dev_id seems to be for IRQ sharing

Forum: Supporting Functions
Re: request_irq(...,void *dev_id) (Robert Wilhelm)
Keywords: request_irq dev_id IRQ-sharing
Date: Tue, 08 Apr 1997 02:11:34 GMT
From: Steven Hunyady <hunyady@kestrel.nmt.edu>

Look in Don Becker's 3c59x.c net driver. Apparently, IRQ sharing amongst like (or dissimilar?) cards developed progressively in the kernel, and this driver, usable in several major kernel versions, shows this ongoing adaptation. Most other device drivers have not yet allowed for multiple use of IRQ lines, hence they simply put "NULL" for this fifth parameter in the function request_irq() and the second in free_irq().