linux-kernel - Re: [PATCH v4 2/3] spi: Add Amlogic SPISG driver

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Message-ID: <aG0hU5nbjLjTWS6p@finisterre.sirena.org.uk>
Date: Tue, 8 Jul 2025 14:50:08 +0100
From: Mark Brown <broonie@...nel.org>
To: Xianwei Zhao <xianwei.zhao@...ogic.com>
Cc: Sunny Luo <sunny.luo@...ogic.com>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	linux-amlogic@...ts.infradead.org, linux-spi@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] spi: Add Amlogic SPISG driver

On Tue, Jul 08, 2025 at 06:34:02PM +0800, Xianwei Zhao wrote:
> On 2025/7/7 21:05, Mark Brown wrote:

> > Is it worth having a copybreak such that smaller transfers are done
> > using PIO?  With a lot of controllers that increases performance due to
> > the extra overhead of setting up DMA, talking to the DMA and interrupt
> > controllers can be as expensive as directly accessing the FIFOs.

> If the data volume of a single transfer (xfer) is small, PIO mode does offer
> some advantages. However, since PIO requires the CPU to wait in a busy loop
> for the transfer to complete, it continuously occupies CPU resources. As a
> result, its advantages are not particularly significant.

The CPU overhead tends to be higher (you can avoid some of it with a
dead reckoning sleep), but the latency vastly improved which for many
applications is a worthwhile advantage.  It tends to be things like
accesses to one or two registers on a device with registers where this
wins, 16 bytes or lower would be a common number off the top of my head.

> If PIO is to be implemented, it can only handle one transfer at a time (via
> transfer_one), and not entire messages (which consist of multiple
> transfers). In contrast, when processing messages, the SPI controller can
> handle the entire sequence in one go, which also provides certain benefits.

It's probably worth adding something to the framework to be able to take
a decision at the message level, for writes this tends to all fall out
naturally since the write will tend to be a single transfer anyway.

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