linux-kernel - Re: [PATCH v2 1/2] drm/msm/dsi: Remove spurious IRQF

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Message-ID: <346cb67b-a5bb-b4ad-a15d-d2cfc850ccf5@linaro.org>
Date:   Thu, 10 Feb 2022 01:03:11 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] drm/msm/dsi: Remove spurious IRQF_ONESHOT flag

On 01/02/2022 20:47, Daniel Thompson wrote:
> Quoting the header comments, IRQF_ONESHOT is "Used by threaded interrupts
> which need to keep the irq line disabled until the threaded handler has
> been run.". When applied to an interrupt that doesn't request a threaded
> irq then IRQF_ONESHOT has a lesser known (undocumented?) side effect,
> which it to disable the forced threading of irqs (and for "normal" kernels
> it is a nop). In this case I can find no evidence that suppressing forced
> threading is intentional. Had it been intentional then a driver must adopt
> the raw_spinlock API in order to avoid deadlocks on PREEMPT_RT kernels
> (and avoid calling any kernel API that uses regular spinlocks).
> 
> Fix this by removing the spurious additional flag.
> 
> This change is required for my Snapdragon 7cx Gen2 tablet to boot-to-GUI
> with PREEMPT_RT enabled.
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

> ---
>   drivers/gpu/drm/msm/dsi/dsi_host.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
> index 6b3ced4aaaf5d..3a3f53f0c8ae1 100644
> --- a/drivers/gpu/drm/msm/dsi/dsi_host.c
> +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
> @@ -1877,7 +1877,7 @@ int msm_dsi_host_init(struct msm_dsi *msm_dsi)
>   
>   	/* do not autoenable, will be enabled later */
>   	ret = devm_request_irq(&pdev->dev, msm_host->irq, dsi_host_irq,
> -			IRQF_TRIGGER_HIGH | IRQF_ONESHOT | IRQF_NO_AUTOEN,
> +			IRQF_TRIGGER_HIGH | IRQF_NO_AUTOEN,
>   			"dsi_isr", msm_host);
>   	if (ret < 0) {
>   		dev_err(&pdev->dev, "failed to request IRQ%u: %d\n",


-- 
With best wishes
Dmitry