linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: poll cmdq until it has space

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Message-ID: <c1c10203-ffd3-25f9-f2c6-9cee3458aac9@huawei.com>
Date:   Tue, 21 Sep 2021 17:22:48 +0100
From:   John Garry <john.garry@...wei.com>
To:     Fernand Sieber <sieberf@...zon.com>, <will@...nel.org>,
        <robin.murphy@....com>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu/arm-smmu-v3: poll cmdq until it has space

On 21/09/2021 12:43, Fernand Sieber wrote:
>   	do {

I didn't follow the full logic of this change yet ...

>   		llq->val = READ_ONCE(cmdq->q.llq.val);
> -		if (!queue_full(llq))
> +		if (!queue_has_space(llq, n))

But is the polarity really correct? That is, if we don't have space, 
then exit with success (the function to check for space).

>   			break;
> 
> +		/*
> +		 * We must return here even if there's no space, because the producer
> +		 * having moved forward could mean that the last thread observing the
> +		 * SMMU progress has allocated space in the cmdq and moved on, leaving
> +		 * us in this waiting loop with no other thread updating
> +		 * llq->state->val.

what is llq->state->val?

> +		 */
> +		if (llq->prod != prod)
> +			return -EAGAIN;
> +
>   		ret = queue_poll(&qp);

Thanks,
John