From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?ISO-8859-1?Q?Christian_K=F6nig?= Subject: Re: [RFC PATCH v1 08/16] drm/radeon: use common fence implementation for fences Date: Thu, 15 May 2014 11:42:21 +0200 Message-ID: <53748BFD.1050608@vodafone.de> References: <20140514145134.21163.32350.stgit@patser> <20140514145809.21163.64947.stgit@patser> <53738BCC.2070809@vodafone.de> <5374131D.4010906@canonical.com> <53748702.6070606@vodafone.de> <53748AFA.8010109@canonical.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1"; Format="flowed" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <53748AFA.8010109-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: nouveau-bounces-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org Sender: "Nouveau" To: Maarten Lankhorst , airlied-cv59FeDIM0c@public.gmane.org Cc: nouveau-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org List-Id: dri-devel@lists.freedesktop.org Am 15.05.2014 11:38, schrieb Maarten Lankhorst: > op 15-05-14 11:21, Christian K=F6nig schreef: >> Am 15.05.2014 03:06, schrieb Maarten Lankhorst: >>> op 14-05-14 17:29, Christian K=F6nig schreef: >>>>> + /* did fence get signaled after we enabled the sw irq? */ >>>>> + if = >>>>> (atomic64_read(&fence->rdev->fence_drv[fence->ring].last_seq) >=3D = >>>>> fence->seq) { >>>>> + radeon_irq_kms_sw_irq_put(fence->rdev, fence->ring); >>>>> + return false; >>>>> + } >>>>> + >>>>> + fence->fence_wake.flags =3D 0; >>>>> + fence->fence_wake.private =3D NULL; >>>>> + fence->fence_wake.func =3D radeon_fence_check_signaled; >>>>> + __add_wait_queue(&fence->rdev->fence_queue, &fence->fence_wake); >>>>> + fence_get(f); >>>> That looks like a race condition to me. The fence needs to be added = >>>> to the wait queue before the check, not after. >>>> >>>> Apart from that the whole approach looks like a really bad idea to = >>>> me. How for example is lockup detection supposed to happen with this? = >>> It's not a race condition because fence_queue.lock is held when this = >>> function is called. >> Ah, I see. That's also the reason why you moved the wake_up_all out = >> of the processing function. > Correct. :-) >>> Lockup's a bit of a weird problem, the changes wouldn't allow core = >>> ttm code to handle the lockup any more, >>> but any driver specific wait code would still handle this. I did = >>> this by design, because in future patches the wait >>> function may be called from outside of the radeon driver. The = >>> official wait function takes a timeout parameter, >>> so lockups wouldn't be fatal if the timeout is set to something like = >>> 30*HZ for example, it would still return >>> and report that the function timed out. >> Timeouts help with the detection of the lockup, but not at all with = >> the handling of them. >> >> What we essentially need is a wait callback into the driver that is = >> called in non atomic context without any locks held. >> >> This way we can block for the fence to become signaled with a timeout = >> and can then also initiate the reset handling if necessary. >> >> The way you designed the interface now means that the driver never = >> gets a chance to wait for the hardware to become idle and so never = >> has the opportunity to the reset the whole thing. > You could set up a hangcheck timer like intel does, and end up with a = > reliable hangcheck detection that doesn't depend on cpu waits. :-) Or = > override the default wait function and restore the old behavior. Overriding the default wait function sounds better, please implement it = this way. Thanks, Christian. > > ~Maarten >