Re: [PATCH v2 1/2] drm/bridge/synopsys: dw-hdmi: Handle audio for more clock rates
From: Andrzej Hajda
Date: Wed Jun 26 2019 - 05:56:24 EST
On 25.06.2019 18:26, Doug Anderson wrote:
> On Tue, Jun 25, 2019 at 9:07 AM Andrzej Hajda <a.hajda@xxxxxxxxxxx> wrote:
>> On 19.06.2019 23:07, Douglas Anderson wrote:
>>> Let's add some better support for HDMI audio to dw_hdmi.
>>> 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 pick a more ideal value.
>> Why? I ask because it is against recommendation from HDMI specs.
> The place where it does matter (and why I originally did this work) is
> when you don't have auto-CTS. In such a case you really need "N" and
> "CTS" to make the math work and both be integral. This makes sure
> that you don't slowly accumulate offsets. I'm hoping that this point
> should be non-controversial so I won't argue it more.
> I am an admitted non-expert, but I have a feeling that with Auto-CTS
> either the old number or the new numbers would produce pretty much the
> same experience.
Because Auto-CTS mechanism will alternate between two or more CTS values
every frame, thus it will compensate non-rational clock relationship.
> AKA: anyone using auto-CTS won't notice any change
> at all. I guess the question is: with Auto-CTS should you pick the
> "ideal" 6272 or a value that allows CTS to be the closest to integral
> as possible. By reading between the lines of the spec, I decided that
> it was slightly more important to allow for an integral CTS. If
> achieving an integral CTS wasn't a goal then the spec wouldn't even
> have listed special cases for any of the clock rates. We would just
> be using the ideal N and Auto-CTS and be done with it. The whole
> point of the tables they list is to make CTS integral.
Specification recommends many contradictory things without explicit
prioritization, at least I have not found it.
So we should relay on our intuition.
I guess that with auto-cts N we should follow recommendation - I guess
most sinks have been better tested with recommended values.
So what with non-auto-cts case:
1. How many devices do not have auto-cts? how many alternative TMDS
clocks we have? Maybe it is theoretical problem.
2. Alternating CTS in software is possible, but quite
complicated/annoying, but at least it will follow recommendation :)
>>> 2. The new table has values from the HDMI spec for 297 MHz and 594
>>> 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.
>>> - 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.
>>> - This patch makes much less of a difference after the patch
>>> ("drm/bridge: dw-hdmi: Use automatic CTS generation mode when using
>>> non-AHB audio"), at least if you're using I2S audio. The main goal
>>> of picking a good N is to make it possible to get a nice integral
>>> CTS value, but if CTS is automatic then that's much less critical.
>> As I said above HDMI recommendations are different from those from your
>> patch. Please elaborate why?
>> Btw I've seen your old patches introducing recommended N/CTS calculation
>> helpers in HDMI framework, unfortunately abandoned due to lack of interest.
>> Maybe resurrecting them would be a good idea, with assumption there will
>> be users :)
> I have old patches introducing this into the HDMI framework? I don't
> remember them / can't find them. Can you provide a pointer?