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From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Date: Thu, 25 Apr 2024 01:50:52 +0300
Message-ID: <CAA8EJprLvEt_pt4XzACQG7pU8KYagKbQ71xwtRF9KrLygTT3ow@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] drm/panel: kd101ne3: add new panel driver
To: Hsin-Yi Wang <hsinyi@google.com>
Cc: Doug Anderson <dianders@google.com>, 
	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 Thu, 25 Apr 2024 at 01:15, Hsin-Yi Wang <hsinyi@google.com> wrote:
>
> On Wed, Apr 24, 2024 at 2:49=E2=80=AFPM Dmitry Baryshkov
> <dmitry.baryshkov@linaro.org> wrote:
> >
> > On Thu, 25 Apr 2024 at 00:04, 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@goo=
gle.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. Ca=
n you use
> > > > > > > > > mipi_dsi_generic_write_seq() instead of the huge table? O=
r if you prefer
> > > > > > > > > the table, we should extract this framework to a common h=
elper.
> > > > > > > > > (my preference is shifted towards mipi_dsi_generic_write_=
seq()).
> > > > > > > > >
> > > > > > > > The drawback of mipi_dsi_generic_write_seq() is that it can=
 cause the
> > > > > > > > kernel size grows a lot since every sequence will be expand=
ed.
> > > > > > > >
> > > > > > > > Similar discussion in here:
> > > > > > > > https://lore.kernel.org/dri-devel/CAD=3DFV=3DWju3WS45=3DEpX=
MUg7FjYDh3-=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 m=
odule size?
> > > > > > >
> > > > > > > Yes, let's get a framework done in a useful way.
> > > > > > > I'd say, drop the _INIT_DELAY_CMD. msleep() and usleep_range(=
) should be
> > > > > > > used instead (and it's up to the developer to select correct =
delay
> > > > > > > function).
> > > > > > >
> > > > > > > >
> > > > > > > > > > +
> > > > > > > > > > +static const struct panel_init_cmd kingdisplay_kd101ne=
3_init_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, =
maybe
> > > > > > > instead of adding a single-table based approach we can improv=
e
> > > > > > > mipi_dsi_generic_write_seq() to reduce the bloat. E.g. by mov=
ing the
> > > > > > > error handling to a common part of enable() / prepare() funct=
ion.
> > > > > > >
> > > > > >
> > > > > > For this panel, I think it can also refer to how
> > > > > > panel-kingdisplay-kd097d04.c does. Create the table for init cm=
d data,
> > > > > > not what operation to use, and use mipi_dsi_generic_write_seq()=
 when
> > > > > > looping through the table.
> > > > >
> > > > > Even more similar discussion:
> > > > >
> > > > > https://lore.kernel.org/r/CAD=3DFV=3DUGDbNvAMjzWSOvxybGikQcvW9JsR=
tbxHVg8_97YPEQCA@mail.gmail.com
> > > >
> > > > It seems I skipped that thread.
> > > >
> > > > I'd still suggest a code-based solution compared to table-based one=
, for
> > > > the reasons I've outlined before. Having a tables puts a pressure o=
n the
> > > > 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 th=
e
> > > 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 the=
y
> > > 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...
> >
> >
> > I did a quick check on the boe-tv101wum-nl6 driver by converting the
> > writes to use the following macro:
> >
> > #define mipi_dsi_dcs_write_cmd_seq(dsi, cmd, seq...)
> >              \
> >         do {                                                           =
        \
> >                 static const u8 d[] =3D { cmd, seq };                  =
      \
> >                 ret =3D mipi_dsi_dcs_write_buffer(dsi, d, ARRAY_SIZE(d)=
);    \
> >                 if (ret < 0)                                           =
        \
> >                         goto err;                                      =
        \
> >         } while (0)
> >
> > And then at the end of the init funciton having
> >
> > err:
> >         dev_err(panel->dev,
> >                 "failed to write command %d\n", ret);
> >         return ret;
> > }
> >
>
> I'm not sure about the coding style rule here, would it be considered
> unclear that caller of mipi_dsi_dcs_write_cmd_seq() needs to have err
> block, but the block may not be directly used in that caller and is
> only jumped from the macro?

I'm also not sure here. It was a quick and dirty test.
We might as well do something like

ret =3D mipi_dsi_dcs_write_cmd_seq(dsi, ...);
if (ret)
    goto err;

all over the place.


>
>
> > Size comparison:
> >    text    data     bss     dec     hex filename
> > before
> >   34109   10410      18   44537    adf9
> > ../build-64/drivers/gpu/drm/panel/panel-boe-tv101wum-nl6.o
> > making init data const
> >   44359     184       0   44543    adff
> > ../build-64/drivers/gpu/drm/panel/panel-boe-tv101wum-nl6.o
> > with new macros
> >   44353     184       0   44537    adf9
> > ../build-64/drivers/gpu/drm/panel/panel-boe-tv101wum-nl6.o
> >
> > As you can see, there is literally no size difference with this macro i=
n place.
> > The only drawback is that the init stops on the first write rather
> > than going through the sequence.
> >
> > WDYT? I can turn this into a proper patch if you think this makes sense=
.
> >
> > >
> > >
> > > [1] https://lore.kernel.org/r/CAD=3DFV=3DUFa_AoJQvUT3BTiRs19WCA2xLVeQ=
OU=3D+nYu_HaE0_c6Q@mail.gmail.com
> >
> >
> >
> > --
> > With best wishes
> > Dmitry


--
With best wishes
Dmitry