Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE
To:     Christoph Hellwig <hch@lst.de>
Cc:     =?UTF-8?Q?Horia_Geant=c4=83?= <horia.geanta@nxp.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-imx@nxp.com
References: <20190522072018.10660-1-horia.geanta@nxp.com>
 <20190522123243.GA26390@lst.de>
 <6cbe5470-16a6-17e9-337d-6ba18b16b6e8@arm.com>
 <20190522130921.GA26874@lst.de>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <fdfd7318-7999-1fe6-01b6-ae1fb7ba8c30@arm.com>
Date:   Wed, 22 May 2019 14:25:38 +0100
User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:60.0) Gecko/20100101
 Thunderbird/60.6.1
MIME-Version: 1.0
In-Reply-To: <20190522130921.GA26874@lst.de>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-GB
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 2019-05-22 2:09 pm, Christoph Hellwig wrote:
> On Wed, May 22, 2019 at 01:50:47PM +0100, Robin Murphy wrote:
>> Would that work out any different from the existing DMA_ATTR_SKIP_CPU_SYNC?
>>
>> If drivers are prepared to handle this issue from their end, they can
>> already do so for single mappings by using that attr along with explicit
>> partial syncs via dma_sync_single(). For page/sg mappings we'd still have
>> the problem of identifying what part of "partial" actually matters, and
>> probably having to add some additional new sync operations to cope.
> 
> Except that the same optimization we are tripping over here is also
> present in dma_sync_* - dma_sync_*_to_device with DMA_FROM_DEVICE is a
> no-op in swiotlb.

Sure, but that should be irrelevant since the effective problem here is 
in the sync_*_for_cpu direction, and it's the unmap which nobbles the 
buffer. If the driver does this:

	dma_map_single(whole buffer);
	<device writes to part of buffer>
	dma_unmap_single(whole buffer);
	<contents of rest of buffer now undefined>

then it could instead do this and be happy:

	dma_map_single(whole buffer, SKIP_CPU_SYNC);
	<device writes to part of buffer>
	dma_sync_single_for_cpu(updated part of buffer);
	dma_unmap_single(whole buffer, SKIP_CPU_SYNC);
	<contents of rest of buffer still valid>

Robin.