Could we start deprecating filter functions ? They are unusable anyway, and I propose that for this KPI the filter was always NULL. I.e., remove it from the template, and pass NULL to bus_dma_tag_create() in bus_dma_tag_simple().

You are inconsistent with blank lines: bu_dma_tag_init() does not add blank line after '{'.

One of the biggest mistakes that I feel I've made with the CAM and busdma APIs in the past was in changing the argument signatures of existing functions. I don't want to repeat that mistake and redefine a bus bus_dma_tag_create(). But I also agree with you that filters are useless and the filter arguments are just noise. This new template API is my attempt to essentially deprecate filters as well as the old API signature without actually changing it. I suppose that the filter arguments can also be removed from the template, though, as a signal that filters are no longer a first-class part of the API.

I believe that it is part of style(9) to have a blank line when no local variables are declared.

static void
usage(void)
{
        /* Optional blank line goes here. */

Optionally, insert a blank line at the beginning of functions with no
local variables.  Older versions of this style document required the
blank line convention, so it is widely used in existing code.

I am not proposing to change any functions' signatures. I propose to drop filtfunc/filtfuncarg from the bus_dma_template_t structure, nobody would use them anyway. Pass NULL for bus_dma_tag_create() for filter when called from bus_dma_tag_simple() (or whatever you rename it).

My point is that the patch is not consistent. You added blank line there, but not in bus_dma_tag_init().

From some time, the blank line is optional.