linux-kernel - Re: [RFC PATCH] mmc: sdio: Allow sdio code to be build time disabled

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Message-ID: <CAPDyKFptzJfxCeqNjAMsu2q4nEOiaKg6xFjNud9RR=QS1HQW-w@mail.gmail.com>
Date: Thu, 22 Jan 2026 13:08:25 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Daniel Palmer <daniel@...ngy.jp>
Cc: linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mmc: sdio: Allow sdio code to be build time disabled

On Mon, 5 Jan 2026 at 07:46, Daniel Palmer <daniel@...ngy.jp> wrote:
>
> In some cases the sdio support is not needed but the compiler
> cannot tell this and it ends up in your binary.
>
> This might not matter so much on generic kernels for systems
> with gigabytes of memory but in situations with megabytes
> of memory that just have an SD card attached via bitbanged SPI
> over GPIO this is wasteful.
>
> For amd64 disabling SDIO reduces the size of the kernel by ~26KiB.
>
> Signed-off-by: Daniel Palmer <daniel@...ngy.jp>
> ---
>
> RFC because this is probably incomplete, maybe not a good idea,..
> I have only tested that this removes the unneeded SDIO code on my
> machine with 8MB of RAM and its still able to enumerate and access
> a 64GB SD card it has connected over SPI.

Hmm, in principle I don't mind this. Although, I assume there are lots
of other subsystems that can be split up using additional Kconfigs in
the similar manner.

Doing this for MMC/SD/SDIO alone seems silly unless we have a broader
consensus to do things like this. Do we have that?

In regards to the implementation and from a very brief review, I
observed two issues.
1) We don't want "#ifdef"-s sprinkled all over the place in the c-code
(headers are fine), then better to use stub-functions.
2) We need to be careful about Kconfig dependencies. The SDIO func
drivers are implemented across various subsystems (bluetooth, wifi,
etc), those corresponding Kconfigs probably need a "depends on SDIO"
too.

