Date: Fri, 22 Apr 2022 00:35:37 -0400 From: Farhan Khan <farhan@farhan.codes> To: Yusuf Khan <yusisamerican@gmail.com>, freebsd-drivers@freebsd.org Subject: Re: How do I write a driver and how do I find out what kernel API calls do? Message-ID: <af7b270bfbea1a539dcbaef4b3e610e90d6a58b4.camel@farhan.codes> In-Reply-To: <CAJoG2%2B-fPESZmJaV-ydduH2Vp=oChCFwM1tpy44yupZmQOWw2Q@mail.gmail.com> References: <CAJoG2%2B-fPESZmJaV-ydduH2Vp=oChCFwM1tpy44yupZmQOWw2Q@mail.gmail.com>
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On Thu, 2022-04-21 at 18:17 -0500, Yusuf Khan wrote: > Literally just copy paste the title here. > > Also where do I get documentation? Hi Yusuf! I'm no expert, but I got my (limited) knowledge from a few sources: 1. There is a section in the documentation on this topic: https://docs.freebsd.org/en/books/arch-handbook/driverbasics/ 2. There's a book called "FreeBSD Device Drivers: A Guide for the Intrepid". Its a bit dated but the concepts largely still apply. 3. I read a ton of sample drivers. I'm currently struggling through the USB stack, /usr/src/sys/dev/usb/wlan has a lot of good and simple WiFi drivers. If you want an example of a wifi driver that is both USB and PCI, look at rtwn(4). 4. Each stack has some documentation, so in the case of USB, I read the usbdi man page and looked at examples. 5. Look at other implementations in other BSDs - or even illumos. Ultimately its not FreeBSD but it helps provide some context, especially for someone like me who has to learn multiple layers/stack prior to understanding what is happening, specifically what is device- specific and what is OS-specific. There's a ton of overlap and in some cases, such as WiFi, they maintain a slightly older version and therefore simpler version of the same stack. In terms of the details of a specific device, I'm honestly just copying previous implementations, namely Linux and OpenBSD. There's a great presentation by an OpenBSD developer on how he wrote wifi device drivers with limited context. You can find it on YouTube as: "Getting started with OpenBSD device driver development, by Stefan Sperling (EuroBSDcon 2017)" I was hoping to write some documentation of my experiences, but I have no immediate plans. If you struggle with the same thing, maybe write your experiences for the benefit of the community. I hope this helps! btw, nice last name! - Farhan Khan
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