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

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Message-ID: <6219398.I55JWXAmVF@jernej-laptop>
Date:   Tue, 18 Jun 2019 19:23:18 +0200
From:   Jernej Škrabec <jernej.skrabec@...l.net>
To:     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, narmstrong@...libre.com,
        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

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