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

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Date:   Fri, 18 Nov 2016 10:09:32 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, David Airlie <airlied@...ux.ie>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/sun4i: tcon: Initialize regmap after enabling bus clocks

On Fri, Nov 18, 2016 at 3:05 AM, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> 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.

That makes sense. I'll send a v2.

ChenYu