Date: Mon, 13 May 2024 08:48:19 -0700
From: Doug Anderson <dianders@...omium.org>
To: Maxime Ripard <mripard@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, 
	lvzhaoxiong@...qin.corp-partner.google.com, 
	Jani Nikula <jani.nikula@...ux.intel.com>, Hsin-Yi Wang <hsinyi@...gle.com>, 
	Linus Walleij <linus.walleij@...aro.org>, Javier Martinez Canillas <javierm@...hat.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, 
	Cong Yang <yangcong5@...qin.corp-partner.google.com>, Sam Ravnborg <sam@...nborg.org>, 
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, Joel Selvaraj <jo@...amily.in>, 
	Brian Norris <briannorris@...omium.org>, Daniel Vetter <daniel@...ll.ch>, 
	David Airlie <airlied@...il.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/9] drm/mipi-dsi: Reduce driver bloat of mipi_dsi_*_write_seq()

Hi,

On Mon, May 13, 2024 at 2:30 AM Maxime Ripard <mripard@...nel.org> wrote:
>
> Hi,
>
> On Wed, May 08, 2024 at 01:51:46PM -0700, Douglas Anderson wrote:
> > Through a cooperative effort between Hsin-Yi Wang and Dmitry
> > Baryshkov, we have realized the dev_err() in the
> > mipi_dsi_*_write_seq() macros was causing quite a bit of bloat to the
> > kernel. Let's hoist this call into drm_mipi_dsi.c by adding a "chatty"
> > version of the functions that includes the print. While doing this,
> > add a bit more comments to these macros making it clear that they
> > print errors and also that they return out of _the caller's_ function.
> >
> > Without any changes to clients this gives a nice savings. Specifically
> > the macro was inlined and thus the error report call was inlined into
> > every call to mipi_dsi_dcs_write_seq() and
> > mipi_dsi_generic_write_seq(). By using a call to a "chatty" function,
> > the usage is reduced to one call in the chatty function and a function
> > call at the invoking site.
> >
> > Building with my build system shows one example:
> >
> > $ scripts/bloat-o-meter \
> >   .../before/panel-novatek-nt36672e.ko \
> >   .../after/panel-novatek-nt36672e.ko
> > add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-4404 (-4404)
> > Function                                     old     new   delta
> > nt36672e_1080x2408_60hz_init               10640    6236   -4404
> > Total: Before=15055, After=10651, chg -29.25%
> >
> > Note that given the change in location of the print it's harder to
> > include the "cmd" in the printout for mipi_dsi_dcs_write_seq() since,
> > theoretically, someone could call the new chatty function with a
> > zero-size array and it would be illegal to dereference data[0].
> > There's a printk format to print the whole buffer and this is probably
> > more useful for debugging anyway. Given that we're doing this for
> > mipi_dsi_dcs_write_seq(), let's also print the buffer for
> > mipi_dsi_generic_write_seq() in the error case.
> >
> > It should be noted that the current consensus of DRM folks is that the
> > mipi_dsi_*_write_seq() should be deprecated due to the non-intuitive
> > return behavior. A future patch will formally mark them as deprecated
> > and provide an alternative.
> >
> > Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> > Reviewed-by: Neil Armstrong <neil.armstrong@...aro.org>
> > Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > ---
> >
> > Changes in v4:
> > - Update wording as per Linus W.
> >
> > Changes in v3:
> > - Rebased upon patch to remove ratelimit of prints.
> >
> > Changes in v2:
> > - Add some comments to the macros about printing and returning.
> > - Change the way err value is handled in prep for next patch.
> > - Modify commit message now that this is part of a series.
> > - Rebased upon patches to avoid theoretical int overflow.
> >
> >  drivers/gpu/drm/drm_mipi_dsi.c | 56 ++++++++++++++++++++++++++++++++++
> >  include/drm/drm_mipi_dsi.h     | 47 +++++++++++++++-------------
> >  2 files changed, 82 insertions(+), 21 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
> > index 795001bb7ff1..8593d9ed5891 100644
> > --- a/drivers/gpu/drm/drm_mipi_dsi.c
> > +++ b/drivers/gpu/drm/drm_mipi_dsi.c
> > @@ -764,6 +764,34 @@ ssize_t mipi_dsi_generic_write(struct mipi_dsi_device *dsi, const void *payload,
> >  }
> >  EXPORT_SYMBOL(mipi_dsi_generic_write);
> >
> > +/**
> > + * mipi_dsi_generic_write_chatty() - mipi_dsi_generic_write() w/ an error log
> > + * @dsi: DSI peripheral device
> > + * @payload: buffer containing the payload
> > + * @size: size of payload buffer
> > + *
> > + * Like mipi_dsi_generic_write() but includes a dev_err_ratelimited()
>
> You mention in both functions that it's calling dev_err_ratelimited() ...
>
> > + * call for you and returns 0 upon success, not the number of bytes sent.
> > + *
> > + * Return: 0 on success or a negative error code on failure.
> > + */
> > +int mipi_dsi_generic_write_chatty(struct mipi_dsi_device *dsi,
> > +                               const void *payload, size_t size)
> > +{
> > +     struct device *dev = &dsi->dev;
> > +     ssize_t ret;
> > +
> > +     ret = mipi_dsi_generic_write(dsi, payload, size);
> > +     if (ret < 0) {
> > +             dev_err(dev, "sending generic data %*ph failed: %zd\n",
> > +                     (int)size, payload, ret);
>
> ... but it doesn't.

Whoops, thanks for catching this! I'll plan to send a v5 tomorrow to
fix this and then I'll still plan to land the series on Thursday
unless anything major comes up.

-Doug

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