Re: [PATCH] drm/sun4i: tcon: Support keeping dclk rate upon ancestor clock changes

From: Jernej Škrabec
Date: Wed Mar 13 2024 - 15:03:18 EST

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In reply to: Frank Oltmanns: "Re: [PATCH] drm/sun4i: tcon: Support keeping dclk rate upon ancestor clock changes"
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Hi Frank!

Thanks on tackling this issue.

Dne nedelja, 10. marec 2024 ob 14:32:29 CET je Frank Oltmanns napisal(a):
> Allow the dclk to reset its rate when a rate change is initiated from an
> ancestor clock. This makes it possible to no longer to get an exclusive
> lock. As a consequence, it is now possible to set new rates if
> necessary, e.g. when an external display is connected.
>
> The first user of this functionality is the A64 because PLL-VIDEO0 is an
> ancestor for both HDMI and TCON0. This allows to select an optimal rate
> for TCON0 as long as there is no external HDMI connection. Once a change
> in PLL-VIDEO0 is performed when an HDMI connection is established, TCON0
> can react gracefully and select an optimal rate based on this the new
> constraint.
>
> Signed-off-by: Frank Oltmanns <frank@xxxxxxxxxxxx>
> ---
> I would like to make the Allwinner A64's data-clock keep its rate
> when its ancestor's (pll-video0) rate changes. Keeping data-clock's rate
> is required, to let the A64 drive both an LCD and HDMI display at the
> same time, because both have pll-video0 as an ancestor.
>
> TCONs that use this flag store the ideal rate for their data-clock and
> subscribe to be notified when data-clock changes. When rate setting has
> finished (indicated by a POST_RATE_CHANGE event) the call back function
> schedules delayed work to set the data-clock's rate to the initial value
> after 100 ms. Using delayed work maks sure that the clock setting is
> finished.
>
> I've implemented this functionality as a quirk, so that it is possible
> to use it only for the A64.
>
> This patch supersedes [1].
>
> This work is inspired by an out-of-tree patchset [2] [3] [4].
> Unfortunately, the patchset uses clk_set_rate() directly in a notifier
> callback, which the following comment on clk_notifier_register()
> forbids: "The callbacks associated with the notifier must not re-enter
> into the clk framework by calling any top-level clk APIs." [5]
> Furthermore, that out-of-tree patchset no longer works since 6.6,
> because setting pll-mipi is now also resetting pll-video0 and therefore
> causes a race condition.
>
> Thank you for considering this contribution,
> Frank
>
> [1] https://lore.kernel.org/lkml/20230825-pll-mipi_keep_rate-v1-0-35bc43570730@xxxxxxxxxxxx/
> [2] https://codeberg.org/megi/linux/commit/a37cda2fff41a67a2bacf82b1594e10335d0bd8a
> [3] https://codeberg.org/megi/linux/commit/24dc09128d2c8efc6ddf19249420e9798e967a46
> [4] https://codeberg.org/megi/linux/commit/728a93d46f99f0eb231ed6fa8971a45f97c7182c
> [5] https://elixir.bootlin.com/linux/v6.7.9/source/drivers/clk/clk.c#L4669
> ---
> drivers/gpu/drm/sun4i/sun4i_tcon.c | 70 ++++++++++++++++++++++++++++++++++----
> drivers/gpu/drm/sun4i/sun4i_tcon.h | 12 +++++++
> 2 files changed, 76 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.c b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> index a1a2c845ade0..b880bd44049a 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_tcon.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_tcon.c
> @@ -108,9 +108,11 @@ static void sun4i_tcon_channel_set_status(struct sun4i_tcon *tcon, int channel,
>
> if (enabled) {
> clk_prepare_enable(clk);
> - clk_rate_exclusive_get(clk);
> + if (!tcon->quirks->restores_rate)
> + clk_rate_exclusive_get(clk);
> } else {
> - clk_rate_exclusive_put(clk);
> + if (!tcon->quirks->restores_rate)
> + clk_rate_exclusive_put(clk);
> clk_disable_unprepare(clk);
> }
> }
> @@ -343,6 +345,53 @@ static void sun4i_tcon0_mode_set_dithering(struct sun4i_tcon *tcon,
> regmap_write(tcon->regs, SUN4I_TCON_FRM_CTL_REG, val);
> }
>
> +static void sun4i_rate_reset_notifier_delayed_update(struct work_struct *work)
> +{
> + struct sun4i_rate_reset_nb *rate_reset = container_of(work, struct sun4i_rate_reset_nb,
> + reset_rate_work.work);
> +
> + clk_set_rate(rate_reset->target_clk, rate_reset->saved_rate);
> +}
> +
> +static int sun4i_rate_reset_notifier_cb(struct notifier_block *nb,
> + unsigned long event, void *data)
> +{
> + struct sun4i_rate_reset_nb *rate_reset = to_sun4i_rate_reset_nb(nb);
> +
> + if (event == POST_RATE_CHANGE)
> + schedule_delayed_work(&rate_reset->reset_rate_work, msecs_to_jiffies(100));

