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On 2025/7/8 21:50, Mark Brown wrote: > Subject: > Re: [PATCH v4 2/3] spi: Add Amlogic SPISG driver > From: > Mark Brown > Date: > 2025/7/8 21:50 > > To: > Xianwei Zhao > CC: > Sunny Luo , Rob Herring , > Krzysztof Kozlowski , Conor Dooley > , linux-amlogic@lists.infradead.org, > linux-spi@vger.kernel.org, devicetree@vger.kernel.org, > linux-kernel@vger.kernel.org > > > > 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. I will try to add new API message_can_dma for framework, and implement PIO for message.