linux-kernel - Re: [PATCH 3/4] drm/msm: Add SYSPROF param

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Date:   Thu, 3 Mar 2022 14:36:17 -0800
From:   Stephen Boyd <swboyd@...omium.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Jordan Crouse <jordan@...micpenguin.net>,
        Akhil P Oommen <quic_akhilpo@...cinc.com>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Marek <jonathan@...ek.ca>,
        Emma Anholt <emma@...olt.net>,
        Yangtao Li <tiny.windzz@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] drm/msm: Add SYSPROF param

Quoting Rob Clark (2022-03-03 13:47:14)
> On Thu, Mar 3, 2022 at 1:17 PM Rob Clark <robdclark@...il.com> wrote:
> >
> > On Thu, Mar 3, 2022 at 12:47 PM Stephen Boyd <swboyd@...omium.org> wrote:
> > >
> > > Quoting Rob Clark (2022-03-03 11:46:47)
> > > > +
> > > > +       /* then apply new value: */
> > >
> > > It would be safer to swap this. Otherwise a set when the values are at
> > > "1" would drop to "zero" here and potentially trigger some glitch,
> > > whereas incrementing one more time and then dropping the previous state
> > > would avoid that short blip.
> > >
> > > > +       switch (sysprof) {
> > > > +       default:
> > > > +               return -EINVAL;
> > >
> > > This will become more complicated though.
> >
> > Right, that is why I took the "unwind first and then re-apply"
> > approach.. in practice I expect userspace to set the value before it
> > starts sampling counter values, so I wasn't too concerned about this
> > racing with a submit and clearing the counters.  (Plus any glitch if
> > userspace did decide to change it dynamically would just be transient
> > and not really a big deal.)
>
> Actually I could just swap the two switch's.. the result would be that
> an EINVAL would not change the state instead of dropping the state to
> zero.  Maybe that is better anyways
>

Yeah it isn't clear to me what should happen if the new state is
invalid. Outright rejection is probably better than replacing the
previous state with an invalid state.