Re: [PATCH v4 3/4] gpio: shared-proxy: Drive the default value when no votes are cast

[Bartosz Golaszewski]

On Wed, 28 Oct 2026 08:01:01 +0100, Shawn Guo
<shengchao.guo@xxxxxxxxxxxxxxxx> said:
> On Tue, Oct 20, 2026 at 09:17:00AM -0400, Bartosz Golaszewski wrote:
>> On Sat, 17 Oct 2026 06:36:04 +0200, Shawn Guo
>> <shengchao.guo@xxxxxxxxxxxxxxxx> said:
>> > A consumer that requests a shared line with GPIOD_ASIS never calls
>> > direction_output(), so def_val keeps its initial value of 0. If the
>> > line was left high, for instance by the bootloader, a later
>> > gpiod_set_value(0) from that consumer matches def_val and the proxy
>> > treats it as withdrawing a vote it never cast. It returns success
>> > without writing the hardware, and the line stays high.
>> >
>> > The consumer has no way to tell. Its request to drive the line low is
>> > silently ignored until it first votes for high and then withdraws
>> > that vote.
>> >
>> > When the requested value is the default and no votes are
>> > outstanding, write it to the hardware instead of returning early.
>> > For lines configured via direction_output() this is a redundant
>> > write of the value the line already has. With votes outstanding the
>> > request remains a no-op, as the line must stay at the voted value.
>> >
>>
>> Wouldn't this only matter for a line requested as-is? We could avoid the
>> redundant write if we only did it for such lines instead of for all
>> unconditionally.
>
> It also matters for a line configured via direction_output(), after
> patch 2. When a proxy releases the line and restoring the default value
> fails, gpio_shared_proxy_free() still drops its vote. The line is then
> at the non-default value with votecnt == 0, so "no votes outstanding"
> no longer implies "line is at the default" either way. The unconditional
> write lets any remaining user that sets the default bring the line back.
> Gating it on as-is lines would leave it stuck there.
>
> The proxy also has no direct notion of as-is; it would need a flag in
> gpio_shared_desc tracking whether def_val has ever been written to the
> hardware, set and cleared in the init paths of direction_output() and
> direction_input().
>
> The write itself is cheap and rare: it only happens when a proxy that
> hasn't voted sets the default while no votes are outstanding.
>
> If you still prefer the gating, I can do that in v5 and keep the
> free-failure case recoverable some other way. Otherwise I'll add the
> reasoning above to the commit message.
>

No, that's alright, let's keep the write.

Bart