linux-kernel - Re: [PATCH v4 4/9] drm/mipi-dsi: Reduce driver bloat of mipi_dsi_*_write

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Date: Mon, 13 May 2024 11:29:58 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Douglas Anderson <dianders@...omium.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 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.

Maxime

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