Hi Andi,
thanks for the feedback.
Am Donnerstag, 9. November 2023, 00:38:09 CET schrieb Andi Shyti:
> Hi Alexander,
>
> On Tue, Nov 07, 2023 at 03:12:01PM +0100, Alexander Stein wrote:
> > From: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxxxx>
> >
> > Instead of repeatedly calling clk_get_rate for each transfer, register
> > a clock notifier to update the cached divider value each time the clock
> > rate actually changes.
> > There is also a lockdep splat if a I2C based clock provider needs to
> > access the i2c bus while in recalc_rate(). "prepare_lock" is about to
> > be locked recursively.
> >
> > Fixes: a55fa9d0e42e ("i2c: imx-lpi2c: add low power i2c bus driver")
>
> What's the exact fix here? Is it the lockdep warning? Is it
> actually causing a real deadlock?
We've seen actual deadlocks on our imx8qxp based hardware using a downstream
kernel, so we have implemented as similar fix [1]. This happened using
tlv320aic32x4 audio codec.
The backtrace is similar, a i2c based clock provider is trying t issue an i2c
transfer while adding the clock, thus 'prepare_lock' is already locked.
Lockdep raises an error both for downstream kernel as well as mainline, both
for tlv320aic32x4 or pcf85063.
[1] https://github.com/tq-systems/linux-tqmaxx/commit/
b0339ff83a979f2ea066012b9209ea7c54f2b4e7
> > Signed-off-by: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxxxx>
> > Reviewed-by: Alexander Stein <alexander.stein@xxxxxxxxxxxxxxx>
> > Signed-off-by: Alexander Stein <alexander.stein@xxxxxxxxxxxxxxx>
> > ---
> > Changes in v7:
> > * Use dev_err_probe
> > * Reworked/Shortened the commit message
> >
> > It is not about saving CPU cycles, but to avoid locking the clk subsystem
> > upon each transfer.
> >
> > drivers/i2c/busses/i2c-imx-lpi2c.c | 40 +++++++++++++++++++++++++++++-
> > 1 file changed, 39 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/i2c/busses/i2c-imx-lpi2c.c
> > b/drivers/i2c/busses/i2c-imx-lpi2c.c index 678b30e90492a..3070e605fd8ff
> > 100644
> > --- a/drivers/i2c/busses/i2c-imx-lpi2c.c
> > +++ b/drivers/i2c/busses/i2c-imx-lpi2c.c
> > @@ -5,6 +5,7 @@
> >
> > * Copyright 2016 Freescale Semiconductor, Inc.
> > */
> >
> > +#include <linux/atomic.h>
> >
> > #include <linux/clk.h>
> > #include <linux/completion.h>
> > #include <linux/delay.h>
> >
> > @@ -99,6 +100,8 @@ struct lpi2c_imx_struct {
> >
> > __u8 *rx_buf;
> > __u8 *tx_buf;
> > struct completion complete;
> >
> > + struct notifier_block clk_change_nb;
> > + atomic_t rate_per;
> >
> > unsigned int msglen;
> > unsigned int delivered;
> > unsigned int block_data;
> >
> > @@ -197,6 +200,20 @@ static void lpi2c_imx_stop(struct lpi2c_imx_struct
> > *lpi2c_imx)>
> > } while (1);
> >
> > }
> >
> > +static int lpi2c_imx_clk_change_cb(struct notifier_block *nb,
> > + unsigned long action, void *data)
> > +{
> > + struct clk_notifier_data *ndata = data;
> > + struct lpi2c_imx_struct *lpi2c_imx = container_of(nb,
> > + struct
lpi2c_imx_struct,
> > +
clk_change_nb);
> > +
> > + if (action & POST_RATE_CHANGE)
> > + atomic_set(&lpi2c_imx->rate_per, ndata->new_rate);
> > +
> > + return NOTIFY_OK;
> > +}
> > +
> >
> > /* CLKLO = I2C_CLK_RATIO * CLKHI, SETHOLD = CLKHI, DATAVD = CLKHI/2 */
> > static int lpi2c_imx_config(struct lpi2c_imx_struct *lpi2c_imx)
> > {
> >
> > @@ -207,7 +224,7 @@ static int lpi2c_imx_config(struct lpi2c_imx_struct
> > *lpi2c_imx)>
> > lpi2c_imx_set_mode(lpi2c_imx);
> >
> > - clk_rate = clk_get_rate(lpi2c_imx->clks[0].clk);
> > + clk_rate = atomic_read(&lpi2c_imx->rate_per);
> >
> > if (!clk_rate)
> >
> > return -EINVAL;
>
> Doesn't seem like EINVAL, defined as "Invalid argument", is the
> correct number here. As we are failing to read the clock rate, do
> you think EIO is better?
Well, this is already the current error code. In both the old and new code I
would consider this error case as 'no clock rate provided' rather than failing
to read. I would reject EIO as there is no IO transfer for retrieving the
clock rate.
> > @@ -590,6 +607,27 @@ static int lpi2c_imx_probe(struct platform_device
> > *pdev)>
> > if (ret)
> >
> > return ret;
> >
> > + lpi2c_imx->clk_change_nb.notifier_call = lpi2c_imx_clk_change_cb;
> > + ret = devm_clk_notifier_register(&pdev->dev, lpi2c_imx->clks[0].clk,
> > + &lpi2c_imx->clk_change_nb);
> > + if (ret)
> > + return dev_err_probe(&pdev->dev, ret,
> > + "can't register peripheral clock
notifier\n");
>
> can't we fall back to how things were instead of failing the
> probe? But I'm not sure it would remove the lockdep warning,
> though. I can live with it.
I don't see a reason why you would want to continue if
devm_clk_notifier_register() failed. It's either ENOMEM, EINVAL (if you pass
NULL for clk or notifier block) or EEXIST if registered twice. So in reality
only ENOMEM is possible, but then things went south already.
Best regards,
Alexander
> > + /*
> > + * Lock the clock rate to avoid rate change between clk_get_rate()
and
> > + * atomic_set()
> > + */
> > + ret = clk_rate_exclusive_get(lpi2c_imx->clks[0].clk);
> > + if (ret)
> > + return dev_err_probe(&pdev->dev, ret,
> > + "can't lock I2C peripheral clock
rate\n");
> > +
> > + atomic_set(&lpi2c_imx->rate_per, clk_get_rate(lpi2c_imx-
>clks[0].clk));
> > + clk_rate_exclusive_put(lpi2c_imx->clks[0].clk);
> > + if (!atomic_read(&lpi2c_imx->rate_per))
> > + return dev_err_probe(&pdev->dev, -EINVAL,
> > + "can't get I2C peripheral clock
rate\n");
> > +
>
> Not a bad patch, would be nice if all the above was provided by
> the library so that other drivers can use it.
>
> Andi
>
> > pm_runtime_set_autosuspend_delay(&pdev->dev, I2C_PM_TIMEOUT);
> > pm_runtime_use_autosuspend(&pdev->dev);
> > pm_runtime_get_noresume(&pdev->dev);
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