linux-kernel - Re: [PATCH v10 2/4] drm/panthor: Extend IRQ helpers for mask modification/restoration

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Message-ID: <20260116144158.4a3fcdcc@fedora>
Date: Fri, 16 Jan 2026 14:41:58 +0100
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
Cc: Steven Price <steven.price@....com>, Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, Chia-I Wu
 <olvaffe@...il.com>, Karunika Choo <karunika.choo@....com>,
 kernel@...labora.com, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v10 2/4] drm/panthor: Extend IRQ helpers for mask
 modification/restoration

On Fri, 16 Jan 2026 13:57:31 +0100
Nicolas Frattaroli <nicolas.frattaroli@...labora.com> wrote:

> The current IRQ helpers do not guarantee mutual exclusion that covers
> the entire transaction from accessing the mask member and modifying the
> mask register.
> 
> This makes it hard, if not impossible, to implement mask modification
> helpers that may change one of these outside the normal
> suspend/resume/isr code paths.
> 
> Add a spinlock to struct panthor_irq that protects both the mask member
> and register. Acquire it in all code paths that access these, but drop
> it before processing the threaded handler function. Then, add the
> aforementioned new helpers: enable_events, and disable_events. They work
> by ORing and NANDing the mask bits.
> 
> resume is changed to no longer have a mask passed, as pirq->mask is
> supposed to be the user-requested mask now, rather than a mirror of the
> INT_MASK register contents. Users of the resume helper are adjusted
> accordingly, including a rather painful refactor in panthor_mmu.c.
> 
> In panthor_mmu.c, the bespoke mask modification is excised, and replaced
> with enable_events/disable_events in as_enable/as_disable.
> 
> Co-developed-by: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Boris Brezillon <boris.brezillon@...labora.com>
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@...labora.com>

Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>

Just one question below.

> ---
>  drivers/gpu/drm/panthor/panthor_device.h | 86 ++++++++++++++++++++++++++------
>  drivers/gpu/drm/panthor/panthor_fw.c     |  3 +-
>  drivers/gpu/drm/panthor/panthor_gpu.c    |  2 +-
>  drivers/gpu/drm/panthor/panthor_mmu.c    | 47 ++++++++---------
>  drivers/gpu/drm/panthor/panthor_pwr.c    |  2 +-
>  5 files changed, 98 insertions(+), 42 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_device.h b/drivers/gpu/drm/panthor/panthor_device.h
> index 8597b388cc40..8664adb1febf 100644
> --- a/drivers/gpu/drm/panthor/panthor_device.h
> +++ b/drivers/gpu/drm/panthor/panthor_device.h
> @@ -84,9 +84,19 @@ struct panthor_irq {
>  	/** @irq: IRQ number. */
>  	int irq;
>  
> -	/** @mask: Current mask being applied to xxx_INT_MASK. */
> +	/** @mask: Values to write to xxx_INT_MASK if active. */
>  	u32 mask;
>  
> +	/**
> +	 * @mask_lock: protects modifications to _INT_MASK and @mask.
> +	 *
> +	 * In paths where _INT_MASK is updated based on a state
> +	 * transition/check, it's crucial for the state update/check to be
> +	 * inside the locked section, otherwise it introduces a race window
> +	 * leading to potential _INT_MASK inconsistencies.
> +	 */
> +	spinlock_t mask_lock;
> +
>  	/** @state: one of &enum panthor_irq_state reflecting the current state. */
>  	atomic_t state;
>  };
> @@ -425,13 +435,14 @@ static irqreturn_t panthor_ ## __name ## _irq_raw_handler(int irq, void *data)
>  	if (!gpu_read(ptdev, __reg_prefix ## _INT_STAT))					\
>  		return IRQ_NONE;								\
>  												\
> +	guard(spinlock_irqsave)(&pirq->mask_lock);						\
> +	gpu_write(ptdev, __reg_prefix ## _INT_MASK, 0);						\
>  	old_state = atomic_cmpxchg(&pirq->state,						\
>  				   PANTHOR_IRQ_STATE_ACTIVE,					\
>  				   PANTHOR_IRQ_STATE_PROCESSING);				\
>  	if (old_state != PANTHOR_IRQ_STATE_ACTIVE)						\
>  		return IRQ_NONE;								\
>  												\
> -	gpu_write(ptdev, __reg_prefix ## _INT_MASK, 0);						\

Is moving this INT_MASK=0 before the atomic_cmpxchg() is really
required. It's harmless of course, because of the lock surrounding the
state + INT_MASK update, but I was wondering if there was another
reason for doing that that I'm missing.

>  	return IRQ_WAKE_THREAD;									\
>  }												\