Re: [PATCH 05/16] dma-mapping: Introduce dma_map_sg_p2pdma()

[Logan Gunthorpe]

On 2021-05-02 3:23 p.m., John Hubbard wrote:
> On 4/8/21 10:01 AM, Logan Gunthorpe wrote:
>> dma_map_sg() either returns a positive number indicating the number
>> of entries mapped or zero indicating that resources were not available
>> to create the mapping. When zero is returned, it is always safe to retry
>> the mapping later once resources have been freed.
>>
>> Once P2PDMA pages are mixed into the SGL there may be pages that may
>> never be successfully mapped with a given device because that device may
>> not actually be able to access those pages. Thus, multiple error
>> conditions will need to be distinguished to determine weather a retry
>> is safe.
>>
>> Introduce dma_map_sg_p2pdma[_attrs]() with a different calling
>> convention from dma_map_sg(). The function will return a positive
>> integer on success or a negative errno on failure.
>>
>> ENOMEM will be used to indicate a resource failure and EREMOTEIO to
>> indicate that a P2PDMA page is not mappable.
>>
>> The __DMA_ATTR_PCI_P2PDMA attribute is introduced to inform the lower
>> level implementations that P2PDMA pages are allowed and to warn if a
>> caller introduces them into the regular dma_map_sg() interface.
>>
>> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
>> ---
>>   include/linux/dma-mapping.h | 15 +++++++++++
>>   kernel/dma/mapping.c        | 52 ++++++++++++++++++++++++++++++++-----
>>   2 files changed, 61 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
>> index 2a984cb4d1e0..50b8f586cf59 100644
>> --- a/include/linux/dma-mapping.h
>> +++ b/include/linux/dma-mapping.h
>> @@ -60,6 +60,12 @@
>>    * at least read-only at lesser-privileged levels).
>>    */
>>   #define DMA_ATTR_PRIVILEGED		(1UL << 9)
>> +/*
>> + * __DMA_ATTR_PCI_P2PDMA: This should not be used directly, use
>> + * dma_map_sg_p2pdma() instead. Used internally to indicate that the
>> + * caller is using the dma_map_sg_p2pdma() interface.
>> + */
>> +#define __DMA_ATTR_PCI_P2PDMA		(1UL << 10)
>>
> 
> As mentioned near the top of this file,
> Documentation/core-api/dma-attributes.rst also needs to be updated, for
> this new item.

As this attribute is not meant to be used by anyone outside the dma
functions, I don't think it should be documented here. (That's why it
has a double underscource prefix).

>>   /*
>>    * A dma_addr_t can hold any valid DMA or bus address for the platform.  It can
>> @@ -107,6 +113,8 @@ void dma_unmap_page_attrs(struct device *dev, dma_addr_t addr, size_t size,
>>   		enum dma_data_direction dir, unsigned long attrs);
>>   int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
>>   		enum dma_data_direction dir, unsigned long attrs);
>> +int dma_map_sg_p2pdma_attrs(struct device *dev, struct scatterlist *sg,
>> +		int nents, enum dma_data_direction dir, unsigned long attrs);
>>   void dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sg,
>>   				      int nents, enum dma_data_direction dir,
>>   				      unsigned long attrs);
>> @@ -160,6 +168,12 @@ static inline int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>>   {
>>   	return 0;
>>   }
>> +static inline int dma_map_sg_p2pdma_attrs(struct device *dev,
>> +		struct scatterlist *sg, int nents, enum dma_data_direction dir,
>> +		unsigned long attrs)
>> +{
>> +	return 0;
>> +}
>>   static inline void dma_unmap_sg_attrs(struct device *dev,
>>   		struct scatterlist *sg, int nents, enum dma_data_direction dir,
>>   		unsigned long attrs)
>> @@ -392,6 +406,7 @@ static inline void dma_sync_sgtable_for_device(struct device *dev,
>>   #define dma_map_single(d, a, s, r) dma_map_single_attrs(d, a, s, r, 0)
>>   #define dma_unmap_single(d, a, s, r) dma_unmap_single_attrs(d, a, s, r, 0)
>>   #define dma_map_sg(d, s, n, r) dma_map_sg_attrs(d, s, n, r, 0)
>> +#define dma_map_sg_p2pdma(d, s, n, r) dma_map_sg_p2pdma_attrs(d, s, n, r, 0)
> 
> This hunk is fine, of course.
> 
> But, about pre-existing issues: note to self, or to anyone: send a patch to turn
> these into inline functions. The macro redirection here is not adding value, but
> it does make things just a little bit worse.
> 
> 
>>   #define dma_unmap_sg(d, s, n, r) dma_unmap_sg_attrs(d, s, n, r, 0)
>>   #define dma_map_page(d, p, o, s, r) dma_map_page_attrs(d, p, o, s, r, 0)
>>   #define dma_unmap_page(d, a, s, r) dma_unmap_page_attrs(d, a, s, r, 0)
>> diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
>> index b6a633679933..923089c4267b 100644
>> --- a/kernel/dma/mapping.c
>> +++ b/kernel/dma/mapping.c
>> @@ -177,12 +177,8 @@ void dma_unmap_page_attrs(struct device *dev, dma_addr_t addr, size_t size,
>>   }
>>   EXPORT_SYMBOL(dma_unmap_page_attrs);
>>   
>> -/*
>> - * dma_maps_sg_attrs returns 0 on error and > 0 on success.
>> - * It should never return a value < 0.
>> - */
> 
> It would be better to leave the comment in place, given the non-standard
> return values. However, looking around here, it would be better if we go
> with the standard -ERRNO for error, and >0 for sucess.

