Re: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

2022-07-04 Thread Robin Murphy

On 2022-06-15 17:12, Logan Gunthorpe wrote:

Make use of the third free LSB in scatterlist's page_link on 64bit systems.

The extra bit will be used by dma_[un]map_sg_p2pdma() to determine when a
given SGL segments dma_address points to a PCI bus address.
dma_unmap_sg_p2pdma() will need to perform different cleanup when a
segment is marked as a bus address.

The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means
PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the
majority of P2PDMA use cases are restricted to newer root complexes and
roughly require the extra address space for memory BARs used in the
transactions.


Another thought that's hit me slightly late; depending on CONFIG_64BIT 
also means that we've got a whole 4 bytes of padding in struct 
scatterlist to play with, so at that point maybe it's worth considering 
carrying new extra DMA mapping properties in their own field(s). For 
instance it would also be really helpful to flag whether a segment is 
bounce-buffered or not.


Robin.


Signed-off-by: Logan Gunthorpe 
Reviewed-by: Chaitanya Kulkarni 
---
  drivers/pci/Kconfig |  5 +
  include/linux/scatterlist.h | 44 -
  2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 133c73207782..5cc7cba1941f 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -164,6 +164,11 @@ config PCI_PASID
  config PCI_P2PDMA
bool "PCI peer-to-peer transfer support"
depends on ZONE_DEVICE
+   #
+   # The need for the scatterlist DMA bus address flag means PCI P2PDMA
+   # requires 64bit
+   #
+   depends on 64BIT
select GENERIC_ALLOCATOR
help
  Enableѕ drivers to do PCI peer-to-peer transactions to and from
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 7ff9d6386c12..6561ca8aead8 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -64,12 +64,24 @@ struct sg_append_table {
  #define SG_CHAIN  0x01UL
  #define SG_END0x02UL
  
+/*

+ * bit 2 is the third free bit in the page_link on 64bit systems which
+ * is used by dma_unmap_sg() to determine if the dma_address is a
+ * bus address when doing P2PDMA.
+ */
+#ifdef CONFIG_PCI_P2PDMA
+#define SG_DMA_BUS_ADDRESS 0x04UL
+static_assert(__alignof__(struct page) >= 8);
+#else
+#define SG_DMA_BUS_ADDRESS 0x00UL
+#endif
+
  /*
   * We overload the LSB of the page pointer to indicate whether it's
   * a valid sg entry, or whether it points to the start of a new scatterlist.
   * Those low bits are there for everyone! (thanks mason :-)
   */
-#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END)
+#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END | SG_DMA_BUS_ADDRESS)
  
  static inline unsigned int __sg_flags(struct scatterlist *sg)

  {
@@ -91,6 +103,11 @@ static inline bool sg_is_last(struct scatterlist *sg)
return __sg_flags(sg) & SG_END;
  }
  
+static inline bool sg_is_dma_bus_address(struct scatterlist *sg)

+{
+   return __sg_flags(sg) & SG_DMA_BUS_ADDRESS;
+}
+
  /**
   * sg_assign_page - Assign a given page to an SG entry
   * @sg:   SG entry
@@ -245,6 +262,31 @@ static inline void sg_unmark_end(struct scatterlist *sg)
sg->page_link &= ~SG_END;
  }
  
+/**

+ * sg_dma_mark_bus address - Mark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist
+ *
+ * Description:
+ *   Marks the passed in sg entry to indicate that the dma_address is
+ *   a bus address and doesn't need to be unmapped.
+ **/
+static inline void sg_dma_mark_bus_address(struct scatterlist *sg)
+{
+   sg->page_link |= SG_DMA_BUS_ADDRESS;
+}
+
+/**
+ * sg_unmark_pci_p2pdma - Unmark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist
+ *
+ * Description:
+ *   Clears the bus address mark.
+ **/
+static inline void sg_dma_unmark_bus_address(struct scatterlist *sg)
+{
+   sg->page_link &= ~SG_DMA_BUS_ADDRESS;
+}
+
  /**
   * sg_phys - Return physical address of an sg entry
   * @sg:SG entry

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Re: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

