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 17:22:46 -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 14:09, Tomi Valkeinen wrote:
> On 22/03/2024 18:18, Sean Anderson wrote:
>> 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.
> 
> I like it. We can't use IRQF_ONESHOT, as that would keep the irq masked while the threaded handler is being ran. I don't think that's a problem, but just something to keep in mind that both handlers can run concurrently.

Actually, I'm not sure we can do it like this. Imagine we have something
like

CPU 0                      CPU 1
zynqmp_dp_thread_handler()
  atomic_xchg()                      
			   __handle_irq_event_percpu 
                             zynqmp_dp_irq_handler()
                               atomic_or()
                               return IRQ_WAIT_THREAD
                             __irq_wake_thread()
                               test_and_set_bit(IRQTF_RUNTHREAD, ...)
                               return
  return IRQ_HANDLED

and whoops we now have bits set in dp->status but the thread isn't
running. I don't think there's a way to fix this without locking (or two
works). TBH I am leaning towards just having two works; it is a clean
implementation. We can also convert to use work_struct instead of
delayed_work, since we never set a delay.

--Sean