RE: Re: [PATCH] spi: pxa2xx: Add CS control clock quirk

["Srivastava, Shobhit" <shobhit.srivastava@xxxxxxxxx>]

> Hi
> 
> + Andy
> 
> On 2/12/20 12:34 AM, Rajat Jain wrote:
> > From: Evan Green <evgreen@xxxxxxxxxxxx>
> >
> > Date: Wed, 29 Jan 2020 13:54:16 -0800
> > Subject: [PATCH] spi: pxa2xx: Add CS control clock quirk
> >
> This patch subject is missing from mail subject.
> 
Added

> > In some circumstances on Intel LPSS controllers, toggling the LPSS CS
> > control register doesn't actually cause the CS line to toggle.
> > This seems to be failure of dynamic clock gating that occurs after
> > going through a suspend/resume transition, where the controller is
> > sent through a reset transition. This ruins SPI transactions that
> > either rely on delay_usecs, or toggle the CS line without sending
> > data.
> >
> > Whenever CS is toggled, momentarily set the clock gating register to
> > "Force On" to poke the controller into acting on CS.
> >
> Could you share the test case how to trigger this? What's the platform here?
> I'd like to check does this reproduce on other Intel LPSS platforms so is there
> need to add quirk for them too.
> 
This is on a CometLake platform. We are probing the SPI_CS line on a scope. 
Even though the writes to SPI_CS_CONTROL register are successful, it doesn’t toggle the CS line.
Hence checking on a scope is better.

Easy way to test this is to program the cs control register via iotools and see if the CS line toggles. 
This has to be done after one cycle of S0ix.

> > Signed-off-by: Evan Green <evgreen@xxxxxxxxxxxx>
> > Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>
> > ---
> >   drivers/spi/spi-pxa2xx.c | 23 +++++++++++++++++++++++
> >   1 file changed, 23 insertions(+)
> >
> > diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c index
> > 4c7a71f0fb3e..2e318158fca9 100644
> > --- a/drivers/spi/spi-pxa2xx.c
> > +++ b/drivers/spi/spi-pxa2xx.c
> > @@ -70,6 +70,10 @@ MODULE_ALIAS("platform:pxa2xx-spi");
> >   #define LPSS_CAPS_CS_EN_SHIFT			9
> >   #define LPSS_CAPS_CS_EN_MASK			(0xf <<
> LPSS_CAPS_CS_EN_SHIFT)
> >
> > +#define LPSS_PRIV_CLOCK_GATE 0x38
> > +#define LPSS_PRIV_CLOCK_GATE_CLK_CTL_MASK 0x3 #define
> > +LPSS_PRIV_CLOCK_GATE_CLK_CTL_FORCE_ON 0x3
> > +
> >   struct lpss_config {
> >   	/* LPSS offset from drv_data->ioaddr */
> >   	unsigned offset;
> > @@ -86,6 +90,8 @@ struct lpss_config {
> >   	unsigned cs_sel_shift;
> >   	unsigned cs_sel_mask;
> >   	unsigned cs_num;
> > +	/* Quirks */
> > +	unsigned cs_clk_stays_gated : 1;
> >   };
> >
> >   /* Keep these sorted with enum pxa_ssp_type */ @@ -156,6 +162,7 @@
> > static const struct lpss_config lpss_platforms[] = {
> >   		.tx_threshold_hi = 56,
> >   		.cs_sel_shift = 8,
> >   		.cs_sel_mask = 3 << 8,
> > +		.cs_clk_stays_gated = true,
> >   	},
> >   };
> >
> > @@ -383,6 +390,22 @@ static void lpss_ssp_cs_control(struct spi_device
> *spi, bool enable)
> >   	else
> >   		value |= LPSS_CS_CONTROL_CS_HIGH;
> >   	__lpss_ssp_write_priv(drv_data, config->reg_cs_ctrl, value);
> > +	if (config->cs_clk_stays_gated) {
> > +		u32 clkgate;
> > +
> > +		/*
> > +		 * Changing CS alone when dynamic clock gating is on won't
> > +		 * actually flip CS at that time. This ruins SPI transfers
> > +		 * that specify delays, or have no data. Toggle the clock mode
> > +		 * to force on briefly to poke the CS pin to move.
> > +		 */
> > +		clkgate = __lpss_ssp_read_priv(drv_data,
> LPSS_PRIV_CLOCK_GATE);
> > +		value = (clkgate & ~LPSS_PRIV_CLOCK_GATE_CLK_CTL_MASK)
> |
> > +			LPSS_PRIV_CLOCK_GATE_CLK_CTL_FORCE_ON;
> > +
> > +		__lpss_ssp_write_priv(drv_data, LPSS_PRIV_CLOCK_GATE,
> value);
> > +		__lpss_ssp_write_priv(drv_data, LPSS_PRIV_CLOCK_GATE,
> clkgate);
> > +	}
> >   }
> >
> I wonder is it enough to have this quick toggling only or is time or actually
> number of clock cycles dependent? Now there is no delay between but I'm
> thinking if it needs certain number cycles does this still work when using low
> ssp_clk rates similar than in commit d0283eb2dbc1 ("spi:
> pxa2xx: Add output control for multiple Intel LPSS chip selects").
> 
> I'm thinking can this be done only once after resume and may other LPSS
> blocks need the same? I.e. should this be done in drivers/mfd/intel-lpss.c?
> 
This behavior is seen after S0ix resume, but it is not seen after S3 resume.
I am thinking that it happens because we are not enabling the SSP after resume. 
It is deferred until we need to send data. By enabling the SSP in resume, I don’t see the issue.
For S3, I think BIOS re-enables the SSP in resume flow.

> Jarkko

Regards,
Shobhit