linux-kernel - Re: [PATCH 02/20] dma-mapping: provide a generic dma-noncoherent implementation

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Date:   Fri, 18 May 2018 10:28:09 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     "hch@....de" <hch@....de>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>,
        "monstr@...str.eu" <monstr@...str.eu>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linux-c6x-dev@...ux-c6x.org" <linux-c6x-dev@...ux-c6x.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
        "linux-hexagon@...r.kernel.org" <linux-hexagon@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>,
        "openrisc@...ts.librecores.org" <openrisc@...ts.librecores.org>,
        "green.hu@...il.com" <green.hu@...il.com>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
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        "deanbo422@...il.com" <deanbo422@...il.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 02/20] dma-mapping: provide a generic dma-noncoherent
 implementation

On 05/18/2018 06:23 AM, hch@....de wrote:
>   Fri, May 18, 2018 at 01:03:46PM +0000, Alexey Brodkin wrote:
>> Note mmc_get_dma_dir() is just "data->flags & MMC_DATA_WRITE ? DMA_TO_DEVICE : DMA_FROM_DEVICE".
>> I.e. if we're preparing for sending data dma_noncoherent_map_sg() will have DMA_TO_DEVICE which
>> is quite OK for passing to dma_noncoherent_sync_sg_for_device() but in case of reading we'll have
>> DMA_FROM_DEVICE which we'll pass to dma_noncoherent_sync_sg_for_device() in dma_noncoherent_map_sg().
>>
>> I'd say this is not entirely correct because IMHO arch_sync_dma_for_cpu() is supposed to only be used
>> in case of DMA_FROM_DEVICE and arch_sync_dma_for_device() only in case of DMA_TO_DEVICE.
> arc overrides the dir paramter of the dma_sync_single_for_device/
> dma_sync_single_for_cpu calls.  My patches dropped that, and I have
> restored that, and audit for the other architectures is pending.

Right, for now lets retain that and do a sweeping audit of @direction - to me it 
seems extraneous (as it did 10 years ago), but I'm not an expert in this are so 
perhaps it is needed for some device / arches and it would be good to understand 
that finally.

> That being said the existing arc code still looks rather odd as it
> didn't do the same thing for the scatterlist versions of the calls.
> I've thrown in a few patches into my new tree to make the sg versions
> make the normal calls, and to clean up the area a bit.

Not calling names or anything here, but it doesn't exist for sg variants, because 
I didn't write that code :-)
It was introduced by your commi:

2016-01-20 052c96dbe33b arc: convert to dma_map_ops