SPI 问题

embeddedlwp

spi_transfer结构中有一个变量cs_change，它什么时候为0，什么时候为1啊

amarant

找一個標量在哪改變過用cscope
在vim下使用
: cs f s cs_change
就能看見這個變量在哪些代碼裏被改變過

asuka2001

include/linux/spi/spi.h中的注释说的很清楚，我偷懒直接摘抄了其中一段，如果希望了解的更清楚，可以去看spi_transfer的注释!

* All SPI transfers start with the relevant chipselect active.  Normally
* it stays selected until after the last transfer in a message.  Drivers
* can affect the chipselect signal using cs_change.
*
* (i) If the transfer isn't the last one in the message, this flag is
* used to make the chipselect briefly go inactive in the middle of the
* message.  Toggling chipselect in this way may be needed to terminate
* a chip command, letting a single spi_message perform all of group of
* chip transactions together.
*
* (ii) When the transfer is the last one in the message, the chip may
* stay selected until the next transfer.  On multi-device SPI busses
* with nothing blocking messages going to other devices, this is just
* a performance hint; starting a message to another device deselects
* this one.  But in other cases, this can be used to ensure correctness.
* Some devices need protocol transactions to be built from a series of
* spi_message submissions, where the content of one message is determined
* by the results of previous messages and where the whole transaction
* ends when the chipselect goes intactive.

笼统点说，cs_change其实是指示在bit流传输完毕后是否将片选设置为无效