2022-06-29 Thread Logan Gunthorpe



On 2022-06-29 12:02, Robin Murphy wrote:
> On 2022-06-29 16:39, Logan Gunthorpe wrote:
>> On 2022-06-29 03:05, Robin Murphy wrote:
>>> On 2022-06-15 17:12, Logan Gunthorpe wrote:
>>> Does this serve any useful purpose? If a page is determined to be device
>>> memory, it's not going to suddenly stop being device memory, and if the
>>> underlying sg is recycled to point elsewhere then sg_assign_page() will
>>> still (correctly) clear this flag anyway. Trying to reason about this
>>> beyond superficial API symmetry - i.e. why exactly would a caller need
>>> to call it, and what would the implications be of failing to do so -
>>> seems to lead straight to confusion.
>>>
>>> In fact I'd be inclined to have sg_assign_page() be responsible for
>>> setting the flag automatically as well, and thus not need
>>> sg_dma_mark_bus_address() either, however I can see the argument for
>>> doing it this way round to not entangle the APIs too much, so I don't
>>> have any great objection to that.
>>
>> Yes, I think you misunderstand what this is for. The SG_DMA_BUS_ADDDRESS
>> flag doesn't mark the segment for the page, but for the dma address. It
>> cannot be set in sg_assign_page() seeing it's not a property of the page
>> but a property of the dma_address in the sgl.
>>
>> It's not meant for use by regular SG users, it's only meant for use
>> inside DMA mapping implementations. The purpose is to know whether a
>> given dma_address in the SGL is a bus address or regular memory because
>> the two different types must be unmapped differently. We can't rely on
>> the page because, as you know, many dma_map_sg() the dma_address entry
>> in the sgl does not map to the same memory as the page. Or to put it
>> another way: is_pci_p2pdma_page(sg->page) does not imply that
>> sg->dma_address points to a bus address.
>>
>> Does that make sense?
> 
> Ah, you're quite right, in trying to take in the whole series at once
> first thing in the morning I did fail to properly grasp that detail, so
> indeed the sg_assign_page() thing couldn't possibly work, but as I said
> that's fine anyway. I still think the lifecycle management is a bit off
> though - equivalently, a bus address doesn't stop being a bus address,
> so it would seem appropriate to update this flag appropriately whenever
> sg_dma_address() is assigned to, and not when it isn't.

Yes, that's pretty much the way the code is now. The only two places
sg_dma_mark_bus_address() is called are in the two pci_p2pdma helpers
that set the dma address to the bus address. The lines before both calls
set the dma_address and dma_len.

Logan

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Re: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

2022-06-29 Thread Robin Murphy

On 2022-06-29 16:39, Logan Gunthorpe wrote:




On 2022-06-29 03:05, Robin Murphy wrote:

On 2022-06-15 17:12, Logan Gunthorpe wrote:

Make use of the third free LSB in scatterlist's page_link on 64bit
systems.

The extra bit will be used by dma_[un]map_sg_p2pdma() to determine when a
given SGL segments dma_address points to a PCI bus address.
dma_unmap_sg_p2pdma() will need to perform different cleanup when a
segment is marked as a bus address.

The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means
PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the
majority of P2PDMA use cases are restricted to newer root complexes and
roughly require the extra address space for memory BARs used in the
transactions.

Signed-off-by: Logan Gunthorpe 
Reviewed-by: Chaitanya Kulkarni 
---
   drivers/pci/Kconfig |  5 +
   include/linux/scatterlist.h | 44 -
   2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 133c73207782..5cc7cba1941f 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -164,6 +164,11 @@ config PCI_PASID
   config PCI_P2PDMA
   bool "PCI peer-to-peer transfer support"
   depends on ZONE_DEVICE
+    #
+    # The need for the scatterlist DMA bus address flag means PCI P2PDMA
+    # requires 64bit
+    #
+    depends on 64BIT
   select GENERIC_ALLOCATOR
   help
     Enableѕ drivers to do PCI peer-to-peer transactions to and from
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 7ff9d6386c12..6561ca8aead8 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -64,12 +64,24 @@ struct sg_append_table {
   #define SG_CHAIN    0x01UL
   #define SG_END    0x02UL
   +/*
+ * bit 2 is the third free bit in the page_link on 64bit systems which
+ * is used by dma_unmap_sg() to determine if the dma_address is a
+ * bus address when doing P2PDMA.
+ */
+#ifdef CONFIG_PCI_P2PDMA
+#define SG_DMA_BUS_ADDRESS    0x04UL
+static_assert(__alignof__(struct page) >= 8);
+#else
+#define SG_DMA_BUS_ADDRESS    0x00UL
+#endif
+
   /*
    * We overload the LSB of the page pointer to indicate whether it's
    * a valid sg entry, or whether it points to the start of a new
scatterlist.
    * Those low bits are there for everyone! (thanks mason :-)
    */
-#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END)
+#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END | SG_DMA_BUS_ADDRESS)
     static inline unsigned int __sg_flags(struct scatterlist *sg)
   {
@@ -91,6 +103,11 @@ static inline bool sg_is_last(struct scatterlist *sg)
   return __sg_flags(sg) & SG_END;
   }
   +static inline bool sg_is_dma_bus_address(struct scatterlist *sg)
+{
+    return __sg_flags(sg) & SG_DMA_BUS_ADDRESS;
+}
+
   /**
    * sg_assign_page - Assign a given page to an SG entry
    * @sg:    SG entry
@@ -245,6 +262,31 @@ static inline void sg_unmark_end(struct
scatterlist *sg)
   sg->page_link &= ~SG_END;
   }
   +/**
+ * sg_dma_mark_bus address - Mark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist


entryScatterlist?


+ *
+ * Description:
+ *   Marks the passed in sg entry to indicate that the dma_address is
+ *   a bus address and doesn't need to be unmapped.
+ **/
+static inline void sg_dma_mark_bus_address(struct scatterlist *sg)
+{
+    sg->page_link |= SG_DMA_BUS_ADDRESS;
+}
+
+/**
+ * sg_unmark_pci_p2pdma - Unmark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist
+ *
+ * Description:
+ *   Clears the bus address mark.
+ **/
+static inline void sg_dma_unmark_bus_address(struct scatterlist *sg)
+{
+    sg->page_link &= ~SG_DMA_BUS_ADDRESS;
+}


Does this serve any useful purpose? If a page is determined to be device
memory, it's not going to suddenly stop being device memory, and if the
underlying sg is recycled to point elsewhere then sg_assign_page() will
still (correctly) clear this flag anyway. Trying to reason about this
beyond superficial API symmetry - i.e. why exactly would a caller need
to call it, and what would the implications be of failing to do so -
seems to lead straight to confusion.

In fact I'd be inclined to have sg_assign_page() be responsible for
setting the flag automatically as well, and thus not need
sg_dma_mark_bus_address() either, however I can see the argument for
doing it this way round to not entangle the APIs too much, so I don't
have any great objection to that.


Yes, I think you misunderstand what this is for. The SG_DMA_BUS_ADDDRESS
flag doesn't mark the segment for the page, but for the dma address. It
cannot be set in sg_assign_page() seeing it's not a property of the page
but a property of the dma_address in the sgl.

It's not meant for use by regular SG users, it's only meant for use
inside DMA mapping implementations. The purpose is to know whether a
given dma_address in the SGL is a bus address or regular memory because
the two different 