Kind regards
Uffe

>
>  drivers/mmc/core/Kconfig    | 12 +++++++++++-
>  drivers/mmc/core/Makefile   |  6 +++---
>  drivers/mmc/core/core.h     |  7 +++++++
>  drivers/mmc/core/host.c     |  2 ++
>  drivers/mmc/core/sdio_bus.h |  5 +++++
>  drivers/mmc/core/sdio_cis.h |  4 ++++
>  drivers/mmc/core/sdio_ops.h | 14 ++++++++++++++
>  7 files changed, 46 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mmc/core/Kconfig b/drivers/mmc/core/Kconfig
> index 14d2ecbb04d3..4112242a6c98 100644
> --- a/drivers/mmc/core/Kconfig
> +++ b/drivers/mmc/core/Kconfig
> @@ -64,9 +64,19 @@ config MMC_BLOCK_MINORS
>
>           If unsure, say 8 here.
>
> +config MMC_SDIO
> +       bool "Support SDIO in MMC core" if EXPERT
> +       default y
> +       help
> +         Enable SDIO support in the MMC core. If you will never use
> +         SDIO because your hardware can't support it or your usecase
> +         will never need it i.e. you have one SD host and it has fixed
> +         eMMC attached to it you can set this to N and remove some
> +         unneeded code.
> +
>  config SDIO_UART
>         tristate "SDIO UART/GPS class support"
> -       depends on TTY
> +       depends on MMC_SDIO && TTY
>         help
>           SDIO function driver for SDIO cards that implements the UART
>           class, as well as the GPS class which appears like a UART.
> diff --git a/drivers/mmc/core/Makefile b/drivers/mmc/core/Makefile
> index 15b067e8b0d1..efc354150e8a 100644
> --- a/drivers/mmc/core/Makefile
> +++ b/drivers/mmc/core/Makefile
> @@ -6,9 +6,9 @@
>  obj-$(CONFIG_MMC)              += mmc_core.o
>  mmc_core-y                     := core.o bus.o host.o \
>                                    mmc.o mmc_ops.o sd.o sd_ops.o \
> -                                  sdio.o sdio_ops.o sdio_bus.o \
> -                                  sdio_cis.o sdio_io.o sdio_irq.o sd_uhs2.o\
> -                                  slot-gpio.o regulator.o
> +                                  sd_uhs2.o slot-gpio.o regulator.o
> +mmc_core-$(CONFIG_MMC_SDIO)    += sdio.o sdio_ops.o sdio_bus.o \
> +                                  sdio_cis.o sdio_io.o sdio_irq.o
>  mmc_core-$(CONFIG_OF)          += pwrseq.o
>  obj-$(CONFIG_PWRSEQ_SIMPLE)    += pwrseq_simple.o
>  obj-$(CONFIG_PWRSEQ_SD8787)    += pwrseq_sd8787.o
> diff --git a/drivers/mmc/core/core.h b/drivers/mmc/core/core.h
> index a028b48be164..8e2a1abc5523 100644
> --- a/drivers/mmc/core/core.h
> +++ b/drivers/mmc/core/core.h
> @@ -85,7 +85,14 @@ int mmc_detect_card_removed(struct mmc_host *host);
>
>  int mmc_attach_mmc(struct mmc_host *host);
>  int mmc_attach_sd(struct mmc_host *host);
> +#ifdef CONFIG_MMC_SDIO
>  int mmc_attach_sdio(struct mmc_host *host);
> +#else
> +static inline int mmc_attach_sdio(struct mmc_host *host)
> +{
> +       return -EOPNOTSUPP;
> +}
> +#endif
>  int mmc_attach_sd_uhs2(struct mmc_host *host);
>
>  /* Module parameters */
> diff --git a/drivers/mmc/core/host.c b/drivers/mmc/core/host.c
> index 88c95dbfd9cf..0216953b8906 100644
> --- a/drivers/mmc/core/host.c
> +++ b/drivers/mmc/core/host.c
> @@ -563,7 +563,9 @@ struct mmc_host *mmc_alloc_host(int extra, struct device *dev)
>         spin_lock_init(&host->lock);
>         init_waitqueue_head(&host->wq);
>         INIT_DELAYED_WORK(&host->detect, mmc_rescan);
> +#ifdef CONFIG_MMC_SDIO
>         INIT_WORK(&host->sdio_irq_work, sdio_irq_work);
> +#endif
>         timer_setup(&host->retune_timer, mmc_retune_timer, 0);
>
>         INIT_WORK(&host->supply.uv_work, mmc_undervoltage_workfn);
> diff --git a/drivers/mmc/core/sdio_bus.h b/drivers/mmc/core/sdio_bus.h
> index 27b8069a72ab..ad14784b37d1 100644
> --- a/drivers/mmc/core/sdio_bus.h
> +++ b/drivers/mmc/core/sdio_bus.h
> @@ -14,8 +14,13 @@ struct sdio_func *sdio_alloc_func(struct mmc_card *card);
>  int sdio_add_func(struct sdio_func *func);
>  void sdio_remove_func(struct sdio_func *func);
>
> +#ifdef CONFIG_MMC_SDIO
>  int sdio_register_bus(void);
>  void sdio_unregister_bus(void);
> +#else
> +#define sdio_register_bus() 0
> +#define sdio_unregister_bus()
> +#endif
>
>  #endif
>
> diff --git a/drivers/mmc/core/sdio_cis.h b/drivers/mmc/core/sdio_cis.h
> index 6d76f6fa608c..486eeb802162 100644
> --- a/drivers/mmc/core/sdio_cis.h
> +++ b/drivers/mmc/core/sdio_cis.h
> @@ -13,10 +13,14 @@
>  struct mmc_card;
>  struct sdio_func;
>
> +#ifdef CONFIG_MMC_SDIO
>  int sdio_read_common_cis(struct mmc_card *card);
>  void sdio_free_common_cis(struct mmc_card *card);
>
>  int sdio_read_func_cis(struct sdio_func *func);
>  void sdio_free_func_cis(struct sdio_func *func);
> +#else
> +#define sdio_free_common_cis(card)
> +#endif
>
>  #endif
> diff --git a/drivers/mmc/core/sdio_ops.h b/drivers/mmc/core/sdio_ops.h
> index 37f79732a206..dc16e80aa361 100644
> --- a/drivers/mmc/core/sdio_ops.h
> +++ b/drivers/mmc/core/sdio_ops.h
> @@ -12,6 +12,7 @@
>  #include <linux/mmc/sdio.h>
>
>  struct mmc_host;
> +#ifdef CONFIG_MMC_SDIO
>  struct mmc_card;
>  struct work_struct;
>
> @@ -33,6 +34,19 @@ static inline bool sdio_is_io_busy(u32 opcode, u32 arg)
>                 (opcode == SD_IO_RW_DIRECT &&
>                 !(addr == SDIO_CCCR_ABORT || addr == SDIO_CCCR_SUSPEND)));
>  }
> +#else
> +/* These are referenced in code outside of the sdio files so define dummy versions */
> +static inline int sdio_reset(struct mmc_host *host)
> +{
> +       return 0;
> +}
> +
> +static inline bool sdio_is_io_busy(u32 opcode, u32 arg)
> +{
> +       return false;
> +}
> +#endif
> +
>
>  #endif
>
> --
> 2.51.0
>