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Sat, 23 Mar 2024 09:54:29 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1711184069; bh=kcPY+uAYhpR11SyqWUdaVLpYzTaSDlEU7e/g5QTEhqA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Cn8GFEYHJEqI2jPkHtG9YZnYFWD9F5zv1+n/8hzXO3VbE2H3S1i6Mz9VX4pStrDl0 jlANx7jWO6cSjKS7qQXyI/yaH2cGzVLOJOo2Y2moReY04YjLTlJNC4ZNwGzJE9+xbX TtVSEwoJyshSZgroEWQg4wHbPJrVnXY69IYfiFtc= Message-ID: Date: Sat, 23 Mar 2024 10:54:55 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 5/8] drm: zynqmp_dp: Don't retrain the link in our IRQ Content-Language: en-US To: Sean Anderson Cc: Michal Simek , David Airlie , linux-kernel@vger.kernel.org, Daniel Vetter , linux-arm-kernel@lists.infradead.org, Laurent Pinchart , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , dri-devel@lists.freedesktop.org References: <20240319225122.3048400-1-sean.anderson@linux.dev> <20240319225122.3048400-6-sean.anderson@linux.dev> <53b2df23-d5ea-498b-a501-b64f753c0074@linux.dev> <0514ef71-5baa-4989-9b7d-8bd9526c4d8d@ideasonboard.com> <16ccf678-270c-4770-8cc9-f676b4fabf09@linux.dev> <1f27ce69-9ea6-4df4-9147-332d74febdf0@ideasonboard.com> <2dbf138f-5112-48e1-85a6-9e3ad84ec4a6@linux.dev> <305a8e43-4d65-490c-9f83-afce6490bc83@ideasonboard.com> <7a4c332b-a044-4c82-a5b2-cb4b318f5110@linux.dev> From: Tomi Valkeinen Autocrypt: addr=tomi.valkeinen@ideasonboard.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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