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

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Date:   Mon, 16 May 2022 18:26:38 +0530
From:   Joel Selvaraj <jo@...amily.in>
To:     Linus Walleij <linus.walleij@...aro.org>
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

Hi Linus Walleij,

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

> 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?

> So this utility macro should be in a generic file such as
> include/drm/drm_mipi_dsi.h. (Can be added in a separate
> patch.)

I agree. Could be done in another patch.

> Third I think you need only one macro (see below).
> 
>> +static int ebbg_ft8719_on(struct ebbg_ft8719 *ctx)
>> +{
>> +       struct mipi_dsi_device *dsi = ctx->dsi;
>> +       struct device *dev = &dsi->dev;
>> +       int ret;
>> +
>> +       dsi->mode_flags |= MIPI_DSI_MODE_LPM;
>> +
>> +       dsi_dcs_write_seq(dsi, 0x00, 0x00);
>> +       dsi_generic_write_seq(dsi, 0xff, 0x87, 0x19, 0x01);
> 
> 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?

> 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.

 > Doesn't it work to combine them into one call for each
 > pair?
 >> +       dsi_dcs_write_seq(dsi, 0x00, 0x80);
 >> +       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.

> 
>> +       if (ctx->prepared)
>> +               return 0;
> (...)
>> +       ctx->prepared = true;
>> +       return 0;
> (...)
>> +       if (!ctx->prepared)
>> +               return 0;
> (...)
>> +       ctx->prepared = false;
>> +       return 0;
> 
> Drop this state variable it is a reimplementation of something
> that the core will track for you.

ok. Will drop them in the next version.

> The rest looks nice!

Thanks for your review!

> Yours,
> Linus Walleij

Best Regards,
Joel Selvaraj