Re: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

2022-06-29 Thread Logan Gunthorpe



On 2022-06-29 03:05, Robin Murphy wrote:
> On 2022-06-15 17:12, Logan Gunthorpe wrote:
>> Make use of the third free LSB in scatterlist's page_link on 64bit
>> systems.
>>
>> The extra bit will be used by dma_[un]map_sg_p2pdma() to determine when a
>> given SGL segments dma_address points to a PCI bus address.
>> dma_unmap_sg_p2pdma() will need to perform different cleanup when a
>> segment is marked as a bus address.
>>
>> The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means
>> PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the
>> majority of P2PDMA use cases are restricted to newer root complexes and
>> roughly require the extra address space for memory BARs used in the
>> transactions.
>>
>> Signed-off-by: Logan Gunthorpe 
>> Reviewed-by: Chaitanya Kulkarni 
>> ---
>>   drivers/pci/Kconfig |  5 +
>>   include/linux/scatterlist.h | 44 -
>>   2 files changed, 48 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
>> index 133c73207782..5cc7cba1941f 100644
>> --- a/drivers/pci/Kconfig
>> +++ b/drivers/pci/Kconfig
>> @@ -164,6 +164,11 @@ config PCI_PASID
>>   config PCI_P2PDMA
>>   bool "PCI peer-to-peer transfer support"
>>   depends on ZONE_DEVICE
>> +    #
>> +    # The need for the scatterlist DMA bus address flag means PCI P2PDMA
>> +    # requires 64bit
>> +    #
>> +    depends on 64BIT
>>   select GENERIC_ALLOCATOR
>>   help
>>     Enableѕ drivers to do PCI peer-to-peer transactions to and from
>> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
>> index 7ff9d6386c12..6561ca8aead8 100644
>> --- a/include/linux/scatterlist.h
>> +++ b/include/linux/scatterlist.h
>> @@ -64,12 +64,24 @@ struct sg_append_table {
>>   #define SG_CHAIN    0x01UL
>>   #define SG_END    0x02UL
>>   +/*
>> + * bit 2 is the third free bit in the page_link on 64bit systems which
>> + * is used by dma_unmap_sg() to determine if the dma_address is a
>> + * bus address when doing P2PDMA.
>> + */
>> +#ifdef CONFIG_PCI_P2PDMA
>> +#define SG_DMA_BUS_ADDRESS    0x04UL
>> +static_assert(__alignof__(struct page) >= 8);
>> +#else
>> +#define SG_DMA_BUS_ADDRESS    0x00UL
>> +#endif
>> +
>>   /*
>>    * We overload the LSB of the page pointer to indicate whether it's
>>    * a valid sg entry, or whether it points to the start of a new
>> scatterlist.
>>    * Those low bits are there for everyone! (thanks mason :-)
>>    */
>> -#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END)
>> +#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END | SG_DMA_BUS_ADDRESS)
>>     static inline unsigned int __sg_flags(struct scatterlist *sg)
>>   {
>> @@ -91,6 +103,11 @@ static inline bool sg_is_last(struct scatterlist *sg)
>>   return __sg_flags(sg) & SG_END;
>>   }
>>   +static inline bool sg_is_dma_bus_address(struct scatterlist *sg)
>> +{
>> +    return __sg_flags(sg) & SG_DMA_BUS_ADDRESS;
>> +}
>> +
>>   /**
>>    * sg_assign_page - Assign a given page to an SG entry
>>    * @sg:    SG entry
>> @@ -245,6 +262,31 @@ static inline void sg_unmark_end(struct
>> scatterlist *sg)
>>   sg->page_link &= ~SG_END;
>>   }
>>   +/**
>> + * sg_dma_mark_bus address - Mark the scatterlist entry as a bus address
>> + * @sg: SG entryScatterlist
> 
> entryScatterlist?
> 
>> + *
>> + * Description:
>> + *   Marks the passed in sg entry to indicate that the dma_address is
>> + *   a bus address and doesn't need to be unmapped.
>> + **/
>> +static inline void sg_dma_mark_bus_address(struct scatterlist *sg)
>> +{
>> +    sg->page_link |= SG_DMA_BUS_ADDRESS;
>> +}
>> +
>> +/**
>> + * sg_unmark_pci_p2pdma - Unmark the scatterlist entry as a bus address
>> + * @sg: SG entryScatterlist
>> + *
>> + * Description:
>> + *   Clears the bus address mark.
>> + **/
>> +static inline void sg_dma_unmark_bus_address(struct scatterlist *sg)
>> +{
>> +    sg->page_link &= ~SG_DMA_BUS_ADDRESS;
>> +}
> 
> Does this serve any useful purpose? If a page is determined to be device
> memory, it's not going to suddenly stop being device memory, and if the
> underlying sg is recycled to point elsewhere then sg_assign_page() will
> still (correctly) clear this flag anyway. Trying to reason about this
> beyond superficial API symmetry - i.e. why exactly would a caller need
> to call it, and what would the implications be of failing to do so -
> seems to lead straight to confusion.
> 
> In fact I'd be inclined to have sg_assign_page() be responsible for
> setting the flag automatically as well, and thus not need
> sg_dma_mark_bus_address() either, however I can see the argument for
> doing it this way round to not entangle the APIs too much, so I don't
> have any great objection to that.

Yes, I think you misunderstand what this is for. The SG_DMA_BUS_ADDDRESS
flag doesn't mark the segment for the page, but for the dma address. It
cannot be set in sg_assign_page() seeing it's not a property of the page
but a 

Re: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

2022-06-29 Thread Robin Murphy

On 2022-06-15 17:12, Logan Gunthorpe wrote:

Make use of the third free LSB in scatterlist's page_link on 64bit systems.

The extra bit will be used by dma_[un]map_sg_p2pdma() to determine when a
given SGL segments dma_address points to a PCI bus address.
dma_unmap_sg_p2pdma() will need to perform different cleanup when a
segment is marked as a bus address.

The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means
PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the
majority of P2PDMA use cases are restricted to newer root complexes and
roughly require the extra address space for memory BARs used in the
transactions.

