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

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Date: Sat, 23 Mar 2024 10:54:55 +0200
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Sean Anderson <sean.anderson@...ux.dev>
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 22/03/2024 23:22, Sean Anderson wrote:
> 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

In your example above, the IRQTF_RUNTHREAD has been cleared by the 
threaded-irq before calling zynqmp_dp_thread_handler. So the hard-irq 
will set that flag before the zynqmp_dp_thread_handler() returns.

When zynqmp_dp_thread_handler() returns, the execution will go to 
irq_wait_for_interrupt(). That function will notice the IRQTF_RUNTHREAD 
flag (and clear it), and run the zynqmp_dp_thread_handler() again.

So if I'm not mistaken, when the hard-irq function returns 
IRQ_WAKE_THREAD, it's always guaranteed that a "fresh" run of the 
threaded handler will be ran.

I think that makes sense, as I'm not sure how threaded handlers without 
IRQF_ONESHOT could be used if that were not the case. I hope I'm right 
in my analysis =).

  Tomi