Message-ID: <CACb=7PXKTkpPA-cSwwoa3a2H5-iR7uNCWH9X0DHaFWmZd001Ow@mail.gmail.com>
Date: Wed, 24 Apr 2024 14:18:33 -0700
From: Hsin-Yi Wang <hsinyi@...gle.com>
To: Doug Anderson <dianders@...gle.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, 
	lvzhaoxiong <lvzhaoxiong@...qin.corp-partner.google.com>, mripard@...nel.org, 
	airlied@...il.com, daniel@...ll.ch, robh@...nel.org, 
	krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org, 
	dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	cong yang <yangcong5@...qin.corp-partner.google.com>
Subject: Re: [PATCH v1 2/2] drm/panel: kd101ne3: add new panel driver

On Wed, Apr 24, 2024 at 2:05 PM Doug Anderson <dianders@...gle.com> wrote:
>
> Hi,
>
> On Tue, Apr 23, 2024 at 2:20 PM Dmitry Baryshkov
> <dmitry.baryshkov@...aro.org> wrote:
> >
> > On Tue, Apr 23, 2024 at 01:41:59PM -0700, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Tue, Apr 23, 2024 at 11:10 AM Hsin-Yi Wang <hsinyi@...gle.com> wrote:
> > > >
> > > > > > > > +#define _INIT_DCS_CMD(...) { \
> > > > > > > > +     .type = INIT_DCS_CMD, \
> > > > > > > > +     .len = sizeof((char[]){__VA_ARGS__}), \
> > > > > > > > +     .data = (char[]){__VA_ARGS__} }
> > > > > > > > +
> > > > > > > > +#define _INIT_DELAY_CMD(...) { \
> > > > > > > > +     .type = DELAY_CMD,\
> > > > > > > > +     .len = sizeof((char[]){__VA_ARGS__}), \
> > > > > > > > +     .data = (char[]){__VA_ARGS__} }
> > > > > > >
> > > > > > > This is the third panel driver using the same appoach. Can you 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 helper.
> > > > > > > (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 expanded.
> > > > > >
> > > > > > Similar discussion in here:
> > > > > > https://lore.kernel.org/dri-devel/CAD=FV=Wju3WS45=EpXMUg7FjYDh3-=mvm_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 module 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_kd101ne3_init_cmd[] = {
> > > > > > > > +     _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 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 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=FV=UGDbNvAMjzWSOvxybGikQcvW9JsRtbxHVg8_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 on 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 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=FV=UFa_AoJQvUT3BTiRs19WCA2xLVeQOU=+nYu_HaE0_c6Q@mail.gmail.com

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