linux-kernel - Re: [PATCH] ALSA: hda: Use maple tree register cache

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Date:   Tue, 13 Jun 2023 16:24:28 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Mark Brown <broonie@...nel.org>
Cc:     Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.com>,
        alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ALSA: hda: Use maple tree register cache

On Tue, 13 Jun 2023 15:59:14 +0200,
Mark Brown wrote:
> 
> On Tue, Jun 13, 2023 at 09:36:34AM +0200, Takashi Iwai wrote:
> 
> > This is an error from regache_sync_val(), and it indicates that the
> > synced register is write-only; regcache_maple_sync() tries to sync all
> > cached values no matter whether it's writable or not, then hitting
> > this.
> 
> BTW I was just looking at reg_raw_update_once() and I can't figure out
> why it's trying to do what it's doing - it does a read to check if it's
> seen the register before and then does an _update_bits() if the register
> hasn't been cached yet, apparently trying suppress duplicate writes but
> possibly deliberately discarding changes to multiple bitfields in the
> same register.  That's not what the non-regmap path does, it'll only
> discard noop changes to the same bitfield.

Yes, it's a quite hackish way of optimization of the initialization.

Since HD-audio codec has no known default values unlike normal codecs,
it needs to initialize itself only at the first access, and this
helper does it.


Takashi