linux-kernel - Re: [PATCH 1/2] firmware: cs_dsp: Remove async regmap writes

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Message-ID: <e2f3397b-05c0-42f7-9950-29c4d9b24350@sirena.org.uk>
Date: Tue, 25 Feb 2025 15:15:15 +0000
From: Mark Brown <broonie@...nel.org>
To: Richard Fitzgerald <rf@...nsource.cirrus.com>
Cc: tiwai@...e.com, linux-sound@...r.kernel.org,
	linux-kernel@...r.kernel.org, patches@...nsource.cirrus.com
Subject: Re: [PATCH 1/2] firmware: cs_dsp: Remove async regmap writes

On Tue, Feb 25, 2025 at 01:18:42PM +0000, Richard Fitzgerald wrote:

> The async writes are a 12-year-old workaround for inefficiency of
> synchronous writes in the SPI subsystem. The SPI subsystem has since
> been changed to avoid the overheads, so this workaround should not be
> necessary.

That wasn't really the issue, it was that on systems with low numbers of
slower cores (especially single core but there were some dual core
systems too) there was a surprisingly low threashold where it was useful
to overlap the marshalling of data to send to the device with
transmitting already marshalled data.  SMP adds locking requirements
which loose a lot of the gains, and it's controller dependent where the
cuttoff points for interrupts and/or DMA are.

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