GPLv2 forbids the additional non-free restrictions added in GPLv3 so they can't be mixed together.
It isn't permitted to use Linux kernel code in GNU projects or vice versa.
GPL is why Linux users don't have a nice mainline ZFS implementation.
This hardly qualifies as freedom.
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GPL is a non-free software license. It's avoided by projects with strict requirements for free software licenses like OpenBSD.
Free Software means the freedom to use it for any purpose, including building a device with an immutable root of trust or mixing it with other software.
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The restrictions regularly get in my way as a developer and a user of software.
I would use ZFS on my workstation if the Linux kernel used a free software license rather than GPLv2. It's too problematic to use an out-of-tree filesystem.
GPLv3 gets in the way far more often.
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If there's a license incompatibility, then at least one of the licenses doesn't support the freedom to use the software for any purpose. A true Free Software license doesn't have license incompatibilities.
The people who regularly contact me pushing GPL helped me realize this.
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