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: <CANwerB2Xhd_Pfg2bKX9TUzAkvc56urtk4-4CbxvP0OG9A5qzTA@mail.gmail.com>
Date:   Fri, 5 Jan 2018 22:08:33 +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 21:03, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> On Fri, Jan 05, 2018 at 09:44:39AM +1100, Jonathan Liu wrote:
>> 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.
>
> Ok. That explanation must be part of your commit log, or at least
> which problem you're trying to address and in which situation it will
> trigger.

Ok. I have added amended the commit message with explanation for v3.

Regards,
Jonathan