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

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Date:   Mon, 11 Oct 2021 15:04:05 +0800
From:   Cai Huoqing <caihuoqing@...du.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     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] scsi: ibmvscsi_tgt: Use dma_alloc_coherent() instead of
 get_zeroed_page/dma_map_single()

On 11 10月 21 07:42:28, Christoph Hellwig wrote:
> On Mon, Oct 11, 2021 at 12:00:53AM +0800, Cai Huoqing wrote:
> > Replacing get_zeroed_page/free_page/dma_map_single/dma_unmap_single()
> > with dma_alloc_coherent/dma_free_coherent() helps to reduce
> > code size, and simplify the code, and coherent DMA will not
> > clear the cache every time.
> 
> This explanation does not make any sense whatsoever.  Please explain
> why it would an show numbers.
Hi Christoph,
thanks for your feedback.

In this case, just simplify the code.
10 insertions(+), 31 deletions(-)

dma_sync_ API is not called, I think the hardware may keep cache coherent
directly or is a no cache system. No need to make perfermance compare.

thanks
Cai