From e257b8e50459d65fff5c1bed4d38523b810a3ba7 Mon Sep 17 00:00:00 2001 From: Douglas Anderson Date: Mon, 29 Jun 2020 16:41:06 -0700 Subject: [PATCH] spi: Avoid setting the chip select if we don't need to On some SPI controllers (like spi-geni-qcom) setting the chip select is a heavy operation. For instance on spi-geni-qcom, with the current code, is was measured as taking upwards of 20 us. Even on SPI controllers that aren't as heavy, setting the chip select is at least something like a MMIO operation over some peripheral bus which isn't as fast as a RAM access. While it would be good to find ways to mitigate problems like this in the drivers for those SPI controllers, it can also be noted that the SPI framework could also help out. Specifically, in some situations, we can see the SPI framework calling the driver's set_cs() with the same parameter several times in a row. This is specifically observed when looking at the way the Chrome OS EC SPI driver (cros_ec_spi) works but other drivers likely trip it to some extent. Let's solve this by caching the chip select state in the core and only calling into the controller if there was a change. We check not only the "enable" state but also the chip select mode (active high or active low) since controllers may care about both the mode and the enable flag in their callback. Signed-off-by: Douglas Anderson Link: https://lore.kernel.org/r/20200629164103.1.Ied8e8ad8bbb2df7f947e3bc5ea1c315e041785a2@changeid Signed-off-by: Mark Brown --- drivers/spi/spi.c | 11 +++++++++++ include/linux/spi/spi.h | 4 ++++ 2 files changed, 15 insertions(+) diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c index 6fa56590bba27..d4ba723a30da0 100644 --- a/drivers/spi/spi.c +++ b/drivers/spi/spi.c @@ -778,6 +778,17 @@ static void spi_set_cs(struct spi_device *spi, bool enable) { bool enable1 = enable; + /* + * Avoid calling into the driver (or doing delays) if the chip select + * isn't actually changing from the last time this was called. + */ + if ((spi->controller->last_cs_enable == enable) && + (spi->controller->last_cs_mode_high == (spi->mode & SPI_CS_HIGH))) + return; + + spi->controller->last_cs_enable = enable; + spi->controller->last_cs_mode_high = spi->mode & SPI_CS_HIGH; + if (!spi->controller->set_cs_timing) { if (enable1) spi_delay_exec(&spi->controller->cs_setup, NULL); diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h index b4917df796374..0e67a9a3a1d35 100644 --- a/include/linux/spi/spi.h +++ b/include/linux/spi/spi.h @@ -368,6 +368,8 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv) * @cur_msg_prepared: spi_prepare_message was called for the currently * in-flight message * @cur_msg_mapped: message has been mapped for DMA + * @last_cs_enable: was enable true on the last call to set_cs. + * @last_cs_mode_high: was (mode & SPI_CS_HIGH) true on the last call to set_cs. * @xfer_completion: used by core transfer_one_message() * @busy: message pump is busy * @running: message pump is running @@ -604,6 +606,8 @@ struct spi_controller { bool auto_runtime_pm; bool cur_msg_prepared; bool cur_msg_mapped; + bool last_cs_enable; + bool last_cs_mode_high; bool fallback; struct completion xfer_completion; size_t max_dma_len; -- 2.39.5