Re: [PATCH] i2c: at91: handle TXRDY interrupt spam

From: Raag Jadav
Date: Mon May 06 2019 - 12:03:32 EST

On Mon, May 06, 2019 at 08:19:01AM +0000, Eugen.Hristev@xxxxxxxxxxxxx wrote:
>
>
> On 04.05.2019 02:58, Raag Jadav wrote:
>
> > On Thu, May 02, 2019 at 04:01:16PM +0200, Ludovic Desroches wrote:
> >> On Tue, Apr 30, 2019 at 04:03:32AM +0530, Raag Jadav wrote:
> >>> External E-Mail
> >>>
> >>>
> >>> On Mon, Apr 29, 2019 at 11:00:05AM +0200, Ludovic Desroches wrote:
> >>>> Hello Raag,
> >>>>
> >>>> On Tue, Apr 23, 2019 at 01:06:48PM +0530, Raag Jadav wrote:
> >>>>> External E-Mail
> >>>>>
> >>>>>
> >>>>> Performing i2c write operation while SDA or SCL line is held
> >>>>> or grounded by slave device, we go into infinite at91_twi_write_next_byte
> >>>>> loop with TXRDY interrupt spam.
> >>>>
> >>>> Sorry but I am not sure to have the full picture, the controller is in
> >>>> slave or master mode?
> >>>>
> >>>> SVREAD is only used in slave mode. When SVREAD is set, it means that a read
> >>>> access is performed and your issue concerns the write operation.
> >>>>
> >>>> Regards
> >>>>
> >>>> Ludovic
> >>>
> >>> Yes, even though the datasheet suggests that SVREAD is irrelevant in master mode,
> >>> TXRDY and SVREAD are the only ones being set in status register upon reproducing the issue.
> >>> Couldn't think of a better way to handle such strange behaviour.
> >>> Any suggestions would be appreciated.
> >>
> >> I have the confirmation that you can't rely on the SVREAD flag when in
> >> master mode. This flag should always have the same value.
> >>
> >> I am trying to understand what could lead to your situation. Can you
> >> give me more details. What kind of device it is? What does lead to this
> >> situation? Does it happen randomly or not?
> >
> > One of the sama5d2 based board I worked on, was having trouble complete its boot
> > because of a faulty i2c device, which was randomly holding down the SDA line
> > on i2c write operation, not allowing the controller to complete its transmission,
> > causing a massive TXRDY interrupt spam, ultimately hanging the processor.
> >
> > Another strange observation was that SVREAD was being set in the status register
> > along with TXRDY, every time I reproduced the issue.
> > You can reproduce it by simply grounding the SDA line and performing i2c write
> > on the bus.
> >
> > Note that NACK, LOCK or TXCOMP are never set as the transmission never completes.
> > I'm not sure why slave bits are being set in master mode,
> > but it's been working reliably for me.
> >
> > This patch doesn't recover the SDA line. It just prevents the processor from
> > getting hanged in case of i2c bus lockup.
>
> Hello,
>
> I have noticed the same hanging at some points... In my case it is
> because of this patch:
>
> commit e8f39e9fc0e0b7bce24922da925af820bacb8ef8
> Author: David Engraf <david.engraf@xxxxxxxxx>
> Date: Thu Apr 26 11:53:14 2018 +0200
>
>
> diff --git a/drivers/i2c/busses/i2c-at91.c b/drivers/i2c/busses/i2c-at91.c
> index bfd1fdf..3f3e8b3 100644
> --- a/drivers/i2c/busses/i2c-at91.c
> +++ b/drivers/i2c/busses/i2c-at91.c
> @@ -518,8 +518,16 @@ static irqreturn_t atmel_twi_interrupt(int irq,
> void *dev_id)
> * the RXRDY interrupt first in order to not keep garbage data
> in the
> * Receive Holding Register for the next transfer.
> */
> - if (irqstatus & AT91_TWI_RXRDY)
> - at91_twi_read_next_byte(dev);
> + if (irqstatus & AT91_TWI_RXRDY) {
> + /*
> + * Read all available bytes at once by polling RXRDY
> usable w/
> + * and w/o FIFO. With FIFO enabled we could also read
> RXFL and
> + * avoid polling RXRDY.
> + */
> + do {
> + at91_twi_read_next_byte(dev);
> + } while (at91_twi_read(dev, AT91_TWI_SR) & AT91_TWI_RXRDY);
> + }
>
>
> In my opinion having a do/while with an exit condition relying solely on
> a bit read from hardware is unacceptable in IRQ context - kernel can
> hang here.
> A timeout would be a solution...
>
> For me, reverting this patch solves hanging issues.
>
> Hope this helps,
>
> Eugen

Thank you for your input, but my issue concerns i2c write operation.
I haven't noticed any issue with i2c read yet.
But given the same scenario, it could be true for RXRDY as well.

Cheers,
Raag

>
> >
> > Cheers,
> > Raag
> >
> >>
> >> Regards
> >>
> >> Ludovic
> >>
> >>>
> >>> Cheers,
> >>> Raag
> >>>
> >>>>
> >>>>>
> >>>>> Signed-off-by: Raag Jadav <raagjadav@xxxxxxxxx>
> >>>>> ---
> >>>>> drivers/i2c/busses/i2c-at91.c | 6 +++++-
> >>>>> 1 file changed, 5 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/drivers/i2c/busses/i2c-at91.c b/drivers/i2c/busses/i2c-at91.c
> >>>>> index 3f3e8b3..b2f5fdb 100644
> >>>>> --- a/drivers/i2c/busses/i2c-at91.c
> >>>>> +++ b/drivers/i2c/busses/i2c-at91.c
> >>>>> @@ -72,6 +72,7 @@
> >>>>> #define AT91_TWI_TXCOMP BIT(0) /* Transmission Complete */
> >>>>> #define AT91_TWI_RXRDY BIT(1) /* Receive Holding Register Ready */
> >>>>> #define AT91_TWI_TXRDY BIT(2) /* Transmit Holding Register Ready */
> >>>>> +#define AT91_TWI_SVREAD BIT(3) /* Slave Read */
> >>>>> #define AT91_TWI_OVRE BIT(6) /* Overrun Error */
> >>>>> #define AT91_TWI_UNRE BIT(7) /* Underrun Error */
> >>>>> #define AT91_TWI_NACK BIT(8) /* Not Acknowledged */
> >>>>> @@ -571,7 +572,10 @@ static irqreturn_t atmel_twi_interrupt(int irq, void *dev_id)
> >>>>> at91_disable_twi_interrupts(dev);
> >>>>> complete(&dev->cmd_complete);
> >>>>> } else if (irqstatus & AT91_TWI_TXRDY) {
> >>>>> - at91_twi_write_next_byte(dev);
> >>>>> + if ((status & AT91_TWI_SVREAD) && (dev->buf_len == 0))
> >>>>> + at91_twi_write(dev, AT91_TWI_IDR, AT91_TWI_TXRDY);
> >>>>> + else
> >>>>> + at91_twi_write_next_byte(dev);
> >>>>> }
> >>>>>
> >>>>> /* catch error flags */
> >>>>> --
> >>>>> 2.7.4
> >>>>>
> >>>>>
> >>>
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> >>>
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