linux-kernel - Re: [PATCH] drm/bridge/synopsys: dw-hdmi: Handle audio for more clock rates

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Date:   Wed, 19 Jun 2019 17:22:56 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Jernej Škrabec <jernej.skrabec@...l.net>,
        Douglas Anderson <dianders@...omium.org>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        seanpaul@...omium.org, heiko@...ech.de, jonas@...boo.se,
        linux-rockchip@...ts.infradead.org, dgreid@...omium.org,
        cychiang@...omium.org, David Airlie <airlied@...ux.ie>,
        Zheng Yang <zhengyang@...k-chips.com>,
        Sam Ravnborg <sam@...nborg.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH] drm/bridge/synopsys: dw-hdmi: Handle audio for more clock
 rates

On 18/06/2019 19:23, Jernej Škrabec wrote:
> Hi!
> 
> Dne torek, 18. junij 2019 ob 01:55:58 CEST je Douglas Anderson napisal(a):
>> Let's add some better support for HDMI audio to dw_hdmi.
>> Specifically:
>>
>> 1. For 44.1 kHz audio the old code made the assumption that an N of
>> 6272 was right most of the time.  That wasn't true and the new table
>> should give better 44.1 kHz audio for many more rates.
>>
>> 2. The new table has values from the HDMI spec for 297 MHz and 594
>> MHz.
>>
>> 3. There is now code to try to come up with a more idea N/CTS for
>> clock rates that aren't in the table.  This code is a bit slow because
>> it iterates over every possible value of N and picks the best one, but
>> it should make a good fallback.
>>
>> 4. The new code allows for platforms that know they make a clock rate
>> slightly differently to pick different N/CTS values.  For instance on
>> rk3288 we can make 25,176,471 Hz instead of 25,174,825.1748... Hz
>> (25.2 MHz / 1.001).  A future patch to the rk3288 platform code could
>> enable support for this clock rate and specify the N/CTS that would be
>> ideal.
>>
>> NOTE: the oddest part of this patch comes about because computing the
>> ideal N/CTS means knowing the _exact_ clock rate, not a rounded
>> version of it.  The drm framework makes this harder by rounding rates
>> to kHz, but even if it didn't there might be cases where the ideal
>> rate could only be calculated if we knew the real (non-integral) rate.
>> This means that in cases where we know (or believe) that the true rate
>> is something other than the rate we are told by drm.
>>
>> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> 
> Which bus is used for audio transfer on your device? If it is I2S, which is 
> commonly used, then please be aware of this patch:
> https://lists.freedesktop.org/archives/dri-devel/2019-June/221539.html
> 
> It avoids exact N/CTS calculation by enabling auto detection. It is well 
> tested on multiple SoCs from Allwinner, Amlogic and Rockchip.
> 
> Best regards,
> Jernej
> 
> 
Hi Douglas,

Thanks for your work !

If you could rebase on top of https://lists.freedesktop.org/archives/dri-devel/2019-June/221539.html
so we can make use of your extended N table with automatic CTS HW calculation, it would be great !

Finally could you add the plat_data tmds table as a separate patch to simplify review ?

Thanks,
Neil