On 12/03/21 1:07 pm, Guenter Roeck wrote: > On 3/11/21 3:47 PM, Chris Packham wrote: >> On 12/03/21 10:34 am, Guenter Roeck wrote: >>> On 3/11/21 1:17 PM, Chris Packham wrote: >>>> On 11/03/21 9:18 pm, Wolfram Sang wrote: >>>>>> Bummer. What is really weird is that you see clock stretching under >>>>>> CPU load. Normally clock stretching is triggered by the device, not >>>>>> by the host. >>>>> One example: Some hosts need an interrupt per byte to know if they >>>>> should send ACK or NACK. If that interrupt is delayed, they stretch the >>>>> clock. >>>>> >>>> It feels like something like that is happening. Looking at the T2080 >>>> Reference manual there is an interesting timing diagram (Figure 14-2 if >>>> someone feels like looking it up). It shows SCL low between the ACK for >>>> the address and the data byte. I think if we're delayed in sending the >>>> next byte we could violate Ttimeout or Tlow:mext from the SMBUS spec. >>>> >>> I think that really leaves you only two options that I can see: >>> Rework the driver to handle critical actions (such as setting TXAK, >>> and everything else that might result in clock stretching) in the >>> interrupt handler, or rework the driver to handle everything in >>> a high priority kernel thread. >> One thing I've found that does seem to avoid the problem is to disable >> preemption, use polling and replace the schedule() in i2c_wait() with >> udelay(50). That's kind of like the kernel thread option. > It is kind of hackish, though, especially since it makes the "loaded system" > situation even worse by adding even more active wait loops. No -ish about it :). But it might put out one fire for me while I'm looking at doing some kind of interrupt driven state machine.
