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67c15187d4
When SPI transfer is being prepared, the spi-s3c64xx driver will call
clk_set_rate() to change the rate of SPI source clock (IPCLK). But IPCLK
is a gate (leaf) clock, so it must propagate the rate change up the
clock tree, so that corresponding DIV clocks can actually change their
divider values. Add CLK_SET_RATE_PARENT flag to corresponding clocks for
all SPI instances in Exynos850 (spi_0, spi_1 and spi_2) to make it
possible. This change involves next clocks:
usi_spi_0:
Clock Block Div range
--------------------------------------------
gout_spi0_ipclk CMU_PERI -
dout_peri_spi0 CMU_PERI /1..32
mout_peri_spi_user CMU_PERI -
dout_peri_ip CMU_TOP /1..16
usi_cmgp0:
Clock Block Div range
--------------------------------------------
gout_cmgp_usi0_ipclk CMU_CMGP -
dout_cmgp_usi0 CMU_CMGP /1..32
mout_cmgp_usi0 CMU_CMGP -
gout_clkcmu_cmgp_bus CMU_APM -
dout_apm_bus CMU_APM /1..8
usi_cmgp1:
Clock Block Div range
--------------------------------------------
gout_cmgp_usi1_ipclk CMU_CMGP -
dout_cmgp_usi1 CMU_CMGP /1..32
mout_cmgp_usi1 CMU_CMGP -
gout_clkcmu_cmgp_bus CMU_APM -
dout_apm_bus CMU_APM /1..8
With input clock of 400 MHz, this scheme provides next IPCLK rate range,
for each SPI block:
SPI0: 781 kHz ... 400 MHz
SPI1/2: 1.6 MHz ... 400 MHz
Accounting for internal /4 divider in SPI blocks, and because the max
SPI frequency is limited at 50 MHz, it gives us next SPI SCK rates:
SPI0: 200 kHz ... 49.9 MHz
SPI1/2: 400 kHz ... 49.9 MHz
Which should cover all possible applications of SPI bus. Of course,
setting SPI frequency to values as low as 500 kHz will also affect the
common bus dividers (dout_apm_bus or dout_peri_ip), which in turn
effectively lowers the rates for all leaf bus clocks derived from those
dividers, like HSI2C and I3C clocks. But at least it gives the board
designer a choice, whether to keep all clocks (SPI/HSI2C/I3C) at high
frequencies, or make all those clocks have lower frequencies. Not
propagating the rate change to those common dividers would limit this
choice to "only high frequencies are allowed for SPI/HSI2C/I3C" option,
making the common dividers useless. This decision follows the "Worse is
better" approach, relying on the users/engineers to know the system
internals when working with such low-level features, instead of trying
to account for all possible use-cases.
Fixes:
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.