Re: [PATCH] MMC:dw_mmc:when enable biu_clk should check whether this clk is normal

From: Sebastian Reichel
Date: Sun Jan 04 2026 - 09:48:33 EST

Hi,

On Sun, Jan 04, 2026 at 03:38:42PM +0800, Shawn Lin wrote:
> Hi Sevastian,
>
> 在 2026/01/04 星期日 14:01, Sebastian Reichel 写道:
> > Hi,
> >
> > On Sun, Jan 04, 2026 at 10:55:48AM +0800, Shawn Lin wrote:
> > > 在 2026/01/04 星期日 10:38, ping.gao 写道:
> > > > some vendor doesn't use biu_clk , enable it will panic
> > > > log is below:
> > >
> > > I still don't understand the real issue you tried to solve.
> > >
> > > host->biu_clk is got via devm_clk_get() in dw_mci_probe(). If
> > > succeed, immediately it's enabled a few lines below. Otherwise,
> > > dw_mci_probe() returns with failure. Seems you are not enabling it in
> > > runtime callbacks, but why clk_prepare_enable(host->biu_clk) is able
> > > to work when driver probed?
> >
> > Based on the error and this patch, I believe what happens is:
> >
> > 1. dw_mci_probe() calls devm_clk_get(host->dev, "biu")
> > 2. The clock does not exist on the platform from Ping Gao, so
> > host->biu_clk gets an error pointer assigned
> > 3. "biu clock not available" gets printed at debug level
> > 4. the error is ignored, but the error pointer is still assigned to
> > host->biu_clk
> > 5. in the runtime PM handler clk_prepare_enable is called for
> > host->biu_clk and thus for an error pointer
> >
>
> Thanks for the details. For example,
> 1. dw_mci_exynos_probe() enables runtime PM before calling
> dw_mci_pltfm_register()->dw_mci_probe().
> 2. biu_clk gets an error and propagate it to dw_mci_pltfm_register(),

The error propagation does not happen. This is the current code in
dw_mci_probe():

host->biu_clk = devm_clk_get(host->dev, "biu");
if (IS_ERR(host->biu_clk)) {
dev_dbg(host->dev, "biu clock not available\n");
ret = PTR_ERR(host->biu_clk);
if (ret == -EPROBE_DEFER)
return ret;
/* --- no return here --- */
} else {
ret = clk_prepare_enable(host->biu_clk);
if (ret) {
dev_err(host->dev, "failed to enable biu clock\n");
return ret;
}
}

This code continues probing for any error code that is not
-EPROBE_DEFER. My understanding is, that this has been done
intensionally, since the clock does not exist on some platforms.

Effectively this is an open coded devm_clk_get_optional with
the difference that you may not use any clk_* operations on the
error code. A minimal fix would be to assign "host->biu_clk = NULL;"
at the end of the error handling code, which would open code
more of the devm_clk_get_optional() behaviour.

Obviously there is no point in having this open coded and the
helper existed since ages, so I strongly suggest to just use
that. Either devm_clk_get_optional() or even better
devm_clk_get_optional_enabled(). This results in:

- less code, which is easier to understand
- fixing the bug at hand
- proper error handling for errors that are neither -EPROBE_DEFER
nor -ENOENT.
- better error/debug messaging behaviour for -EPROBE_DEFER when
using dev_err_probe() instead of the dev_dbg print.

