For DT systems the dynamically allocated IDs start at the maximum positive ID and work down so in practice it is vanishingly unlikely that there will be a collision as idiomatic static DT IDs would be low integers.
Yes, this algorithm seems really bullet-proof. However, it isn't actually used now. The ID allocation algorithm using atomic_dec_return call had been introduced 2006-01-08 as [1]. It was being used in the mainline kernel (with some improvements) up to 2017-08-16, when it has been replaced with the newer algorithm using Linux idr, accordingly [2].
Since idr_alloc call works incrementally, the situation of a 'fixed' ID squatting by a driver with 'dynamic ID' seems more than possible. Therefore it would be justified to use a hardcoded constant SPI_DYN_FIRST_BUS_NUM (that was introduced in [2] and eliminated in [3]), but with a sufficiently greater value of the constant.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v4.18&id=8ae12a0d85987dc138f8c944cb78a92bf466cea0 [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v4.18&id=9b61e302210eba55768962f2f11e96bb508c2408 [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v4.18&id=42bdd7061a6e24d7b21d3d21973615fecef544ef