Do we need that delay though? Since clock is set exclusive on TV TCONs, then
it shouldn't be changed. Alternative, simpler variation would be something
like this:
https://elixir.bootlin.com/linux/v6.8/source/drivers/tty/serial/8250/8250_dw.c#L333

> +
> + return NOTIFY_DONE;
> +}
> +
> +static void sun4i_rate_reset_notifier_register(struct sun4i_rate_reset_nb *rate_reset_nb)
> +{
> + if (rate_reset_nb->is_registered)
> + return;
> +
> + rate_reset_nb->clk_nb.notifier_call = sun4i_rate_reset_notifier_cb;
> +
> + INIT_DELAYED_WORK(&rate_reset_nb->reset_rate_work,
> + sun4i_rate_reset_notifier_delayed_update);
> +
> + if (!clk_notifier_register(rate_reset_nb->target_clk,
> + &rate_reset_nb->clk_nb))
> + rate_reset_nb->is_registered = true;
> +}
> +
> +static struct sun4i_rate_reset_nb tcon_rate_reset_tcon0_nb;

Is there any specific reason for global variable? Note that R40 and T507 have
2 LCD and 2 TV TCONs. If it's ever used there, it won't fly. Please move to
TCON struct. You can drop a few fields for doing so.

> +
> +static void sun4i_tcon0_set_dclk_rate(struct sun4i_tcon *tcon, unsigned long rate)
> +{
> + clk_set_rate(tcon->dclk, rate);
> +
> + if (tcon->quirks->restores_rate) {
> + tcon_rate_reset_tcon0_nb.target_clk = tcon->dclk;
> + tcon_rate_reset_tcon0_nb.saved_rate = rate;
> + sun4i_rate_reset_notifier_register(&tcon_rate_reset_tcon0_nb);

Can't be registration done at TCON init time?

