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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. >>> >> >> I tried to implement PIO mode in the driver, but it turned out to be too >> slow. Due to the lack of an internal FIFO, data could only be >> transmitted one word at a time, and each transmission required >> reconfiguring the corresponding registers. As a result, the efficiency >> was quite low. > > One of the use cases is for SPI-based displays and it transfers one > word via PIO and then a lot of words via DMA. I see PIO as beneficial > for this common use case for SPI. > The efficiency does not need to be high for this one word but reducing > the latency for DMA setup is a significant gain. > For the new SPISG driver, regardless of the mode(PIO, DMA(MEM or sg), the configuration registers are the same; the only difference value in the mode selection. see reg cfg_start. >> >> The only simplifications provided by PIO mode were in two areas: >> >> 1. The allocation and release of the transfer descriptor >> 2. The DMA mapping and unmapping process >> >> Therefore, I suggest not implementing PIO mode in this driver. I will >> document clearly in the code that PIO mode is not supported and explain >> the reasons behind this decision. >> >> _______________________________________________ >> linux-amlogic mailing list >> linux-amlogic@lists.infradead.org >> http://lists.infradead.org/mailman/listinfo/linux-amlogic