linux-kernel - Re: [PATCH 3/3] drm/panel: introduce ebbg,ft8719 panel

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Date:   Thu, 19 May 2022 11:09:41 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Joel Selvaraj <jo@...amily.in>
Cc:     devicetree@...r.kernel.org, Hao Fang <fanghao11@...wei.com>,
        David Airlie <airlied@...ux.ie>,
        Shawn Guo <shawnguo@...nel.org>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Rob Herring <robh+dt@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Corentin Labbe <clabbe@...libre.com>,
        phone-devel@...r.kernel.org, Sam Ravnborg <sam@...nborg.org>,
        Stanislav Jakubek <stano.jakubek@...il.com>,
        ~postmarketos/upstreaming@...ts.sr.ht
Subject: Re: [PATCH 3/3] drm/panel: introduce ebbg,ft8719 panel

On Mon, May 16, 2022 at 2:56 PM Joel Selvaraj <jo@...amily.in> wrote:
> On 13/05/22 03:21, Linus Walleij wrote:
> > On Fri, May 6, 2022 at 2:18 PM Joel Selvaraj <jo@...amily.in> wrote:
> >> +#define dsi_dcs_write_seq(dsi, seq...) do {                            \
> >> +               static const u8 d[] = { seq };                          \
> >> +               int ret;                                                \
> >> +               ret = mipi_dsi_dcs_write_buffer(dsi, d, ARRAY_SIZE(d)); \
> >> +               if (ret < 0)                                            \
> >> +                       return ret;                                     \
> >> +       } while (0)
> >
> > First I don't see what the do {} while (0) buys you, just use
> > a basic block {}.
>
> The do {} while (0) in macro ensures there is a semicolon when it's
> used. With normal blocking, it would have issues with if conditions?
> I read about them here: https://stackoverflow.com/a/2381339

Hm that seems true, it enforces the semicolon ; at the end of the
statement which is nice. I suppose we should fix the other macro
as well.

Noralf added this ({}) form in 02dd95fe31693, so maybe he wants
to chip in.

> > Second look at mipi_dbi_command() in include/drm/drm_mipi_dbi.h
> > this is very similar.
>
> Does the ({..}) style blocking used in mipi_dbi_command help workaround
> the semicolon issue I mentioned above?

Nope. But add the rate limited error print please!

> > It's dubious that you always have dsi_dcs_write_seq()
> > followed by dsi_generic_write_seq().
> >
> > That means mipi_dsi_generic_write() followed by
> > mipi_dsi_dcs_write_buffer(). But if you look at these
> > commands in drivers/gpu/drm/drm_mipi_dsi.c
> > you see that they do the same thing!
>
> They almost do the same thing except for the msg.type values? Mostly the
> msg.type value is used to just check whether it's a long or short write
> in the msm dsi_host code. However, in mipi_dsi_create_packet function,
> the msg->type value is used to calculate packet->header[0] as follows:
>
> packet->header[0] = ((msg->channel & 0x3) << 6) | (msg->type & 0x3f);
>
> Wouldn't the difference between the mipi_dsi_dcs_write_buffer's and
> mipi_dsi_generic_write's msg.type values cause issue here?
>
> I tried using mipi_dsi_dcs_write_buffer for all commands and the panel
> worked fine, but I am not sure if it's correct to do so?

I think it's fine? The only issue would be if there is a DSI host controller
that only supports short writes, and in that case it should emulate
long writes by breaking long messages apart. (My amateur view at least.)

> > Lots of magic numbers. You don't have a datasheet do you?
> > So you could #define some of the magic?
>
> Unfortunately, I don't have a datasheet and the power on sequence is
> taken from downstream android dts. It works pretty well though. So I
> don't think I can #define any of these magic.

If you know which display controller the display is using (usually
Novatek nnnnn, Ilitek nnnn etc someting like that) there is often
a datasheet for the display controller available but the display per
se often obscures the display controller.

>  > Doesn't it work to combine them into one call for each
>  > pair?
>  >> +       dsi_dcs_write_seq(dsi, );
>  >> +       dsi_generic_write_seq(dsi, 0xff, 0x87, 0x19);
>
> By using a macro? We can... but I am not sure what (0x00, 0x80), (0x00,
> 0xa0),etc type of commands signify without the datasheet, so I am not
> sure what to name them in the macro and make any sensible meaning out of it.

I meant just sending dsi_generic_write_seq() with everything in
it:

dsi_generic_write_seq(dsi, 0x00, 0x80, 0xff, 0x87, 0x19);

Instead of two writes. Doesn't this work?

Yours,
Linus Walleij