linux-kernel - Re: [PATCH RFC v2 2/2] ufs: core: delegate the interrupt service routine to a threaded irq handler

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Message-ID: <4a5efc8e-ec61-40c8-9b36-59e185b0fdd5@acm.org>
Date: Thu, 27 Mar 2025 07:56:15 -0400
From: Bart Van Assche <bvanassche@....org>
To: Neil Armstrong <neil.armstrong@...aro.org>,
 Alim Akhtar <alim.akhtar@...sung.com>, Avri Altman <avri.altman@....com>,
 "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
 "Martin K. Petersen" <martin.petersen@...cle.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
 linux-arm-msm@...r.kernel.org, linux-scsi@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 2/2] ufs: core: delegate the interrupt service
 routine to a threaded irq handler

On 3/26/25 4:36 AM, Neil Armstrong wrote:
 > When MCQ & Interrupt Aggregation are supported, the interrupt
 > are directly handled in the "hard" interrupt routine to
 > keep IOPs high since queues handling is done in separate
 > per-queue interrupt routines.

The above explanation suggests that I/O completions are handled by the
modified interrupt handler. This is not necessarily the case. With MCQ,
I/O completions are either handled by dedicated interrupts or by the
legacy interrupt handler.

> Reported bandwidth is not affected on various tests.

This kind of patch can only affect command completion latency but not
the bandwidth, isn't it?

> +/**
> + * ufshcd_intr - Main interrupt service routine
> + * @irq: irq number
> + * @__hba: pointer to adapter instance
> + *
> + * Return:
> + *  IRQ_HANDLED     - If interrupt is valid
> + *  IRQ_WAKE_THREAD - If handling is moved to threaded handled
> + *  IRQ_NONE        - If invalid interrupt
> + */
> +static irqreturn_t ufshcd_intr(int irq, void *__hba)
> +{
> +	struct ufs_hba *hba = __hba;
> +
> +	/*
> +	 * Move interrupt handling to thread when MCQ is not supported
> +	 * or when Interrupt Aggregation is not supported, leading to
> +	 * potentially longer interrupt handling.
> +	 */
> +	if (!is_mcq_supported(hba) || !ufshcd_is_intr_aggr_allowed(hba))
> +		return IRQ_WAKE_THREAD;
> +
> +	/* Directly handle interrupts since MCQ handlers does the hard job */
> +	return ufshcd_sl_intr(hba, ufshcd_readl(hba, REG_INTERRUPT_STATUS) &
> +				   ufshcd_readl(hba, REG_INTERRUPT_ENABLE));
> +}

Where has ufshcd_is_intr_aggr_allowed() been defined? I can't find this
function.

For the MCQ case, this patch removes the loop from around
ufshcd_sl_intr() without explaining in the patch description why this 
change has been made. Please explain all changes in the patch
description.

Thanks,

Bart.