linux-kernel - Re: [PATCH 6/8] nvmet: pci-epf: Use dmaengine_prep_slave_single

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Message-ID: <20251210225455.GA3544539@bhelgaas>
Date: Wed, 10 Dec 2025 16:54:55 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Frank Li <Frank.Li@....com>
Cc: Vinod Koul <vkoul@...nel.org>, Manivannan Sadhasivam <mani@...nel.org>,
	Krzysztof Wilczyński <kwilczynski@...nel.org>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Christoph Hellwig <hch@....de>,
	Sagi Grimberg <sagi@...mberg.me>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Nicolas Ferre <nicolas.ferre@...rochip.com>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	Claudiu Beznea <claudiu.beznea@...on.dev>,
	Koichiro Den <den@...inux.co.jp>, Niklas Cassel <cassel@...nel.org>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org,
	mhi@...ts.linux.dev, linux-arm-msm@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	imx@...ts.linux.dev
Subject: Re: [PATCH 6/8] nvmet: pci-epf: Use
 dmaengine_prep_slave_single_config() API

On Mon, Dec 08, 2025 at 12:09:45PM -0500, Frank Li wrote:
> Use the new dmaengine_prep_slave_single_config() API to combine the
> configuration and descriptor preparation into a single call.
> 
> Since dmaengine_prep_slave_single_config() performs the configuration
> and preparation atomically, the mutex can be removed.

> @@ -386,22 +386,16 @@ static int nvmet_pci_epf_dma_transfer(struct nvmet_pci_epf *nvme_epf,
>  		return -EINVAL;
>  	}
>  
> -	mutex_lock(lock);
> -
>  	dma_dev = dmaengine_get_dma_device(chan);
>  	dma_addr = dma_map_single(dma_dev, seg->buf, seg->length, dir);
>  	ret = dma_mapping_error(dma_dev, dma_addr);
>  	if (ret)
> -		goto unlock;
> -
> -	ret = dmaengine_slave_config(chan, &sconf);
> -	if (ret) {
> -		dev_err(dev, "Failed to configure DMA channel\n");
> -		goto unmap;
> -	}
> +		return ret;
>  
> -	desc = dmaengine_prep_slave_single(chan, dma_addr, seg->length,
> -					   sconf.direction, DMA_CTRL_ACK);
> +	desc = dmaengine_prep_slave_single_config(chan, dma_addr, seg->length,
> +						  sconf.direction,
> +						  DMA_CTRL_ACK,
> +						  &sconf);
>  	if (!desc) {
>  		dev_err(dev, "Failed to prepare DMA\n");
>  		ret = -EIO;
> @@ -423,9 +417,6 @@ static int nvmet_pci_epf_dma_transfer(struct nvmet_pci_epf *nvme_epf,
>  unmap:
>  	dma_unmap_single(dma_dev, dma_addr, seg->length, dir);
>  
> -unlock:
> -	mutex_unlock(lock);
> -

I don't know the dmaengine code, but it's not obvious to me what makes
dmaengine_prep_slave_single_config() itself atomic, since it doesn't
contain any locking.

Bjorn