Still can't use spi clock over 12MHz (5.4 kernel)

jhp

We applied lowlatency to the 5.4 kernel and tested spi.
It was confirmed that the time from the point where the CS pin becomes low to the point where the clock starts is longer than before. (0.42us --> 14us)
However, if the clock is still set to 12MHz or higher, the clock stops in the middle just like the 5.0 kernel.
Please note that I have inquired about this problem before.
https://forum.up-community.org/discussion/4406/up-xtreme-spi-speed-problem

DCleri

Hello @jhp

We are looking into the issue and we will come back as soon as possible with an update

jhp

As a result of the measurement, the clock stop phenomenon occurs intermittently from 12MHz, and it seems to continue to occur when it exceeds 15MHz.
SPI communication speed should be 25MHz for control period and control speed, and PREEMPT-RT is required.

And when fully PREEMPT RT is applied using a generic kernel, GPIO cannot be used, but only SPI is available.
I am curious about the reason that only SPI can be used, and I would appreciate it if you let me know how to apply the HAT driver to the generic kernel.

Finally, I am curious if there is a plan to support fully PREEMPT RT.
Currently, fully PREEMPT RT is indispensable to ensure real-time performance during robot control.

DCleri

Hello @jhp

From our tests the SPI works fine up to 20Mhz, but not above and as the SPI controller is inside the Intel chip, the only thing that can be done is raise the issue on the kernel bugzilla website.

We tested with kernel 5.4 for UP Board that includes the pinctrl patches to enable the multi function capabilities of our I/O, these features won't work without our patches and that is why you cannot use a generic kernel.

At the moment we don't have a supported fully preemptive kernel in the community, but we have other users and customers using it.
For example, recently, this user has posted on github all the information to install and configure RT patches on our UP 5.4 Kernel for Ubuntu: https://github.com/raess1/UPxtreme-RT-kernel

If you need support for specific configuration you can contact us via our up-shop.org to discuss a service engagement for your project.

jayman

From our tests the SPI works fine up to 20Mhz, but not above and as the SPI controller is inside the Intel chip, the only thing that can be done is raise the issue on the kernel bugzilla website.

The GSPI ports on the WHL PCH are specified as "Supports bit rates up to 20 Mbits/s", so there's no issue to be raised.