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Date:   Tue, 18 Feb 2020 10:30:04 -0800
From:   Prashant Malani <pmalani@chromium.org>
To:     Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc:     Gwendal Grignou <gwendal@chromium.org>,
        Jonathan Cameron <jic23@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hartmut Knaack <knaack.h@gmx.de>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
        Benson Leung <bleung@chromium.org>,
        Guenter Roeck <groeck@chromium.org>,
        Lee Jones <lee.jones@linaro.org>,
        Fabien Lahoudere <fabien.lahoudere@collabora.com>,
        "open list:IIO SUBSYSTEM AND DRIVERS" <linux-iio@vger.kernel.org>
Subject: Re: [PATCH v2 10/17] iio: cros_ec: Use cros_ec_cmd()
Message-ID: <20200218183004.GA184561@google.com>
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Hi All,

Just thought I'd ping this thread since it's been a week since the last
email.

On Mon, Feb 10, 2020 at 12:14:01PM -0800, Prashant Malani wrote:
> Hi All (trimming most code parts of the thread for the sake of brevity),
> 
> Thanks for listing the points Enric, Please see my notes inline:
> 
> On Mon, Feb 10, 2020 at 3:03 AM Enric Balletbo i Serra
> <enric.balletbo@collabora.com> wrote:
> >
> > Hi Gwendal, Prashant et all
> >
> > On 7/2/20 19:47, Gwendal Grignou wrote:
> > > On Thu, Feb 6, 2020 at 10:50 AM Prashant Malani <pmalani@chromium.org> wrote:
> > >>
> > >> Hi Enric,
> > >>
> > >> Thanks for taking a look at the patch. Please see my response inline:
> ....
> > >>>>> @@ -171,9 +162,11 @@ int cros_ec_motion_send_host_cmd(struct cros_ec_sensors_core_state *state,
> > >>>>>
> > >>>>>      memcpy(state->msg->data, &state->param, sizeof(state->param));
> > >>>>>
> > >>>>> -    ret = cros_ec_cmd_xfer_status(state->ec, state->msg);
> > >>>>> +    ret = cros_ec_cmd_xfer(state->ec, state->msg);
> > >>>>>      if (ret < 0)
> > >>>>>              return ret;
> > >>>>> +    else if (state->msg->result != EC_RES_SUCCESS)
> > >>>>> +            return -EPROTO;
> > >>>>>
> > >>>
> > >>> There is no way to use the new cros_ec_cmd here?
> > > When the EC does not support sensor fifo,
> > > cros_ec_motion_send_host_cmd() is on the data path. For instance, it
> > > is called 2 times every 10ms by chrome to calculate the lid angle. I
> > > would be reluctant to call malloc. Given it is well encapsulated into
> > > the sensor stack. Does it make sense to call cros_ec_cmd_xfer
> > > directly?
> > >
> >
> > Thanks Gwendal for pointing this, it makes totally sense, and I suspect this can
> > happen on other cases.
> >
> > Just to make clear, my concern is not about not using the new 'cros_ec_cmd'
> > here, is about changing 'cros_ec_cmd_xfer_status' for 'cros_ec_cmd_xfer'. Also,
> > my other concern is how useful is the new 'cros_ec_cmd' replacing what we have
> > now if cannot replace all current uses.
> >
> > My points of view are this:
> >
> > * Actually we have cros_ec_cmd_xfer and cros_ec_cmd_xfer_status, use the second
> > one is better, in fact, we tried to move all the cros_ec_cmd_xfer to the _status
> > version in the past because makes the code and error handling cleaner. So I'm
> > reticent to get back to use cros_ec_cmd_xfer instead of cros_ec_cmd_xfer_status.
> >
> > * The users of the cros-ec protocol sometimes they mallocing/freeing at runtime,
> > and sometimes they don't. IMHO *non* mallocing/freeing is usually better, more
> > efficient and faster. Would be nice to standardize this.
> 
> I think we should look at latency (I am assuming that is one of the
> concerns Gwendal was referring to).
> We should certainly do more rigorous measurements, but I did a crude
> measurement across a devm_kzalloc() used on one of the EC commands
> inside platform/chrome for struct EC command:
> - Used ktime_get_ns() to record time before and after the devm_kzalloc()
> - Used ktime_sub to subtract the "after" and "before" values:
> 
>         struct cros_ec_command *msg;
>         int ret;
> +       ktime_t start, end, diff;
> 
> +       start = ktime_get_ns();
>         msg = kzalloc(sizeof(*msg) + max(outsize, insize), GFP_KERNEL);
> +       end = ktime_get_ns();
>         if (!msg)
>                 return -ENOMEM;
> 
> +       diff = ktime_sub(end, start);
> +       printk("%s(): TEST: kzalloc took: %lld\n", __func__, ktime_to_ns(diff));
> 
> On an i5 1.6 GHz system, across 16 call measurements I got the
> following latency values (in ns):
> - Count, N:16
> - Average: 72.375
> - Std. Dev : 28.768
> - Max: 143
> - Min:  51
> 
> Are these values significant for the various call-sites? I think the
> driver authors might be able to comment better there (unfortunately I
> don't have enough context for each case).
> Of course there will be other overhead (memcpy) but I think this is a
> good starting point for the discussion.
> (My apologies if this measurement method is incorrect/inaccurate.)

