linux-kernel - Re: [PATCH] swiotlb: sync buffer when mapping FROM

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Message-ID: <AM0PR0402MB3476A633E03202C26D15CB5698010@AM0PR0402MB3476.eurprd04.prod.outlook.com>
Date:   Thu, 23 May 2019 17:53:51 +0000
From:   Horia Geanta <horia.geanta@....com>
To:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>
CC:     Robin Murphy <robin.murphy@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: Re: [PATCH] swiotlb: sync buffer when mapping FROM_DEVICE

On 5/23/2019 7:43 PM, Christoph Hellwig wrote:
> On Thu, May 23, 2019 at 07:35:07AM +0200, Marek Szyprowski wrote:
>> Don't we have DMA_BIDIRECTIONAL for such case?
> 
> Not sure if it was intended for that case, but it definitively should
> do the right thing for swiotlb, and it should also do the right thing
> in terms of cache maintainance.
> 
>> Maybe we should update 
>> documentation a bit to point that DMA_FROM_DEVICE expects the whole 
>> buffer to be filled by the device?
> 
> Probably. Horia, can you try to use DMA_BIDIRECTIONAL?
> 
This works, but at the cost of performance - all the cache lines being written
back to memory, just to be overwritten by the device.

Thanks,
Horia