Re: [PATCH -next] spi: Fix double IDR allocation with DT aliases

[Kirill Kapranov]

> 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