linux-kernel - Re: [PATCH 0/2] soundwire: Remove redundant zeroing of page registers

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Message-ID: <002826da-f22f-9ce2-1688-345fe4e3c7ed@linux.intel.com>
Date:   Thu, 1 Dec 2022 12:31:44 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>, vkoul@...nel.org
Cc:     alsa-devel@...a-project.org, patches@...nsource.cirrus.com,
        linux-kernel@...r.kernel.org, sanyog.r.kale@...el.com,
        yung-chuan.liao@...ux.intel.com
Subject: Re: [PATCH 0/2] soundwire: Remove redundant zeroing of page registers



On 12/1/22 08:08, Richard Fitzgerald wrote:
> Writing zero to the page registers after each message transaction can add
> up to a lot of overhead for codecs that need to transfer large amount of
> data - for example a firmware download.
> 
> There's no spec reason I can see for this zeroing. The page registers are
> only used for a paged address. The bus code uses a non-paged address for
> registers in page 0. It always writes the page registers at the start of
> a paged transaction.
> 
> If this zeroing was a workaround for anything, let me know and I will
> re-implement the zeroing as a quirk that can be enabled only when it is
> necessary.

It's a feature, not a bug :-)

The page registers have to be zeroed out so that any bus-management
command hits the page0 instead of using a value that was set by codec
driver for vendor-specific configurations.

The implementation is far from optimal though, and indeed if we have
long transactions that are not interrupted by anything else we could
avoid resetting the page registers.

I tried to implement a 'lazy approach' some time back, but at the time I
didn't see any benefits due to the limited number of configurations.

I can't remember where the code is, but the initial enhancement was
listed here: https://github.com/thesofproject/linux/issues/2881

> 
> Richard Fitzgerald (2):
>   soundwire: bus: Don't zero page registers after every transaction
>   soundwire: bus: Remove unused reset_page_addr() callback
> 
>  drivers/soundwire/bus.c             | 23 -----------------------
>  drivers/soundwire/cadence_master.c  | 14 --------------
>  drivers/soundwire/cadence_master.h  |  3 ---
>  drivers/soundwire/intel_auxdevice.c |  1 -
>  include/linux/soundwire/sdw.h       |  3 ---
>  5 files changed, 44 deletions(-)
>