I’m trying to clean up the defconfig a little bit, take out things that I don’t need and so forth, and I’ve hit a snag. In my defconfig right now I have the Sound driver support enabled, but I don't need it:
Device Drivers --->
<M> Sound card support --->
--- Sound card support
[*] Preclaim OSS device numbers
<M> Advanced Linux Sound Architecture --->
< > Open Sound System (DEPRECATED) ----
and so I was trying to remove this support. Now what happened when I disabled the top level Sound card support was that in the .config file I lost a CONFIG_REGMAP_SPI which I was not expecting.
I didn’t know what a CONFIG_REGMAP_XXX does so I went and did a little research and I don’t think it’s tied to sound at all, it looks like it’s just an abstraction layer for registers so they don’t always have to go back to the CPU in order to get register values and save time on the bus. So I tried to figure out why the REGMAP_SPI was disabled and looking at the menuconfig entry for CONFIG_REGMAP_SPI I see the following logic required to enable it:
BMP085_SPI [=n] && SPI_MASTER [=y] && SYSFS [=y] || SERIAL_MAX310X [=n] && TTY [=y] && HAS_IOMEM [=y] && SPI_MASTER [=y] || MFD_DA9052_SPI [=n] && HAS_IOMEM [=y] && SPI_MASTER [=y] || MFD_MC13XXX_SPI [=n] && HAS_IOMEM [=y] && SPI_MASTER [=y] || MFD_ARIZONA_SPI [=n] && HAS_IOMEM [=y] && SPI_MASTER [=y] || MFD_WM831X_SPI [=n] && HAS_IOMEM [=y] && SPI_MASTER [=y] || SND_SOC [=n] && SOUND [=n] && !M68K && !UML && SND [=n] && SPI_MASTER [=y] || AD5380 [=n] && IIO [=m] && (SPI_MASTER [=y] && I2C [=y]!=m || I2C [=y]) && SPI_MASTER [=y]
My defconfig has most of these non device specific options set to 1 (SPI_MASTER, SYSFS, TTY, HAS_IOMEN, IIO, I2C) and most of the device specific options not set (BMP085_SPI, SERIAL_MAX310X, MFD_DA9052_SPI, MFD_MC13XXX_SPI, MFD_ARIZONA_SPI, MFD_WM831X_SPI, AD5380). This leads me to believe that “is not set” is not the same as “=n”, and also that the SND_SOC line is the only one that was keeping CONFIG_REGMAP_SPI enabled.
Greping in the kernel code base for CONFIG_REGMAP_SPI I see it only used for sound/soc/soc-io.c, so I took a look around for places the "regmap" + "spi" was used and I found a number of drivers that referenced these functions:
MFD’s:
mc13xxx – power/audio
wm5102 – Codecs/Voice DSP
wm831x - PMIC
da9052 - PMIC
arizona - audio
wm5110 – audio chip
Misc:
bmp085 – temperature/pressure/altitude sensor
iio:
ad5380 – DtoA converter
tty:
serial – max310x - UART
and various sound SOC codecs.
I don't use any of these devices, so I think that it's OK for me to disable Sound (and therefore REGMAP_SPI) but I would like confirmation of two things:
1) Given that I don't use any of those drivers and Audio/sound in my design. Should I be safe removing sound (and therefore REGMAP_SPI) from my kernel?
2) Am I correct about REGMAP_SPI being used as a register abstraction layer?
