| Message ID | 1490787091-21008-3-git-send-email-philipp.tomsich@theobroma-systems.com |
|---|---|
| State | Changes Requested |
| Delegated to: | Simon Glass |
| Headers | show |
On 29 March 2017 at 05:31, Philipp Tomsich <philipp.tomsich@theobroma-systems.com> wrote: > For the RK3399, i2c_set_rate (and by extension: our spi_set_rate, > which had been mindlessly following the template of the i2c_set_rate > implementation) miscalculates the rate returned due to a off-by-one > error resulting from the following sequence of events: > 1. calculates 'src_div := src_freq / target_freq' > 2. stores 'src_div - 1' into the register (the actual divider applied > in hardware is biased by adding 1) > 3. returns the result of the DIV_RATE(src_freq, src_div) macro, which > expects the (decremented) divider from the hardware-register and > implictly adds 1 (i.e. 'DIV_RATE(freq, div) := freq / (div + 1)') > > This can be observed with the SPI driver, which sets a rate of 99MHz > based on the GPLL frequency of 594MHz: the hardware generates a clock > of 99MHz (src_div is 6, the bitfield in the register correctly reads 5), > but reports a frequency of 84MHz (594 / 7) on return. > > To fix, we have two options: > * either we bias (i.e. "DIV_RATE(GPLL, src_div - 1)"), which doesn't > make for a particularily nice read > * we simply call the i2c/spi_get_rate function (introducing additional > overhead for the additional register-read), which reads the divider > from the register and then passes it through the DIV_RATE macro > > Given that this code is not time-critical, the more readable solution > (i.e. calling the appropriate get_rate function) is implemented in this > change. > > Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com> > Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com> > > --- > > Changes in v2: > - fixes an off-by-one for the RK3399 that cause the SPI module input > clock to be misstated as 84MHz (even though it was running at 99MHz) > > drivers/clk/rockchip/clk_rk3399.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/clk/rockchip/clk_rk3399.c b/drivers/clk/rockchip/clk_rk3399.c index 9150183..8e0d5af 100644 --- a/drivers/clk/rockchip/clk_rk3399.c +++ b/drivers/clk/rockchip/clk_rk3399.c @@ -602,7 +602,7 @@ static ulong rk3399_i2c_set_clk(struct rk3399_cru *cru, ulong clk_id, uint hz) return -EINVAL; } - return DIV_TO_RATE(GPLL_HZ, src_clk_div); + return rk3399_i2c_get_clk(cru, clk_id); } #define SPI_CLK_REG_MASK(bus) \ @@ -663,7 +663,7 @@ static ulong rk3399_spi_set_clk(struct rk3399_cru *cru, ulong clk_id, uint hz) return -EINVAL; } - return DIV_TO_RATE(GPLL_HZ, src_clk_div); + return rk3399_spi_get_clk(cru, clk_id); } static ulong rk3399_vop_set_clk(struct rk3399_cru *cru, ulong clk_id, u32 hz)
For the RK3399, i2c_set_rate (and by extension: our spi_set_rate, which had been mindlessly following the template of the i2c_set_rate implementation) miscalculates the rate returned due to a off-by-one error resulting from the following sequence of events: 1. calculates 'src_div := src_freq / target_freq' 2. stores 'src_div - 1' into the register (the actual divider applied in hardware is biased by adding 1) 3. returns the result of the DIV_RATE(src_freq, src_div) macro, which expects the (decremented) divider from the hardware-register and implictly adds 1 (i.e. 'DIV_RATE(freq, div) := freq / (div + 1)') This can be observed with the SPI driver, which sets a rate of 99MHz based on the GPLL frequency of 594MHz: the hardware generates a clock of 99MHz (src_div is 6, the bitfield in the register correctly reads 5), but reports a frequency of 84MHz (594 / 7) on return. To fix, we have two options: * either we bias (i.e. "DIV_RATE(GPLL, src_div - 1)"), which doesn't make for a particularily nice read * we simply call the i2c/spi_get_rate function (introducing additional overhead for the additional register-read), which reads the divider from the register and then passes it through the DIV_RATE macro Given that this code is not time-critical, the more readable solution (i.e. calling the appropriate get_rate function) is implemented in this change. Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com> Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com> --- Changes in v2: - fixes an off-by-one for the RK3399 that cause the SPI module input clock to be misstated as 84MHz (even though it was running at 99MHz) drivers/clk/rockchip/clk_rk3399.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)