linux-kernel - Re: [PATCH] drm: tilcdc: add a workaround for failed clk_set

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Date:   Wed, 28 Sep 2016 13:43:50 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Tomi Valkeinen <tomi.valkeinen@...com>
Cc:     Jyri Sarha <jsarha@...com>, David Airlie <airlied@...ux.ie>,
        Kevin Hilman <khilman@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        linux-drm <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, Sekhar Nori <nsekhar@...com>
Subject: Re: [PATCH] drm: tilcdc: add a workaround for failed clk_set_rate()

+ Sekhar

2016-09-28 13:19 GMT+02:00 Tomi Valkeinen <tomi.valkeinen@...com>:
> Hi,
>
> On 27/09/16 18:29, Bartosz Golaszewski wrote:
>> Some architectures don't use the common clock framework and don't
>> implement all the clk interfaces for every clock. This is the case
>> for da850-lcdk where clk_set_rate() only works for PLL0 and PLL1.
>>
>> Trying to set the clock rate for the LCDC clock results in -EINVAL
>> being returned.
>>
>> As a workaround for that: if the call to clk_set_rate() fails, fall
>> back to adjusting the clock divider instead. Proper divider value is
>> calculated by dividing the current clock rate by the required pixel
>> clock rate in HZ.
>>
>> This code is based on a hack initially developed internally for
>> baylibre by Karl Beldan <kbeldan@...libre.com>.
>>
>> Tested with a da850-lcdk with an LCD display connected over VGA.
>>
>> Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>

[snip]

>
> If the clock divider path easily creates pixel clocks that are quite far
> from the requested ones (which I think it does), I think it would make
> sense to have a print here if the final pixel clock is far enough from
> the requested one. And that's a valid print for the current code path
> too, as it's not clear whether lcdc's input clock can be set to an exact
> value.
>
>>       DBG("lcd_clk=%lu, mode clock=%d, div=%u",
>>               lcd_clk, crtc->mode.clock, clkdiv);
>> @@ -664,7 +682,6 @@ void tilcdc_crtc_update_clk(struct drm_crtc *crtc)
>>               tilcdc_set(dev, LCDC_CLK_ENABLE_REG,
>>                               LCDC_V2_DMA_CLK_EN | LCDC_V2_LIDD_CLK_EN |
>>                               LCDC_V2_CORE_CLK_EN);
>> -

Hi Tomi,

how far is far enough to emit a warning? On da850 the requested rate
is 228000000 Hz, while the calculated divider is 6, which results in
the real rate of 225000000 Hz. This is less than 1% difference -
should we take this value as reference?

I'll apply all other requested changes.

Best regards,
Bartosz Golaszewski