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05 April 2024, 19:05:47 UTC
Revision a44648b057f5331fe6c0e863dc693ed335490e7a authored by Anton Vorontsov on 10 August 2007, 20:01:02 UTC, committed by Linus Torvalds on 11 August 2007, 22:47:41 UTC 
spi_mpc83xx: fix prescale modulus calculation
 
Long ago I've noticed (but didn't pay much attention) that
spi_mpc83xx using PM calculations that differs from what
specs describe. I.e.

u8 pm = mpc83xx_spi->spibrg / (spi->max_speed_hz * 4);

While specs says: "The SPI baud rate generator clock source (either
system clock or system clock divided by 16, depending on DIV16 bit) is
divided by 4 * ([PM] + 1), a range from 4 to 64.".

Thus " - 1" is missing in the spi_mpc83xx's formula.

Why nobody noticed that bug? Probably because sysclk usually less then
user expects, e.g. you expect 200 MHz, but real clock is 198 MHz,
and integer rounding helps when this formula is used.

Suppose it's SPI in QE, SYSCLK at 198 MHz, thus SPIBRG at 99MHz, 25 MHz
requested.

PM = (99MHz / ( 25 MHz * 4 )), PM == 0, output SPICLK will be 24.75 MHz

At lower frequencies this bug is more noticeable, though.

And this bug shows itself in all its beauty if SYSCLK is equal or a bit
more than you expect (200 MHz SYSCLK, 100 MHz SPIBRG):
PM = (100MHz / ( 25 MHz * 4 )), PM == 1, output SPICLK will be 12.625 MHz!

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent e24a4d1
Raw File
Tip revision: a44648b057f5331fe6c0e863dc693ed335490e7a authored by Anton Vorontsov on 10 August 2007, 20:01:02 UTC
spi_mpc83xx: fix prescale modulus calculation
Tip revision: a44648b
Kbuild 
#
# Kbuild for top-level directory of the kernel
# This file takes care of the following:
# 1) Generate asm-offsets.h
# 2) Check for missing system calls

#####
# 1) Generate asm-offsets.h
#

offsets-file := include/asm-$(ARCH)/asm-offsets.h

always  := $(offsets-file)
targets := $(offsets-file)
targets += arch/$(ARCH)/kernel/asm-offsets.s
clean-files := $(addprefix $(objtree)/,$(targets))

# Default sed regexp - multiline due to syntax constraints
define sed-y
	"/^->/{s:^->\([^ ]*\) [\$$#]*\([^ ]*\) \(.*\):#define \1 \2 /* \3 */:; s:->::; p;}"
endef
# Override default regexp for specific architectures
sed-$(CONFIG_MIPS) := "/^@@@/{s/^@@@//; s/ \#.*\$$//; p;}"

quiet_cmd_offsets = GEN     $@
define cmd_offsets
	(set -e; \
	 echo "#ifndef __ASM_OFFSETS_H__"; \
	 echo "#define __ASM_OFFSETS_H__"; \
	 echo "/*"; \
	 echo " * DO NOT MODIFY."; \
	 echo " *"; \
	 echo " * This file was generated by Kbuild"; \
	 echo " *"; \
	 echo " */"; \
	 echo ""; \
	 sed -ne $(sed-y) $<; \
	 echo ""; \
	 echo "#endif" ) > $@
endef

# We use internal kbuild rules to avoid the "is up to date" message from make
arch/$(ARCH)/kernel/asm-offsets.s: arch/$(ARCH)/kernel/asm-offsets.c FORCE
	$(Q)mkdir -p $(dir $@)
	$(call if_changed_dep,cc_s_c)

$(obj)/$(offsets-file): arch/$(ARCH)/kernel/asm-offsets.s Kbuild
	$(Q)mkdir -p $(dir $@)
	$(call cmd,offsets)

#####
# 2) Check for missing system calls
#

quiet_cmd_syscalls = CALL    $<
      cmd_syscalls = $(CONFIG_SHELL) $< $(CC) $(c_flags)

PHONY += missing-syscalls
missing-syscalls: scripts/checksyscalls.sh FORCE
	$(call cmd,syscalls)
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