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From: Hsin-Yi Wang <hsinyi@google.com>
Date: Wed, 24 Apr 2024 14:18:33 -0700
Message-ID: <CACb=7PXKTkpPA-cSwwoa3a2H5-iR7uNCWH9X0DHaFWmZd001Ow@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] drm/panel: kd101ne3: add new panel driver
To: Doug Anderson <dianders@google.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>, 
	lvzhaoxiong <lvzhaoxiong@huaqin.corp-partner.google.com>, mripard@kernel.org, 
	airlied@gmail.com, daniel@ffwll.ch, robh@kernel.org, 
	krzysztof.kozlowski+dt@linaro.org, conor+dt@kernel.org, 
	dri-devel@lists.freedesktop.org, devicetree@vger.kernel.org, 
	linux-kernel@vger.kernel.org, 
	cong yang <yangcong5@huaqin.corp-partner.google.com>
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On Wed, Apr 24, 2024 at 2:05=E2=80=AFPM Doug Anderson <dianders@google.com>=
 wrote:
>
> Hi,
>
> On Tue, Apr 23, 2024 at 2:20=E2=80=AFPM Dmitry Baryshkov
> <dmitry.baryshkov@linaro.org> wrote:
> >
> > On Tue, Apr 23, 2024 at 01:41:59PM -0700, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Tue, Apr 23, 2024 at 11:10=E2=80=AFAM Hsin-Yi Wang <hsinyi@google.=
com> wrote:
> > > >
> > > > > > > > +#define _INIT_DCS_CMD(...) { \
> > > > > > > > +     .type =3D INIT_DCS_CMD, \
> > > > > > > > +     .len =3D sizeof((char[]){__VA_ARGS__}), \
> > > > > > > > +     .data =3D (char[]){__VA_ARGS__} }
> > > > > > > > +
> > > > > > > > +#define _INIT_DELAY_CMD(...) { \
> > > > > > > > +     .type =3D DELAY_CMD,\
> > > > > > > > +     .len =3D sizeof((char[]){__VA_ARGS__}), \
> > > > > > > > +     .data =3D (char[]){__VA_ARGS__} }
> > > > > > >
> > > > > > > This is the third panel driver using the same appoach. Can yo=
u use
> > > > > > > mipi_dsi_generic_write_seq() instead of the huge table? Or if=
 you prefer
> > > > > > > the table, we should extract this framework to a common helpe=
r.
> > > > > > > (my preference is shifted towards mipi_dsi_generic_write_seq(=
)).
> > > > > > >
> > > > > > The drawback of mipi_dsi_generic_write_seq() is that it can cau=
se the
> > > > > > kernel size grows a lot since every sequence will be expanded.
> > > > > >
> > > > > > Similar discussion in here:
> > > > > > https://lore.kernel.org/dri-devel/CAD=3DFV=3DWju3WS45=3DEpXMUg7=
FjYDh3-=3Dmvm_jS7TF1tsaAzbb4Uw@mail.gmail.com/
> > > > > >
> > > > > > This patch would increase the module size from 157K to 572K.
> > > > > > scripts/bloat-o-meter shows chg +235.95%.
> > > > > >
> > > > > > So maybe the common helper is better regarding the kernel modul=
e size?
> > > > >
> > > > > Yes, let's get a framework done in a useful way.
> > > > > I'd say, drop the _INIT_DELAY_CMD. msleep() and usleep_range() sh=
ould be
> > > > > used instead (and it's up to the developer to select correct dela=
y
> > > > > function).
> > > > >
> > > > > >
> > > > > > > > +
> > > > > > > > +static const struct panel_init_cmd kingdisplay_kd101ne3_in=
it_cmd[] =3D {
> > > > > > > > +     _INIT_DELAY_CMD(50),
> > > > > > > > +     _INIT_DCS_CMD(0xE0, 0x00),
> > > > >
> > > > > [skipped the body of the table]
> > > > >
> > > > > > > > +     _INIT_DCS_CMD(0x0E, 0x48),
> > > > > > > > +
> > > > > > > > +     _INIT_DCS_CMD(0xE0, 0x00),
> > > > >
> > > > > > > > +     _INIT_DCS_CMD(0X11),
> > > > >
> > > > > Also, at least this is mipi_dsi_dcs_exit_sleep_mode().
> > > > >
> > > > > > > > +     /* T6: 120ms */
> > > > > > > > +     _INIT_DELAY_CMD(120),
> > > > > > > > +     _INIT_DCS_CMD(0X29),
> > > > >
> > > > > And this is mipi_dsi_dcs_set_display_on().
> > > > >
> > > > > Having a single table enourages people to put known commands into=
 the
> > > > > table, the practice that must be frowned upon and forbidden.
> > > > >
> > > > > We have functions for some of the standard DCS commands. So, mayb=
e
> > > > > instead of adding a single-table based approach we can improve
> > > > > mipi_dsi_generic_write_seq() to reduce the bloat. E.g. by moving =
the
> > > > > error handling to a common part of enable() / prepare() function.
> > > > >
> > > >
> > > > For this panel, I think it can also refer to how
> > > > panel-kingdisplay-kd097d04.c does. Create the table for init cmd da=
ta,
> > > > not what operation to use, and use mipi_dsi_generic_write_seq() whe=
n
> > > > looping through the table.
> > >
> > > Even more similar discussion:
> > >
> > > https://lore.kernel.org/r/CAD=3DFV=3DUGDbNvAMjzWSOvxybGikQcvW9JsRtbxH=
Vg8_97YPEQCA@mail.gmail.com
> >
> > It seems I skipped that thread.
> >
> > I'd still suggest a code-based solution compared to table-based one, fo=
r
> > the reasons I've outlined before. Having a tables puts a pressure on th=
e
> > developer to put commands there for which we already have a
> > command-specific function.
>
> The problem is that with these panels that need big init sequences the
> code based solution is _a lot_ bigger. If it were a few bytes or a
> 1-2KB then fine, but when Hsin-Yi measured Linus W's attempt to move
> from a table to code it was 100K difference in code [1]. I would also
> say that having these long init sequences done as separate commands
> encourages people to skip checking the return values of each of the
> transfer functions and I don't love that idea.
>
> It would be ideal if these panels didn't need these long init
> sequences, but I don't have any inside knowledge here saying that they
> could be removed. So assume we can't get rid of the init sequences it
> feels like we have to find some way to make the tables work for at
> least the large chunks of init code and encourage people to make the
> tables readable...
>
For the init sequence of the panel from this patch, using the table
approach, we can still use mipi_dsi_generic_write_seq() and not invent
new macro or make the code complicated.

>
> [1] https://lore.kernel.org/r/CAD=3DFV=3DUFa_AoJQvUT3BTiRs19WCA2xLVeQOU=
=3D+nYu_HaE0_c6Q@mail.gmail.com