linux-kernel - Re: [PATCH 2/2] drm/panel: Add Samsung S6E3FA2 DSI panel driver

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Date:   Mon, 26 Jul 2021 10:04:28 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Alexey Minnekhanov <alexeymin@...tmarketos.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Sam Ravnborg <sam@...nborg.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS
        <devicetree@...r.kernel.org>, Hans de Goede <hdegoede@...hat.com>, Andy
        Shevchenko <andy.shevchenko@...il.com>," 
        <~postmarketos/upstreaming@...ts.sr.ht>,
        phone-devel@...r.kernel.org
Subject: Re: [PATCH 2/2] drm/panel: Add Samsung S6E3FA2 DSI panel driver

Hi Alexey,

I had some gmail problems and replied to the very old driver
by Iskren, sorry for the mess.

I overall like this driver a lot. Some of Sam's comments could
be addressed especially for backlight.

I think the driver should indeed handle both the physical
displays like you do here.

On Sun, Jul 25, 2021 at 4:05 PM Alexey Minnekhanov
<alexeymin@...tmarketos.org> wrote:

> Samsung S6E3FA2 panel is amoled 1080x1920 command mode DSI
> panel used in Samsung Galaxy S5 phone. There are 2 known
> variations of panel that were shipped in this phone, and
> this driver handles both of them.
>
> Panel has built-in backlight (like all other AMOLED panels),
> controlled over DSI by some vendor specific commands, some
> of them include sending long byte sequences of what seems
> to be called "smart dimming".
>
> Signed-off-by: Alexey Minnekhanov <alexeymin@...tmarketos.org>

(...)

> +#define dsi_generic_write_seq(dsi, seq...) do {                                \
> +               static const u8 d[] = { seq };                          \
> +               int ret;                                                \
> +               ret = mipi_dsi_generic_write(dsi, d, ARRAY_SIZE(d));    \
> +               if (ret < 0)                                            \
> +                       return ret;                                     \
> +       } while (0)
> +
> +#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)

These look generic as pointed out in other mail.

> +static int s6e3fa2_dsi_dcs_read1(struct mipi_dsi_device *dsi, const u8 cmd,
> +                               u8 *data)
> +{
> +       int ret;
> +
> +       ret = mipi_dsi_dcs_read(dsi, cmd, data, 1);
> +       if (ret < 0) {
> +               dev_err(&dsi->dev, "could not read DCS CMD %02x\n", cmd);
> +               return ret;
> +       }
> +       return 0;
> +}

I don't think this needs a wrapper, just call mipi_dsi_dcs_read() directly.

> +/* Panel variants */
> +#define LCD_ID_S6E3FA2         0x602813
> +#define LCD_ID_EA8064G         0x622872

Interesting use of the "vendor" byte by Samsung here. It seems they are
repurposing the non-standard MTP bytes as they seem fit.

> +/*
> + * Which AID sequence to use for each candela level.
> + * This lookup table is same for both panels.
> + */
> +static const u8 map_candela_to_aid[S6E3FA2_NUM_GAMMA_LEVELS] = {
> +        0,  2,  3,  4,  6,  7,  8, 10, 11, 13, 14, 15,
> +       16, 17, 18, 20, 21, 22, 23, 24, 25, 26, 27, 28,
> +       29, 30, 31, 32, 33, 34, 35, 36, 36, 36, 36, 36,
> +       36, 36, 36, 36, 36, 36, 36, 36, 36, 36, 37, 38,
> +       39, 40, 41, 42, 43, 44, 44, 44, 44, 44, 44, 44
> +};

This and other things hints that we are dealing with the same display
controller.

> +/* Other panel drivers call these commands test_key_enable/disable */
> +static const u8 seq_s6e3fa2_test_key_en[6] = {
> +       0xf0, 0x5a, 0x5a,
> +       0xfc, 0x5a, 0x5a
> +};

0xf0 and 0xfc is obviously some "level 2 unlock" commands.
Maybe #define them as pointed out in other comments.

> +static const u8 seq_s6e3fa2_test_key_dis[6] = {
> +       0xf0, 0xa5, 0xa5,
> +       0xfc, 0xa5, 0xa5
> +};
> +static const u8 seq_ea8064g_test_key_en[6] = {
> +       0xf0, 0x5a, 0x5a,
> +       0xf1, 0x5a, 0x5a
> +};
> +static const u8 seq_ea8064g_test_key_dis[6] = {
> +       0xf1, 0xa5, 0xa5,
> +       0xf0, 0xa5, 0xa5
> +};

The use of two different registers for locking is suspicious, that
may point to different display controllers. :/

This is an icky panel, but it seems they are close enough to
be handled by the same driver IMO.

Yours,
Linus Walleij