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

From: Sean Anderson <sean.anderson@linux.dev>
To: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Cc: Michal Simek <michal.simek@amd.com>,
	David Airlie <airlied@gmail.com>,
	linux-kernel@vger.kernel.org, Daniel Vetter <daniel@ffwll.ch>,
	linux-arm-kernel@lists.infradead.org,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v2 5/8] drm: zynqmp_dp: Don't retrain the link in our IRQ
Date: Fri, 22 Mar 2024 12:18:57 -0400	[thread overview]
Message-ID: <2dbf138f-5112-48e1-85a6-9e3ad84ec4a6@linux.dev> (raw)
In-Reply-To: <d6a8bc5c-aed9-4ef4-adb2-dc171106b44b@ideasonboard.com>

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

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