netdev - Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api dma_addr_to_phys

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Date:   Tue, 22 Oct 2019 13:53:14 +0000
From:   Laurentiu Tudor <laurentiu.tudor@....com>
To:     Robin Murphy <robin.murphy@....com>, "hch@....de" <hch@....de>,
        "joro@...tes.org" <joro@...tes.org>,
        Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Ioana Ciornei <ioana.ciornei@....com>
CC:     Leo Li <leoyang.li@....com>,
        Diana Madalina Craciun <diana.craciun@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Madalin-cristian Bucur <madalin.bucur@....com>
Subject: Re: [RFC PATCH 1/3] dma-mapping: introduce a new dma api
 dma_addr_to_phys_addr()


On 22.10.2019 16:25, Robin Murphy wrote:
> On 22/10/2019 13:55, Laurentiu Tudor wrote:
>> From: Laurentiu Tudor <laurentiu.tudor@....com>
>>
>> Introduce a new dma map op called dma_addr_to_phys_addr() that converts
>> a dma address to the physical address backing it up and add wrapper for
>> it.
> 
> I'd really love it if there was a name which could encapsulate that this 
> is *only* for extreme special cases of constrained descriptors/pagetable 
> entries/etc. where there's simply no practical way to keep track of a 
> CPU address alongside the DMA address, and the only option is this 
> potentially-arbitrarily-complex operation (I mean, on some systems it 
> may end up taking locks and poking hardware).
> 
> Either way it's tricky - much as I don't like adding an interface which 
> is ripe for drivers to misuse, I also really don't want hacks like 
> bdf95923086f shoved into other APIs to compensate, so on balance I'd 
> probably consider this proposal ever so slightly the lesser evil.
> 
>> Signed-off-by: Laurentiu Tudor <laurentiu.tudor@....com>
>> ---
>>   include/linux/dma-mapping.h | 21 +++++++++++++++++++++
>>   1 file changed, 21 insertions(+)
>>
>> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
>> index 4a1c4fca475a..5965d159c9a9 100644
>> --- a/include/linux/dma-mapping.h
>> +++ b/include/linux/dma-mapping.h
>> @@ -132,6 +132,8 @@ struct dma_map_ops {
>>       u64 (*get_required_mask)(struct device *dev);
>>       size_t (*max_mapping_size)(struct device *dev);
>>       unsigned long (*get_merge_boundary)(struct device *dev);
>> +    phys_addr_t (*dma_addr_to_phys_addr)(struct device *dev,
>> +                         dma_addr_t dma_handle);
> 
> I'd be inclined to name the internal callback something a bit snappier 
> like .get_phys_addr.

Alright. Want me to also rename the wrapper to something like 
dma_get_phys_addr()? Sounds a bit nicer to me.

>>   };
>>   #define DMA_MAPPING_ERROR        (~(dma_addr_t)0)
>> @@ -442,6 +444,19 @@ static inline int dma_mapping_error(struct device 
>> *dev, dma_addr_t dma_addr)
>>       return 0;
>>   }
>> +static inline phys_addr_t dma_addr_to_phys_addr(struct device *dev,
>> +                        dma_addr_t dma_handle)
>> +{
>> +    const struct dma_map_ops *ops = get_dma_ops(dev);
>> +
>> +    if (dma_is_direct(ops))
>> +        return (phys_addr_t)dma_handle;
> 
> Well that's not right, is it - remember why you had that namespace 
> collision? ;)
> 

Ugh, correct. Don't know what I was thinking. Will rework the check.

---
Thanks & Best Regards, Laurentiu