linux-kernel - Re: [PATCH] drm/bridge: ps8640: Add double reset T4 and T5 to power-on sequence

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Date:   Wed, 17 Aug 2022 15:54:35 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Hsin-Yi Wang <hsinyi@...omium.org>
Cc:     Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        rock.chiu@...adetech.corp-partner.google.com
Subject: Re: [PATCH] drm/bridge: ps8640: Add double reset T4 and T5 to
 power-on sequence

Hi,

On Mon, Aug 15, 2022 at 2:39 AM Hsin-Yi Wang <hsinyi@...omium.org> wrote:
>
> The double reset power-on sequence is a workaround for the hardware
> flaw in some chip that SPI Clock output glitch and cause internal MPU
> unable to read firmware correctly. The sequence is suggested in ps8640
> application note.
>
> Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> ---
>  drivers/gpu/drm/bridge/parade-ps8640.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> index 49107a6cdac18..d7483c13c569b 100644
> --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> @@ -375,6 +375,11 @@ static int __maybe_unused ps8640_resume(struct device *dev)
>         gpiod_set_value(ps_bridge->gpio_reset, 1);
>         usleep_range(2000, 2500);
>         gpiod_set_value(ps_bridge->gpio_reset, 0);
> +       /* Double reset for T4 and T5 */
> +       msleep(50);
> +       gpiod_set_value(ps_bridge->gpio_reset, 1);
> +       msleep(50);
> +       gpiod_set_value(ps_bridge->gpio_reset, 0);

We really need another 100 ms here? ps8640 is already quite slow at
powering itself up and that has a real user impact. Why was it only
2.5 ms for the first reset and 50 ms for the second?

-Doug