The comment is actually left in place. The diff just makes it look like
it was removed. It is added back lower down in the diff.

> There are pre-existing BUG_ON() and WARN_ON_ONCE() items that are partly
> an attempt to compensate for not being able to return proper -ERRNO
> codes. For example, this:
> 
> 	    BUG_ON(!valid_dma_direction(dir));
> 
> ...arguably should be more like this:
> 
>          if(WARN_ON_ONCE(!valid_dma_direction(dir)))
>                  return -EINVAL;

Yes, but you'll have to see the discussion in the RFC. The complaint was
that the calling convention for dma_map_sg() is not expected to return
anything other than 0 or the number of entries mapped. It can't return a
negative error code. That's why BUG_ON(ents < 0) is in the existing
code. That's also why this series introduces the new dma_map_sg_p2pdma()
function. (Though, Jason has made some suggestions to further change this).

> 
>> -int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
>> -		enum dma_data_direction dir, unsigned long attrs)
>> +static int __dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>> +		int nents, enum dma_data_direction dir, unsigned long attrs)
>>   {
>>   	const struct dma_map_ops *ops = get_dma_ops(dev);
>>   	int ents;
>> @@ -197,6 +193,20 @@ int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
>>   		ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
>>   	else
>>   		ents = ops->map_sg(dev, sg, nents, dir, attrs);
>> +
>> +	return ents;
>> +}
>> +
>> +/*
>> + * dma_maps_sg_attrs returns 0 on error and > 0 on success.
>> + * It should never return a value < 0.
>> + */
>> +int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
>> +		enum dma_data_direction dir, unsigned long attrs)
>> +{
>> +	int ents;
> 
> Pre-existing note, feel free to ignore: the ents and nents in the same
> routines together, are way too close to the each other in naming. Maybe
> using "requested_nents", or "nents_arg", for the incoming value, would
> help.

Ok, will change.

>> +
>> +	ents = __dma_map_sg_attrs(dev, sg, nents, dir, attrs);
>>   	BUG_ON(ents < 0);
>>   	debug_dma_map_sg(dev, sg, nents, ents, dir);
>>   
>> @@ -204,6 +214,36 @@ int dma_map_sg_attrs(struct device *dev, struct scatterlist *sg, int nents,
>>   }
>>   EXPORT_SYMBOL(dma_map_sg_attrs);
>>   
>> +/*
>> + * like dma_map_sg_attrs, but returns a negative errno on error (and > 0
>> + * on success). This function must be used if PCI P2PDMA pages might
>> + * be in the scatterlist.
> 
> Let's turn this into a kernel doc comment block, seeing as how it clearly
> wants to be--you're almost there already. You've even reinvented @Return,
> below. :)

Just trying to follow the convention in this file. But I can make it a
kernel doc.

>> + *
>> + * On error this function may return:
>> + *    -ENOMEM indicating that there was not enough resources available and
>> + *      the transfer may be retried later
>> + *    -EREMOTEIO indicating that P2PDMA pages were included but cannot
>> + *      be mapped by the specified device, retries will always fail
>> + *
>> + * The scatterlist should be unmapped with the regular dma_unmap_sg[_attrs]().
> 
> How about:
> 
> "The scatterlist should be unmapped via dma_unmap_sg[_attrs]()."

Ok

Logan