On 3/11/21 12:18 AM, Wolfram Sang wrote:

quoted

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.

Indeed, the i2c-mpc driver sends TXAK (only) after receiving
that interrupt. Since that is running in the context of the user
process, that may well be delayed substantially on a loaded system.

Maybe the interrupt handler will need to play a more active role
in the i2c-mpc driver. Alternatively, the transfer function could
be handled by a high priority kernel thread.

Guenter