通过跟踪内核代码,发现模拟SPI SCK的频率在$(KERN-DIR)/drivers/spi/spi-bitbang-rxtx.h函数bitbang_txrx_be_cpha1里:
static inline u32
bitbang_txrx_be_cpha0(struct spi_device *spi,
unsigned nsecs, unsigned cpol, unsigned flags,
u32 word, u8 bits)
{
/* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
u32 oldbit = (!(word & (1<<(bits-1)))) << 31;
//printk("<***Liuyh***>: nsecs: %d cpol: %d flags: %d word:%d bits:%d\n",nsecs,cpol,flags,word,bits);
/* clock starts at inactive polarity */
for (word <<= (32 - bits); likely(bits); bits--) {
/* setup MSB (to slave) on trailing edge */
if ((flags & SPI_MASTER_NO_TX) == 0) {
if ((word & (1 << 31)) != oldbit) {
setmosi(spi, word & (1 << 31));
oldbit = word & (1 << 31);
}
}
spidelay(nsecs); /* T(setup) */
setsck(spi, !cpol);
spidelay(nsecs);
/* sample MSB (from slave) on leading edge */
word <<= 1;
if ((flags & SPI_MASTER_NO_RX) == 0)
word |= getmiso(spi);
setsck(spi, cpol);
}
return word;
}
可以发现,我们通过spidev.out -s 125000设置频率参数,最终会换算成nsecs。频率换算成nesec是通过$(KERN-DIR)/drivers/spi/spi-bitbang.c里的spi_bitbang_setup_transfer函数完成:
int spi_bitbang_setup_transfer(struct spi_device *spi, struct spi_transfer *t)
{
struct spi_bitbang_cs *cs = spi->controller_state;
u8 bits_per_word;
u32 hz;
if (t) {
bits_per_word = t->bits_per_word;
hz = t->speed_hz;
} else {
bits_per_word = 0;
hz = 0;
}
/* spi_transfer level calls that work per-word */
if (!bits_per_word)
bits_per_word = spi->bits_per_word;
if (bits_per_word <= 8)
cs->txrx_bufs = bitbang_txrx_8;
else if (bits_per_word <= 16)
cs->txrx_bufs = bitbang_txrx_16;
else if (bits_per_word <= 32)
cs->txrx_bufs = bitbang_txrx_32;
else
return -EINVAL;
/* nsecs = (clock period)/2 */
if (!hz)
hz = spi->max_speed_hz;
if (hz) {
cs->nsecs = (1000000000/2) / hz;
if (cs->nsecs > (MAX_UDELAY_MS * 1000 * 1000))
return -EINVAL;
}
return 0;
}
通过分析上面折代码可以发现,125000 hz频率换算后,结果是4000nsecs,所以在bitbang_txrx_be_cpha0中,最终通过spidelay(nsecs)实现SCK的时钟频率。
接下来进一步分析spidelay函数的定义:
/*
* NOTE: this clocks "as fast as we can". It "should" be a function of the
* requested device clock. Software overhead means we usually have trouble
* reaching even one Mbit/sec (except when we can inline bitops), so for now
* we'll just assume we never need additional per-bit slowdowns.
*/
#define spidelay(nsecs) do {} while (0)
通过分析上面问题,发现频率没有设置成功的原因是出现在spidelay上面。
#if defined (CONFIG_SPI_GPIO_DELAY)
#define spidelay(nsecs) ndelay(nsecs)
#else
/*
* NOTE: this clocks "as fast as we can". It "should" be a function of the
* requested device clock. Software overhead means we usually have trouble
* reaching even one Mbit/sec (except when we can inline bitops), so for now
* we'll just assume we never need additional per-bit slowdowns.
*/
#define spidelay(nsecs) do {} while (0)
#endif