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

[Shawn Guo <shengchao.guo@oss.qualcomm.com>]

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@oss.qualcomm.com> 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.

Shawn