linux-kernel - Re: [PATCH v2] scsi: ibmvscsi_tgt: Use dma_alloc_noncoherent() instead of get_zeroed_page/dma_map

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Date:   Tue, 12 Oct 2021 06:20:32 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Cai Huoqing <caihuoqing@...du.com>
Cc:     hch@...radead.org, Michael Cyr <mikecyr@...ux.ibm.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-scsi@...r.kernel.org, target-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] scsi: ibmvscsi_tgt: Use dma_alloc_noncoherent()
 instead of get_zeroed_page/dma_map_single()

On Tue, Oct 12, 2021 at 11:21:09AM +0800, Cai Huoqing wrote:
> Replacing get_zeroed_page/free_page/dma_map_single/dma_unmap_single()
> with dma_alloc_noncoherent/dma_free_noncoherent() helps to reduce
> code size, and simplify the code, and the hardware keep DMA coherent
> itself.

It might also be worth to mention that this also avoids potential
bounce buffering.  Although for pseries vio devices this probably can't
happen anyway.

> +	vscsi->cmd_q.base_addr = dma_alloc_noncoherent(&vdev->dev, PAGE_SIZE,
> +						       &vscsi->cmd_q.crq_token,
> +						       DMA_BIDIRECTIONAL, GFP_KERNEL);

Please avoid the overly long line.

The same comments also apply to the other patch.