linux-kernel - Re: [PATCH 2/5] regmap/SoundWire: sdw: add support for SoundWire 1.2 MBQ

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Date:   Tue, 24 Nov 2020 14:06:54 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Bard Liao <yung-chuan.liao@...ux.intel.com>
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        tiwai@...e.de, broonie@...nel.org, gregkh@...uxfoundation.org,
        jank@...ence.com, srinivas.kandagatla@...aro.org,
        rander.wang@...ux.intel.com, ranjani.sridharan@...ux.intel.com,
        hui.wang@...onical.com, pierre-louis.bossart@...ux.intel.com,
        sanyog.r.kale@...el.com, mengdong.lin@...el.com,
        bard.liao@...el.com
Subject: Re: [PATCH 2/5] regmap/SoundWire: sdw: add support for SoundWire 1.2
 MBQ

On 04-11-20, 01:22, Bard Liao wrote:
> From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
> 
> The SoundWire 1.1 specification only allowed for reads and writes of
> bytes. The SoundWire 1.2 specification adds a new capability to
> transfer "Multi-Byte Quantities" (MBQ) across the bus. The transfers
> still happens one-byte-at-a-time, but the update is atomic.
> 
> For example when writing a 16-bit volume, the first byte transferred
> is only taken into account when the second byte is successfully
> transferred.
> 
> The mechanism is symmetrical for read and writes:
> - On a read, the address of the last byte to be read is modified by
> setting the MBQ bit
> - On a write, the address of all but the last byte to be written are
> modified by setting the MBQ bit. The address for the last byte relies
> on the MBQ bit being cleared.
> 
> The current definitions for MBQ-based controls in the SDCA draft
> standard are limited to 16 bits for volumes, so for now this is the
> only supported format. An update will be provided if and when support
> for 24-bit and 32-bit values is specified by the SDCA standard.
> 
> One possible objection is that this code could have been handled with
> regmap-sdw.c. However this is a new spec addition not handled by every
> SoundWire 1.1 and non-SDCA device, so there's no reason to load code
> that will never be used.
> 
> Also in practice it's extremely unlikely that CONFIG_REGMAP would not
> be selected with CONFIG_REGMAP_MBQ selected. However there's no
> functional dependency between the two modules so they can be selected
> separately.

Reviewed-by: Vinod Koul <vkoul@...nel.org>

-- 
~Vinod