Any thoughts / comments here?

On an overall note, I think keeping cros_ec_cmd_xfer() and cros_ec_cmd()
might be a good starting point.

In this way, we are not introducing any extra function. Also, we can
begin converting the cros_ec_cmd_xfer() use cases (a few call-sites may
need to be investigated from a latency perspective). The
cros_ec_cmd_xfer() conversions are better handled in separate patch
series.

Best regards,

-Prashant
> 
> >
> > * If we want to introduce a new 'cros_ec_cmd', this should make the code cleaner
> > and ideally should be the way we tell the users they should use to communicate
> > with the cros-ec and not open coding constantly. Ideally, should be a
> > replacement of all current 'cros_ec_cmd_xfer*' versions.
> 
> As I mentioned previously, I think all calls of cros_ec_cmd_xfer() can
> be converted to use cros_ec_cmd() (especially since the new API has a
> *result pointer),
> but I think it should be staged out a bit more (since cases like iio:
> cros_ec driver require non-trivial refactoring which I think is better
> in a patch/series).
> 
> >
> > * If 'cros_ec_cmd' *cannot* replace all the cases, it should be clear to the
> > user in which cases he should use this function and in which cases shouldn't use
> > this function.
> 
> This seems like a good compromise, but my expectation is that if there
> is a "fast" and "slow" version of the same functionality, developers
> would be inclined to use the "fast" version always?
> 
> 
> > * Finally, what pointed Gwendal, what's the best approach to send commands to
> > the EC by default, is better use dynamic memory? or is better use the stack? is
> > it always safe use the stack? is always efficient use allocated memory?
> >
> > As you can see I have a lot of questions still around, but taking in
> > consideration that this will be an important change I think that makes sense
> > spend some time discussing it.
> >
> > What do you think?
> >
> > Enric
> >
> >
> > > Gwendal.
> > >>
> > >> I think it is doable. From looking at the code I felt the factors we
> > >> need to be careful about are:
> > >> - The function cros_ec_motion_send_host_cmd() is called from a few
> > >> other files, each of which set up the struct cros_ec_command
> > >> differently (reference:
> > >> https://elixir.bootlin.com/linux/latest/ident/cros_ec_motion_send_host_cmd)
> > >> - It is not clear to me how readability will be affected by making the
> > >> change to cros_ec_cmd().
> > >>
> > >> Due to the above two factors, but primarily because I wanted to avoid
> > >> making such an involved large change in this 17 patch series, I
> > >> reasoned it would be better to make the transition to cros_ec_cmd()
> > >> for these files in a separate patch/series.
> > >> My plan after this patch series is to work on this driver(perhaps we
> > >> can eliminate cros_ec_motion_send_host_cmd() itself?), and then remove
> > >> cros_ec_cmd_xfer() usage.
> > >>
> > >> WDYT?
> > >>
> > >> Best regards,
> > >>
> > >>
> > >>>
> > >>>
> > >>>>>      if (ret &&
> > >>>>>          state->resp != (struct ec_response_motion_sense *)state->msg->data)
> > >>>>