linux-kernel - Re: [PATCH v5 1/6] drm/vc4: Avoid using vrefresh==0 mode in DSI htotal math.

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Message-ID: <87r2wrm3ub.fsf@eliezer.anholt.net>
Date:   Fri, 04 Aug 2017 14:15:56 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:     dri-devel@...ts.freedesktop.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Thierry Reding <thierry.reding@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/6] drm/vc4: Avoid using vrefresh==0 mode in DSI htotal math.

Boris Brezillon <boris.brezillon@...e-electrons.com> writes:

> On Tue, 18 Jul 2017 14:05:05 -0700
> Eric Anholt <eric@...olt.net> wrote:
>
>> The incoming mode might have a missing vrefresh field if it came from
>> drmModeSetCrtc(), which the kernel is supposed to calculate using
>> drm_mode_vrefresh().  We could either use that or the adjusted_mode's
>> original vrefresh value.
>> 
>> However, we can maintain a more exact vrefresh value (not just the
>> integer approximation), by scaling by the ratio of our clocks.
>> 
>> v2: Use math suggested by Andrzej Hajda instead.
>> 
>> Signed-off-by: Eric Anholt <eric@...olt.net>
>> ---
>>  drivers/gpu/drm/vc4/vc4_dsi.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
>> index 629d372633e6..57213f4e3c72 100644
>> --- a/drivers/gpu/drm/vc4/vc4_dsi.c
>> +++ b/drivers/gpu/drm/vc4/vc4_dsi.c
>> @@ -866,7 +866,8 @@ static bool vc4_dsi_encoder_mode_fixup(struct drm_encoder *encoder,
>>  	adjusted_mode->clock = pixel_clock_hz / 1000 + 1;
>>  
>>  	/* Given the new pixel clock, adjust HFP to keep vrefresh the same. */
>> -	adjusted_mode->htotal = pixel_clock_hz / (mode->vrefresh * mode->vtotal);
>> +	adjusted_mode->htotal = (pixel_clock_hz / 1000 * mode->htotal /
>> +				 mode->clock);
>
> Hm, I'm not sure I understand this. Shouldn't we have something like:
>
> 	adjusted_mode->htotal = (adjusted_mode->clock * mode->htotal) /
> 				mode->clock;
>
> Is there a reason for doing '+ 1' when you calculate the adjusted
> pixel clock rate but not here?

We're actually expecting to get within epsilon of pixel_clock_hz, but we
have to bump our clk_set_rate() to a higher value because the clock
driver will give you a bad divider if you ask for anything less than the
rate it can provide.

How about I don't increment the adjusted_mode->clock (since it'll be
userspace visible I think), and instead move that and the "Round up"
comment to the clk_set_rate()?

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