[PATCH 1/1] i2c: lpi2c: Avoid calling clk_get_rate during transfer

[Alexander Stein]

Hi Uwe,

Am Donnerstag, 18. Januar 2024, 11:22:35 CET schrieb Uwe Kleine-König:
> On Thu, Jan 18, 2024 at 08:43:32AM +0100, Alexander Stein wrote:
> > Instead of repeatedly calling clk_get_rate for each transfer, lock
> > the clock rate and cache the value.
> > A deadlock has been observed while adding tlv320aic32x4 audio codec to
> > the system. When this clock provider adds its clock, the clk mutex is
> > locked already, it needs to access i2c, which in return needs the mutex
> > for clk_get_rate as well.
> > 
> > Signed-off-by: Alexander Stein <alexander.stein at ew.tq-group.com>
> > ---
> > This is an alternative, lightweight approach replacing the patch [1] and
> > depends on [2].
> > The issue to address is still removing the call to clk_get_rate() during
> > each transfer, which might reuslt in a deadlock. lockdep also complains
> > about this call chain.
> > 
> > Instead of adding a clock notifier, lock the peripheral clock rate and
> > cache the peripheral clock rate.
> > Currently LPI2C is available in the following SoC:
> > * i.MX7ULP
> > * i.MX8ULP
> > * i.MX8DXL
> > * i.MX8X
> > * i.MX8
> > * i.MX93
> > 
> > Additionally I expect both i.MX91 and i.MX95 to also use this driver.
> > 
> > This patch assumes the parent clock rate never changes. This is apparently
> > true for i.MX93 as each I2C has it's own lpi2c*_root clock. On i.MX8 and
> > i.MX8X clocks are managed by SCU with it's own dedicated firmware. I
> > can't say if the clock never changes though. I have no idea about the
> > other SoC.
> > 
> > Best regards,
> > Alexander
> > 
> > [1]
> > https://lore.kernel.org/all/20240110120556.519800-1-alexander.stein@ew.tq
> > -group.com/ [2]
> > https://lore.kernel.org/all/20240104225512.1124519-2-u.kleine-koenig@peng
> > utronix.de/> 
> >  drivers/i2c/busses/i2c-imx-lpi2c.c | 17 ++++++++++++++++-
> >  1 file changed, 16 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/i2c/busses/i2c-imx-lpi2c.c
> > b/drivers/i2c/busses/i2c-imx-lpi2c.c index 678b30e90492a..6cbcb27a3b280
> > 100644
> > --- a/drivers/i2c/busses/i2c-imx-lpi2c.c
> > +++ b/drivers/i2c/busses/i2c-imx-lpi2c.c
> > @@ -99,6 +99,7 @@ struct lpi2c_imx_struct {
> > 
> >  	__u8			*rx_buf;
> >  	__u8			*tx_buf;
> >  	struct completion	complete;
> > 
> > +	unsigned long		rate_per;
> > 
> >  	unsigned int		msglen;
> >  	unsigned int		delivered;
> >  	unsigned int		block_data;
> > 
> > @@ -207,7 +208,7 @@ 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 = lpi2c_imx->rate_per;
> > 
> >  	if (!clk_rate)
> >  	
> >  		return -EINVAL;
> 
> After the things you did in lpi2c_imx_probe() you can assume that
> clk_rate is not zero, so you could drop the if here.

As in some cases the clock is not setup by Linux, but externally, I'd rather 
keep that check to ensure it's enabled.

> Otherwise looks good to me (if you want even if you keep the if which is
> only a minor optimisation).
> 
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>

Thanks and best regards,
Alexander

> Best regards
> Uwe


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