Re: [RFC PATCH 00/16] soundwire/ASoC: speed-up downloads with BTP/BRA protocol

[Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>]

On 12/7/23 16:56, Mark Brown wrote:
> On Thu, Dec 07, 2023 at 04:29:28PM -0600, Pierre-Louis Bossart wrote:
> 
>> The MIPI specification and most of the new codecs support the Bulk
>> Transfer Protocol (BTP) and specifically the Bulk Register Access
>> (BRA) configuration. This mode reclaims the 'audio' data space of the
>> SoundWire frame to send firmware/coefficients over the DataPort 0
>> (DP0).
> 
> So the bulk register access is accessing registers that are also visible
> through the one register at at time interface, just faster?

Yes, each frame can transmit a packet with a start address, length and a
bunch of data bytes protected with a CRC. With the default 50x4 frame
size we use, we can send 8 contiguous bytes per frame instead of 1. With
a larger frame you get even more bytes per frame.

Also because we program a large buffer with all the packets
pre-formatted by software, we don't have much software overhead. The
packets are streamed over DMA and inserted in the frame by hardware at
the relevant time. That means waiting for one DMA complete event instead
of dealing with thousands of command/responses with interrupts.

There are limitations though, if the frame is already transmitting audio
data then obviously we have a conflict.