linux-kernel - Re: [PATCH v2 4/8] drm/mipi-dsi: Introduce mipi_dsi_*_write_seq

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Message-ID: <CAA8EJppMf2nAU_=pZz6eUhasQGEzEnz1_i72pNXBsmE9UgV--Q@mail.gmail.com>
Date: Wed, 1 May 2024 18:57:16 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Doug Anderson <dianders@...omium.org>
Cc: Jani Nikula <jani.nikula@...ux.intel.com>, neil.armstrong@...aro.org, 
	dri-devel@...ts.freedesktop.org, Linus Walleij <linus.walleij@...aro.org>, 
	lvzhaoxiong@...qin.corp-partner.google.com, Hsin-Yi Wang <hsinyi@...gle.com>, 
	Javier Martinez Canillas <javierm@...hat.com>, Joel Selvaraj <jo@...amily.in>, 
	Cong Yang <yangcong5@...qin.corp-partner.google.com>, Sam Ravnborg <sam@...nborg.org>, 
	Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/8] drm/mipi-dsi: Introduce mipi_dsi_*_write_seq_multi()

On Wed, 1 May 2024 at 18:49, Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Mon, Apr 29, 2024 at 8:39 AM Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> >
> > On Mon, 29 Apr 2024, Doug Anderson <dianders@...omium.org> wrote:
> > > Hi,
> > >
> > > On Mon, Apr 29, 2024 at 2:38 AM Neil Armstrong
> > > <neil.armstrong@...aro.org> wrote:
> > >>
> > >> > +/**
> > >> > + * struct mipi_dsi_multi_context - Context to call multiple MIPI DSI funcs in a row
> > >> > + * @dsi: Pointer to the MIPI DSI device
> > >> > + * @accum_err: Storage for the accumulated error over the multiple calls. Init
> > >> > + *   to 0. If a function encounters an error then the error code will be
> > >> > + *   stored here. If you call a function and this points to a non-zero
> > >> > + *   value then the function will be a noop. This allows calling a function
> > >> > + *   many times in a row and just checking the error at the end to see if
> > >> > + *   any of them failed.
> > >> > + */
> > >> > +
> > >> > +struct mipi_dsi_multi_context {
> > >> > +     struct mipi_dsi_device *dsi;
> > >> > +     int accum_err;
> > >> > +};
> > >>
> > >> I like the design, but having a context struct seems over-engineered while we could pass
> > >> a single int over without encapsulating it with mipi_dsi_multi_context.
> > >>
> > >> void mipi_dsi_dcs_write_buffer_multi(struct mipi_dsi_multi_context *ctx,
> > >>                                      const void *data, size_t len);
> > >> vs
> > >> void mipi_dsi_dcs_write_buffer_multi(struct mipi_dsi_device *dsi, int *accum_err,
> > >>                                      const void *data, size_t len);
> > >>
> > >> is the same, and it avoids having to declare a mipi_dsi_multi_context and set ctx->dsi,
> > >> and I'll find it much easier to migrate, just add a &ret and make sure ret is initialized to 0.
> > >
> > > Yeah, I had the same reaction when Jani suggested the context style
> > > [1] and I actually coded it up exactly as you suggest above. I then
> > > changed my mind and went with the context. My motivation was that when
> > > I tested it I found that using the context produced smaller code.
> > > Specifically, from the description of this patch we see we end up
> > > with:
> > >
> > > Total: Before=10651, After=9663, chg -9.28%
> > >
> > > ...when I didn't have the context and I had the accum_err then instead
> > > of getting ~9% smaller I believe it actually got ~0.5% bigger. This
> > > makes some sense as the caller has to pass 4 parameters to each call
> > > instead of 3.
> > >
> > > It's not a giant size difference but it was at least some motivation
> > > that helped push me in this direction. I'd also say that when I looked
> > > at the code in the end the style grew on me. It's really not too
> > > terrible to have one line in your functions that looks like:
> > >
> > > struct mipi_dsi_multi_context ctx = { .dsi = boe->dsi };
> > >
> > > ...and if that becomes the canonical way to do it then it's really
> > > hard to accidentally forget to initialize the error value. With the
> > > other API it's _very_ easy to forget to initialize the error value and
> > > the compiler won't yell at you. It also makes it very obvious to the
> > > caller that this function is doing something a little different than
> > > most Linux APIs with that error return.
> > >
> > > So I guess I'd say that I ended up being pretty happy with the
> > > "context" even if it does feel a little over engineered and I'd argue
> > > to keep it that way. That being said, if you feel strongly about it
> > > then we can perhaps get others to chime in to see which style they
> > > prefer? Let me know what you think.
> > >
> > >
> > > [1] https://lore.kernel.org/r/8734r85tcf.fsf@intel.com
> >
> > FWIW, I don't feel strongly about this, and I could be persuaded either
> > way, but I've got this gut feeling that an extensible context parameter
> > might be benefitial future proofing in this case.
>
> I've gone ahead and sent out a v3 where I still leave it as taking
> `struct mipi_dsi_multi_context`. Neil: if you feel strongly about it,
> I have no problem sending out a v4. I still think that the size
> savings of using the context are a good "tiebreaking" factor in
> choosing between the two styles... ;-)

I like the idea of context. If later on we need to add additional data
(like DSC PPS or drm_mode), we can simply extend the context
structure.

>
> -Doug



-- 
With best wishes
Dmitry