linux-kernel - Re: [PATCH v2 1/3] drm/sun4i: hdmi: Check for unset best_parent in sun4i_tmds_determine

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Message-ID: <CANwerB3SKX=H-mhQZ0mLaLXmPiohVwoF+8dbWMhq9Fw7eJA4AA@mail.gmail.com>
Date:   Fri, 5 Jan 2018 09:44:39 +1100
From:   Jonathan Liu <net147@...il.com>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     David Airlie <airlied@...ux.ie>, Chen-Yu Tsai <wens@...e.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH v2 1/3] drm/sun4i: hdmi: Check for unset best_parent in sun4i_tmds_determine_rate

Hi Maxime,

On 5 January 2018 at 06:56, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> On Tue, Dec 26, 2017 at 10:12:25PM +1100, Jonathan Liu wrote:
>> We should check if the best match has been set before comparing it.
>>
>> Fixes: 9c5681011a0c ("drm/sun4i: Add HDMI support")
>> Signed-off-by: Jonathan Liu <net147@...il.com>
>> ---
>>  drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> index dc332ea56f6c..4d235e5ea31c 100644
>> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_tmds_clk.c
>> @@ -102,7 +102,7 @@ static int sun4i_tmds_determine_rate(struct clk_hw *hw,
>>                                       goto out;
>>                               }
>>
>> -                             if (abs(rate - rounded / i) <
>> +                             if (!best_parent || abs(rate - rounded / i) <
>
> Why is that causing any issue?
>
> If best_parent is set to 0...
>
>>                                   abs(rate - best_parent / best_div)) {
>
> ... the value returned here is going to be rate, which is going to be
> higher than the first part of the comparison meaning ...
>
>>                                       best_parent = rounded;
>
> ... that best_parent is going to be set there.

Consider the following:
rate = 83500000
rounded = ideal * 2

It is possible that if "rounded = clk_hw_round_rate(parent, ideal)"
gives high enough values that the condition "abs(rate - rounded / i) <
abs(rate - best_parent / best_div)" is never met.

Then you can end up with:
req->rate = 0
req->best_parent_rate = 0
req->best_parent_hw = ...

Also, the sun4i_tmds_calc_divider function has a similar check.

Regards,
Jonathan