Best regards,
Jernej

> + }
> +}
> +
> static void sun4i_tcon0_mode_set_cpu(struct sun4i_tcon *tcon,
> const struct drm_encoder *encoder,
> const struct drm_display_mode *mode)
> @@ -360,8 +409,8 @@ static void sun4i_tcon0_mode_set_cpu(struct sun4i_tcon *tcon,
> */
> tcon->dclk_min_div = SUN6I_DSI_TCON_DIV;
> tcon->dclk_max_div = SUN6I_DSI_TCON_DIV;
> - clk_set_rate(tcon->dclk, mode->crtc_clock * 1000 * (bpp / lanes)
> - / SUN6I_DSI_TCON_DIV);
> + sun4i_tcon0_set_dclk_rate(tcon, mode->crtc_clock * 1000 * (bpp / lanes)
> + / SUN6I_DSI_TCON_DIV);
>
> /* Set the resolution */
> regmap_write(tcon->regs, SUN4I_TCON0_BASIC0_REG,
> @@ -434,7 +483,7 @@ static void sun4i_tcon0_mode_set_lvds(struct sun4i_tcon *tcon,
>
> tcon->dclk_min_div = 7;
> tcon->dclk_max_div = 7;
> - clk_set_rate(tcon->dclk, mode->crtc_clock * 1000);
> + sun4i_tcon0_set_dclk_rate(tcon, mode->crtc_clock * 1000);
>
> /* Set the resolution */
> regmap_write(tcon->regs, SUN4I_TCON0_BASIC0_REG,
> @@ -516,7 +565,7 @@ static void sun4i_tcon0_mode_set_rgb(struct sun4i_tcon *tcon,
>
> tcon->dclk_min_div = tcon->quirks->dclk_min_div;
> tcon->dclk_max_div = 127;
> - clk_set_rate(tcon->dclk, mode->crtc_clock * 1000);
> + sun4i_tcon0_set_dclk_rate(tcon, mode->crtc_clock * 1000);
>
> /* Set the resolution */
> regmap_write(tcon->regs, SUN4I_TCON0_BASIC0_REG,
> @@ -1503,6 +1552,14 @@ static const struct sun4i_tcon_quirks sun8i_a33_quirks = {
> .supports_lvds = true,
> };
>
> +static const struct sun4i_tcon_quirks sun50i_a64_lcd_quirks = {
> + .supports_lvds = true,
> + .has_channel_0 = true,
> + .restores_rate = true,
> + .dclk_min_div = 1,
> + .setup_lvds_phy = sun6i_tcon_setup_lvds_phy,
> +};
> +
> static const struct sun4i_tcon_quirks sun8i_a83t_lcd_quirks = {
> .supports_lvds = true,
> .has_channel_0 = true,
> @@ -1561,6 +1618,7 @@ const struct of_device_id sun4i_tcon_of_table[] = {
> { .compatible = "allwinner,sun9i-a80-tcon-tv", .data = &sun9i_a80_tcon_tv_quirks },
> { .compatible = "allwinner,sun20i-d1-tcon-lcd", .data = &sun20i_d1_lcd_quirks },
> { .compatible = "allwinner,sun20i-d1-tcon-tv", .data = &sun8i_r40_tv_quirks },
> + { .compatible = "allwinner,sun50i-a64-tcon-lcd", .data = &sun50i_a64_lcd_quirks },
> { }
> };
> MODULE_DEVICE_TABLE(of, sun4i_tcon_of_table);
> diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.h b/drivers/gpu/drm/sun4i/sun4i_tcon.h
> index fa23aa23fe4a..bd4abc90062b 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_tcon.h
> +++ b/drivers/gpu/drm/sun4i/sun4i_tcon.h
> @@ -243,6 +243,7 @@ struct sun4i_tcon_quirks {
> bool needs_edp_reset; /* a80 edp reset needed for tcon0 access */
> bool supports_lvds; /* Does the TCON support an LVDS output? */
> bool polarity_in_ch0; /* some tcon1 channels have polarity bits in tcon0 pol register */
> + bool restores_rate; /* restores the initial rate when rate changes */
> u8 dclk_min_div; /* minimum divider for TCON0 DCLK */
>
> /* callback to handle tcon muxing options */
> @@ -300,4 +301,15 @@ void sun4i_tcon_set_status(struct sun4i_tcon *crtc,
>
> extern const struct of_device_id sun4i_tcon_of_table[];
>
> +struct sun4i_rate_reset_nb {
> + struct notifier_block clk_nb;
> + struct delayed_work reset_rate_work;
> +
> + struct clk *target_clk;
> + unsigned long saved_rate;
> + bool is_registered;
> +};
> +
> +#define to_sun4i_rate_reset_nb(_nb) container_of(_nb, struct sun4i_rate_reset_nb, clk_nb)
> +
> #endif /* __SUN4I_TCON_H__ */
>
> ---
> base-commit: dcb6c8ee6acc6c347caec1e73fb900c0f4ff9806
> change-id: 20240304-tcon_keep_stable_rate-5729c7706343
>
> Best regards,
>

Next message: Kees Cook: "[PATCH] ubsan: Disable signed integer overflow sanitizer on GCC < 8"
Previous message: Dan Carpenter: "[PATCH] staging: vc04_services: fix information leak in create_component()"
In reply to: Frank Oltmanns: "Re: [PATCH] drm/sun4i: tcon: Support keeping dclk rate upon ancestor clock changes"
Next in thread: Frank Oltmanns: "Re: [PATCH] drm/sun4i: tcon: Support keeping dclk rate upon ancestor clock changes"
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