Greetings,

-- Sebastian

> 3. runtime PM is disabled in dw_mci_exynos_probe() later if seeing error.
>
> Does that mean runtime PM handler is called between step 2 and step 3?
> If that is the case, how could we allow this happen because the the
> controller's configuaration isn't been finished yet. Or maybe I
> misunderstand your point?
>
>
> > Instead of hacking this up even further (as this patch does),
> > the proper fix is to cleanup the probe function. I believe
> > the complete biu clock handling in it could be reduced to this:
> >
> > host->biu_clk = devm_clk_get_optional_enabled(host->dev, "biu");
> > ret = PTR_ERR_OR_ZERO(host->biu_clk);
> > if (ret)
> > return dev_err_probe(host->dev, ret, "failed to get biu clock\n");
> >
> > That will assign NULL to host->biu_clk when there is no clock
> > defined and it is fine to run clk_prepare_enable() with NULL
> > as clock pointer. Note, that this will handle the clock enabling
> > as part of devm, so you will also have to drop the
> > clk_disable_unprepare() calls from the err_clk_biu goto label
> > and the extra call from dw_mci_remove().
> >
> > Something similar should probably be done for host->ciu_clk, which
> > seems to have the same incorrect logic.
> >
> > Greetings,
> >
> > -- Sebastian
> >
> > > > [ 438.400868] [7: binder:436_2: 4998] Unable to handle kernel paging request at virtual address fffffffffffffffe
> > > > [ 438.400877] [7: binder:436_2: 4998] Mem abort info:
> > > > [ 438.400881] [7: binder:436_2: 4998] ESR = 0x0000000096000005
> > > > [ 438.400887] [7: binder:436_2: 4998] EC = 0x25: DABT (current EL), IL = 32 bits
> > > > [ 438.400894] [7: binder:436_2: 4998] SET = 0, FnV = 0
> > > > [ 438.400899] [7: binder:436_2: 4998] EA = 0, S1PTW = 0
> > > > [ 438.400904] [7: binder:436_2: 4998] FSC = 0x05: level 1 translation fault
> > > > ...
> > > > [ 438.409424] [7: binder:436_2: 4998] Call trace:
> > > > [ 438.409429] [7: binder:436_2: 4998] clk_prepare+0x10/0x24
> > > > [ 438.409439] [7: binder:436_2: 4998] dw_mci_runtime_resume+0x50/0x2d8 [dw_mmc_samsung cd210e210975263404c28fc89778f369f8398f0c]
> > > > [ 438.409471] [7: binder:436_2: 4998] dw_mci_exynos_runtime_resume+0x18/0x58 [dw_mmc_exynos_samsung 2735a594c7c9c9e8c65b0b87523fbf70dcaabfff]
> > > > [ 438.409496] [7: binder:436_2: 4998] pm_generic_runtime_resume+0x40/0x58
> > > > [ 438.409506] [7: binder:436_2: 4998] pm_runtime_force_resume+0x9c/0x134
> > > > [ 438.409517] [7: binder:436_2: 4998] platform_pm_resume+0x40/0x8c
> > > > [ 438.409529] [7: binder:436_2: 4998] dpm_run_callback+0x64/0x230
> > > > [ 438.409540] [7: binder:436_2: 4998] __device_resume+0x1d8/0x394
> > > > [ 438.409551] [7: binder:436_2: 4998] dpm_resume+0x110/0x2b8
> > > > [ 438.409561] [7: binder:436_2: 4998] dpm_resume_end+0x1c/0x38
> > > > [ 438.409570] [7: binder:436_2: 4998] suspend_devices_and_enter+0x828/0xab0
> > > > [ 438.409582] [7: binder:436_2: 4998] pm_suspend+0x334/0x618
> > > > [ 438.409592] [7: binder:436_2: 4998] state_store+0x104/0x144
> > > > [ 438.409601] [7: binder:436_2: 4998] kobj_attr_store+0x30/0x48
> > > > [ 438.409610] [7: binder:436_2: 4998] sysfs_kf_write+0x54/0x6c
> > > > [ 438.409619] [7: binder:436_2: 4998] kernfs_fop_write_iter+0x104/0x1a8
> > > > [ 438.409628] [7: binder:436_2: 4998] vfs_write+0x24c/0x2f4
> > > > [ 438.409640] [7: binder:436_2: 4998] ksys_write+0x78/0xe8
> > > > [ 438.409652] [7: binder:436_2: 4998] __arm64_sys_write+0x1c/0x2c
> > > > [ 438.409664] [7: binder:436_2: 4998] invoke_syscall+0x58/0x114
> > > > [ 438.409676] [7: binder:436_2: 4998] el0_svc_common+0xac/0xe0
> > > > [ 438.409687] [7: binder:436_2: 4998] do_el0_svc+0x1c/0x28
> > > > [ 438.409698] [7: binder:436_2: 4998] el0_svc+0x38/0x68
> > > > [ 438.409705] [7: binder:436_2: 4998] el0t_64_sync_handler+0x68/0xbc
> > > > [ 438.409712] [7: binder:436_2: 4998] el0t_64_sync+0x1a8/0x1ac
> > > > Signed-off-by: ping.gao <ping.gao@xxxxxxxxxxx>
> > > > ---
> > > > drivers/mmc/host/dw_mmc.c | 12 +++++++++---
> > > > 1 file changed, 9 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
> > > > index c5db92bbb094..61f6986f15ef 100644
> > > > --- a/drivers/mmc/host/dw_mmc.c
> > > > +++ b/drivers/mmc/host/dw_mmc.c
> > > > @@ -3633,9 +3633,15 @@ int dw_mci_runtime_resume(struct device *dev)
> > > > if (host->slot &&
> > > > (mmc_host_can_gpio_cd(host->slot->mmc) ||
> > > > !mmc_card_is_removable(host->slot->mmc))) {
> > > > - ret = clk_prepare_enable(host->biu_clk);
> > > > - if (ret)
> > > > - return ret;
> > > > + if (IS_ERR(host->biu_clk)) {
> > > > + dev_dbg(host->dev, "biu clock not available\n");
> > > > + } else {
> > > > + ret = clk_prepare_enable(host->biu_clk);
> > > > + if (ret) {
> > > > + dev_err(host->dev, "failed to enable biu clock\n");
> > > > + goto err;
> > > > + }
> > > > + }
> > > > }
> > > > ret = clk_prepare_enable(host->ciu_clk);
> > >
> > >
>

Attachment: signature.asc
Description: PGP signature