On Mon, Aug 03, 2020 at 10:02:50PM +0200, Bartosz Golaszewski wrote:

On Sun, Aug 2, 2020 at 5:32 AM Kent Gibson [off-list ref] wrote:

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On Fri, Jul 31, 2020 at 06:05:10PM +0200, Bartosz Golaszewski wrote:

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On Sun, Jul 26, 2020 at 3:12 AM Kent Gibson [off-list ref] wrote:

[snip!]

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+static u64 gpioline_config_flags(struct gpioline_config *lc, int line_idx)
+{
+       int i;
+
+       for (i = lc->num_attrs - 1; i >= 0; i--) {

Much better to read is

unsigned int i = lc->num_attrs;

while (i--) {
 ...
}

Really? I find that the post-decrement in the while makes determining the
bounds of the loop more confusing.

Agreed, Andy: this is too much nit-picking. :)

I was actually hoping for some feedback on the direction of that loop,
as it relates to the handling of multiple instances of the same
attribute associated with a given line.

The reverse loop here implements a last in wins policy, but I'm now
thinking the kernel should be encouraging userspace to only associate a
given attribute with a line once, and that a first in wins would help do
that - as additional associations would be ignored.

Alternatively, the kernel should enforce that an attribute can only be
associated once, but that would require adding more request validation.

I guess this would result in a lot of churn to do validation which is
largely unnecessary? To me the first in wins sounds more consistent.

Fully validating the attrs would involve a lot of tedious looping, which
would be pointless 99.99% of the time, so I was hoping to avoid it.
OTOH we're interacting with hardware so I don't want to be doing
anything that userspace hasn't explicitly requested.

But I would be satisfied with clearly documenting the behaviour - and
in most cases libgpiod will be taking care of it anyway...

Also: I just started going through the patches - nice idea with the
GPIO attributes, I really like it. Although I need to give it a longer
thought tomorrow - I'm wondering if we can maybe unify them and the
flags.

I had an earlier draft that did just that - and that is partially why
the loop is last in wins - I was using slot 0 as the default flags.
But the default flags cover a lot of use cases, including all of v1, and
it was simple and cheap to provide a default - and it simplified the
initial port of libgpiod to v2...

Cheers,
Kent.