Fixing Zephyr CAN Timing: API & Standards Alignment
Hey everyone! Let's dive into something super important for all you embedded gurus and Zephyr OS enthusiasts out there: a critical CAN timing test standards inconsistency we've noticed within the Zephyr RTOS. Specifically, we're talking about the can.timing and can.api test suites. This isn't just some nitpicky detail, guys; it's about ensuring our CAN bus communication is as robust and reliable as possible, which is absolutely vital for countless embedded applications, from automotive systems to industrial automation. The core of the issue boils down to how sampling points are handled and tested, and trust me, getting this right can make or break your CAN implementation. Currently, there's a bit of a mismatch: can.timing allows for a small, reasonable margin of error when can_calc_timing and can_calc_timing_data return sampling point values, which is super practical given real-world clock limitations. But then, can.api steps in with a much stricter, zero-tolerance approach, demanding absolute precision that just isn't always achievable. This inconsistency can lead to unnecessary test failures or, worse, hide potential configuration issues that only surface later in deployment. Imagine spending hours debugging a subtle communication glitch, only to find out it's due to a test suite expecting an impossible perfect sampling point. That's a headache no one needs! Our goal here is to shine a light on this discrepancy, understand its implications, and, most importantly, figure out a path towards a more unified and practical testing methodology that truly reflects the realities of embedded hardware. It's all about making Zephyr even better, more predictable, and easier for you awesome developers to work with. So, let's roll up our sleeves and get into the nitty-gritty of why this sampling point deviation matters and how we can bring harmony to our CAN test standards.
Unpacking the Zephyr CAN Timing Discrepancy
Alright, folks, let's really dig into the technical core of this Zephyr CAN timing discrepancy that's causing some friction. The main beef, as we touched on earlier, is the stark difference in how can.timing and can.api handle what's called the sampling point – that crucial moment within a CAN bit where the state of the line is read. In the can.timing tests, specifically around lines like this one: https://github.com/zephyrproject-rtos/zephyr/blob/8053d7793f13a89bc660381a8eae5e8172e79c7e/tests/drivers/can/timing/src/main.c#L173-L175, you'll notice that the can_calc_timing and can_calc_timing_data functions are allowed to return sampling point values with a small margin of error. This is incredibly pragmatic, because, let's be real, embedded systems rarely operate with perfectly ideal clock frequencies like 20 MHz, 40 MHz, or 80 MHz in every scenario. Sometimes, your system clock or peripheral clock dividers just won't allow for a perfectly precise bit time segmentation, meaning your sampling point might be off by a tiny fraction. And guess what? That's often perfectly fine for stable CAN communication. It accounts for the inherent limitations of hardware and clock divisors, acknowledging that a practical, very close-to-ideal value is often good enough. However, when we hop over to the can.api tests, for example, around here: https://github.com/zephyrproject-rtos/zephyr/blob/8053d7793f13a89bc660381a8eae5e8172e79c7e/tests/drivers/can/api/src/canfd.c#L530-L531, the story changes completely. Here, there's no such tolerance for sampling point deviation. The test expects an exact, precise sampling point, and anything less results in a failure. This rigid approach, while seemingly aiming for perfection, can be a real roadblock. It means that even if your hardware configuration, with its slightly less-than-perfect clock division, would still yield perfectly functional and reliable CAN communication in the real world, the can.api test will flag it as an error. This kind of discrepancy between test standards for what is fundamentally the same underlying timing calculation is not only confusing but can also unfairly penalize perfectly valid driver implementations. It forces developers to jump through hoops to satisfy an overly strict test, or worse, prevents them from using valid hardware configurations because the tests don't allow for a realistic slight sampling point deviation. We need to ensure that our test framework is a helpful guardian, not an arbitrary gatekeeper, especially when it comes to something as fundamental as CAN timing, which is inherently subject to hardware constraints.
Now, let's elaborate a bit on the practical implications of this. Guys, it's not always a picnic when you're trying to achieve ideal clock frequencies in embedded systems. You might be working with an MCU that has a specific crystal oscillator, or perhaps your project's power constraints or other peripheral requirements dictate a non-standard clock setup. In these scenarios, achieving that