linux-kernel - Re: [PATCH 3/6] drm/gpu/xe: Ignore dma_fenc

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Message-ID: <enlnwawk3qtcbgfm7dmuyefb4tx3hxsrmzozkabpeixxsi3y24@pukuuth2zlke>
Date: Thu, 27 Nov 2025 18:19:27 +0100
From: Andi Shyti <andi.shyti@...nel.org>
To: phasta@...nel.org
Cc: Matthew Brost <matthew.brost@...el.com>, 
	Sumit Semwal <sumit.semwal@...aro.org>, Gustavo Padovan <gustavo@...ovan.org>, 
	Christian König <christian.koenig@....com>, Felix Kuehling <Felix.Kuehling@....com>, 
	Alex Deucher <alexander.deucher@....com>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Jani Nikula <jani.nikula@...ux.intel.com>, 
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>, 
	Tvrtko Ursulin <tursulin@...ulin.net>, Huang Rui <ray.huang@....com>, 
	Matthew Auld <matthew.auld@...el.com>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, 
	Lucas De Marchi <lucas.demarchi@...el.com>, 
	Thomas Hellström <thomas.hellstrom@...ux.intel.com>, linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org, 
	intel-gfx@...ts.freedesktop.org, intel-xe@...ts.freedesktop.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 3/6] drm/gpu/xe: Ignore dma_fenc_signal() return code

On Thu, Nov 27, 2025 at 02:51:39PM +0100, Philipp Stanner wrote:
> On Thu, 2025-11-27 at 14:37 +0100, Andi Shyti wrote:
> > On Wed, Nov 26, 2025 at 03:56:32PM -0800, Matthew Brost wrote:
> > > On Wed, Nov 26, 2025 at 11:56:57PM +0100, Andi Shyti wrote:
> > > > > @@ -85,7 +85,6 @@ void xe_hw_fence_irq_finish(struct xe_hw_fence_irq *irq)
> > > > > @@ -93,9 +92,9 @@ void xe_hw_fence_irq_finish(struct xe_hw_fence_irq *irq)
> > > > >  		spin_lock_irqsave(&irq->lock, flags);
> > > > >  		list_for_each_entry_safe(fence, next, &irq->pending, irq_link) {
> > > > >  			list_del_init(&fence->irq_link);
> > > > > -			err = dma_fence_signal_locked(&fence->dma);
> > > > 
> > > > why don't we do
> > > > 
> > > > XE_WARN_ON(dma_fence_signal_locked(..))
> 
> because it's impossible because the series is about removing the return
> codes from the dma_fence_signal_* functions.

oh yes, the last patch. Sorry, I went on reviewing and lost the
final target from sight.

> > > > 
> > > 
> > > IIRC the above statement can compile out. So the patch looks correct to me.
> > 
> > you have defined XE_WARN_ON as WARN_ON that should always
> > evaluate the content and, depending on the configuration, it
> > prints the logs or not.
> > 
> > What I don't like from this patch is that we end up checking
> > twice for the DMA_FENCE_FLAG_SIGNALED_BIT bit.
> 
> Depends on what you mean by "we". The Xe code checks it only once, with
> dma_fence_test_signaled_flag(). The dma_fence backend checks it yet
> again, as it always does, to avoid signaling a signaled fence.
> 
> That's not racy here, however, because the fence lock is already being
> held, as evidenced by the current usage of dma_fence_signal_locked().

I haven't said it's racy, I just didn't like that we are testing
for the DMA_FENCE_FLAG_SIGNALED_BIT twice. On the other hand,
with dma_fence_signal_locked() being void, I wouldn't know how to
do it better. So that I guess it's fine.

Andi