linux-kernel - Re: [PATCH v2 5/8] drm: zynqmp_dp: Don't retrain the link in our IRQ

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Date: Fri, 22 Mar 2024 12:18:57 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Michal Simek <michal.simek@....com>, David Airlie <airlied@...il.com>,
 linux-kernel@...r.kernel.org, Daniel Vetter <daniel@...ll.ch>,
 linux-arm-kernel@...ts.infradead.org,
 Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 5/8] drm: zynqmp_dp: Don't retrain the link in our IRQ

On 3/22/24 01:32, Tomi Valkeinen wrote:
> On 21/03/2024 21:17, Sean Anderson wrote:
>> On 3/21/24 15:08, Tomi Valkeinen wrote:
>>> On 21/03/2024 20:01, Sean Anderson wrote:
>>>> On 3/21/24 13:25, Tomi Valkeinen wrote:
>>>>> On 21/03/2024 17:52, Sean Anderson wrote:
>>>>>> On 3/20/24 02:53, Tomi Valkeinen wrote:
>>>>>>> On 20/03/2024 00:51, Sean Anderson wrote:
>>>>>>> Do we need to handle interrupts while either delayed work is being done?
>>>>>>
>>>>>> Probably not.
>>>>>>
>>>>>>> If we do need a delayed work, would just one work be enough which
>>>>>>> handles both HPD_EVENT and HPD_IRQ, instead of two?
>>>>>>
>>>>>> Maybe, but then we need to determine which pending events we need to
>>>>>> handle. I think since we have only two events it will be easier to just
>>>>>> have separate workqueues.
>>>>>
>>>>> The less concurrency, the better...Which is why it would be nice to do it all in the threaded irq.
>>>>
>>>> Yeah, but we can use a mutex for this which means there is not too much
>>>> interesting going on.
>>>
>>> Ok. Yep, if we get (hopefully) a single mutex with clearly defined fields that it protects, I'm ok with workqueues.
>>>
>>> I'd still prefer just one workqueue, though...
>>
>> Yeah, but then we need a spinlock or something to tell the workqueue what it should do.
> 
> Yep. We could also always look at the HPD (if we drop the big sleeps) in the wq, and have a flag for the HPD IRQ, which would reduce the state to a single bit.

How about something like

zynqmp_dp_irq_handler(...)
{
	/* Read status and handle underflow/overflow/vblank */

	status &= ZYNQMP_DP_INT_HPD_EVENT | ZYNQMP_DP_INT_HPD_IRQ;
	if (status) {
		atomic_or(status, &dp->status);
		return IRQ_WAKE_THREAD;
	}

	return IRQ_HANDLED;
}

zynqmp_dp_thread_handler(...)
{
	status = atomic_xchg(&dp->status, 0);
	/* process HPD stuff */
}

which gets rid of the workqueue too.

--Sean