linux-kernel - Re: [PATCH] drm: xlnx: dp: Reset DisplayPort IP

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Date: Wed, 28 Feb 2024 18:22:24 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Rohit Visavalia <rohit.visavalia@....com>
Cc: gregkh@...uxfoundation.org, maarten.lankhorst@...ux.intel.com,
	mripard@...nel.org, tzimmermann@...e.de, airlied@...il.com,
	daniel@...ll.ch, michal.simek@....com,
	dri-devel@...ts.freedesktop.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: xlnx: dp: Reset DisplayPort IP

Hello Rohit,

Thank you for the patch.

On Fri, Feb 16, 2024 at 04:40:43AM -0800, Rohit Visavalia wrote:
> Assert DisplayPort reset signal before deasserting,
> it is to clear out any registers programmed before booting kernel.
> 
> Signed-off-by: Rohit Visavalia <rohit.visavalia@....com>
> ---
>  drivers/gpu/drm/xlnx/zynqmp_dp.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xlnx/zynqmp_dp.c b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> index 1846c4971fd8..5a40aa1d4283 100644
> --- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
> +++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> @@ -1714,6 +1714,10 @@ int zynqmp_dp_probe(struct zynqmp_dpsub *dpsub)
>  		goto err_free;
>  	}
>  
> +	ret = zynqmp_dp_reset(dp, true);
> +	if (ret < 0)
> +		return ret;
> +

This looks fine to me, so

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

However, looking at zynqmp_dp_reset(), we have

static int zynqmp_dp_reset(struct zynqmp_dp *dp, bool assert)
{
	unsigned long timeout;

	if (assert)
		reset_control_assert(dp->reset);
	else
		reset_control_deassert(dp->reset);

	/* Wait for the (de)assert to complete. */
	timeout = jiffies + msecs_to_jiffies(RST_TIMEOUT_MS);
	while (!time_after_eq(jiffies, timeout)) {
		bool status = !!reset_control_status(dp->reset);

		if (assert == status)
			return 0;

		cpu_relax();
	}

	dev_err(dp->dev, "reset %s timeout\n", assert ? "assert" : "deassert");
	return -ETIMEDOUT;
}

That doesn't seem quite right. Aren't reset_control_assert() and
reset_control_deassert() supposed to be synchronous ? If so there should
be no need to wait, and if there's a need to wait, it could be a bug in
the reset controller driver. This should be fixed, and then the code in
probe could be replaced with a call to reset_control_reset().

>  	ret = zynqmp_dp_reset(dp, false);
>  	if (ret < 0)
>  		goto err_free;

-- 
Regards,

Laurent Pinchart