivers/gpu/drm/sun4i/sun4i_tcon.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.c b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> index c6afb2448655..8c2db65ea229 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_tcon.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> @@ -506,16 +506,16 @@ static int sun4i_tcon_bind(struct device *dev, struct device *master,
>  		return ret;
>  	}
>  
> -	ret = sun4i_tcon_init_regmap(dev, tcon);
> +	ret = sun4i_tcon_init_clocks(dev, tcon);
>  	if (ret) {
> -		dev_err(dev, "Couldn't init our TCON regmap\n");
> +		dev_err(dev, "Couldn't init our TCON clocks\n");
>  		goto err_assert_reset;
>  	}
>  
> -	ret = sun4i_tcon_init_clocks(dev, tcon);
> +	ret = sun4i_tcon_init_regmap(dev, tcon);
>  	if (ret) {
> -		dev_err(dev, "Couldn't init our TCON clocks\n");
> -		goto err_assert_reset;
> +		dev_err(dev, "Couldn't init our TCON regmap\n");
> +		goto err_free_clocks;
>  	}

That won't work either. sun4i_tcon_init_clocks requires the regmap to
be enabled before it calls sun4i_dclk_create.

Maybe we should just move that call outside of sun4i_tcon_init_clocks
and put it directly into the bind then.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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