Signed-off-by: Logan Gunthorpe 
Reviewed-by: Chaitanya Kulkarni 
---
  drivers/pci/Kconfig |  5 +
  include/linux/scatterlist.h | 44 -
  2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 133c73207782..5cc7cba1941f 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -164,6 +164,11 @@ config PCI_PASID
  config PCI_P2PDMA
bool "PCI peer-to-peer transfer support"
depends on ZONE_DEVICE
+   #
+   # The need for the scatterlist DMA bus address flag means PCI P2PDMA
+   # requires 64bit
+   #
+   depends on 64BIT
select GENERIC_ALLOCATOR
help
  Enableѕ drivers to do PCI peer-to-peer transactions to and from
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 7ff9d6386c12..6561ca8aead8 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -64,12 +64,24 @@ struct sg_append_table {
  #define SG_CHAIN  0x01UL
  #define SG_END0x02UL
  
+/*

+ * bit 2 is the third free bit in the page_link on 64bit systems which
+ * is used by dma_unmap_sg() to determine if the dma_address is a
+ * bus address when doing P2PDMA.
+ */
+#ifdef CONFIG_PCI_P2PDMA
+#define SG_DMA_BUS_ADDRESS 0x04UL
+static_assert(__alignof__(struct page) >= 8);
+#else
+#define SG_DMA_BUS_ADDRESS 0x00UL
+#endif
+
  /*
   * We overload the LSB of the page pointer to indicate whether it's
   * a valid sg entry, or whether it points to the start of a new scatterlist.
   * Those low bits are there for everyone! (thanks mason :-)
   */
-#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END)
+#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END | SG_DMA_BUS_ADDRESS)
  
  static inline unsigned int __sg_flags(struct scatterlist *sg)

  {
@@ -91,6 +103,11 @@ static inline bool sg_is_last(struct scatterlist *sg)
return __sg_flags(sg) & SG_END;
  }
  
+static inline bool sg_is_dma_bus_address(struct scatterlist *sg)

+{
+   return __sg_flags(sg) & SG_DMA_BUS_ADDRESS;
+}
+
  /**
   * sg_assign_page - Assign a given page to an SG entry
   * @sg:   SG entry
@@ -245,6 +262,31 @@ static inline void sg_unmark_end(struct scatterlist *sg)
sg->page_link &= ~SG_END;
  }
  
+/**

+ * sg_dma_mark_bus address - Mark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist


entryScatterlist?


+ *
+ * Description:
+ *   Marks the passed in sg entry to indicate that the dma_address is
+ *   a bus address and doesn't need to be unmapped.
+ **/
+static inline void sg_dma_mark_bus_address(struct scatterlist *sg)
+{
+   sg->page_link |= SG_DMA_BUS_ADDRESS;
+}
+
+/**
+ * sg_unmark_pci_p2pdma - Unmark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist
+ *
+ * Description:
+ *   Clears the bus address mark.
+ **/
+static inline void sg_dma_unmark_bus_address(struct scatterlist *sg)
+{
+   sg->page_link &= ~SG_DMA_BUS_ADDRESS;
+}


Does this serve any useful purpose? If a page is determined to be device 
memory, it's not going to suddenly stop being device memory, and if the 
underlying sg is recycled to point elsewhere then sg_assign_page() will 
still (correctly) clear this flag anyway. Trying to reason about this 
beyond superficial API symmetry - i.e. why exactly would a caller need 
to call it, and what would the implications be of failing to do so - 
seems to lead straight to confusion.


In fact I'd be inclined to have sg_assign_page() be responsible for 
setting the flag automatically as well, and thus not need 
sg_dma_mark_bus_address() either, however I can see the argument for 
doing it this way round to not entangle the APIs too much, so I don't 
have any great objection to that.


Thanks,
Robin.


+
  /**
   * sg_phys - Return physical address of an sg entry
   * @sg:SG entry

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Re: [PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

2022-06-29 Thread Christoph Hellwig
On Wed, Jun 15, 2022 at 10:12:13AM -0600, Logan Gunthorpe wrote:
> Make use of the third free LSB in scatterlist's page_link on 64bit systems.
> 
> The extra bit will be used by dma_[un]map_sg_p2pdma() to determine when a
> given SGL segments dma_address points to a PCI bus address.
> dma_unmap_sg_p2pdma() will need to perform different cleanup when a
> segment is marked as a bus address.
> 
> The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means
> PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the
> majority of P2PDMA use cases are restricted to newer root complexes and
> roughly require the extra address space for memory BARs used in the
> transactions.
> 
> Signed-off-by: Logan Gunthorpe 
> Reviewed-by: Chaitanya Kulkarni 

Looks good:

Reviewed-by: Christoph Hellwig 
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[PATCH v7 01/21] lib/scatterlist: add flag for indicating P2PDMA segments in an SGL

2022-06-15 Thread Logan Gunthorpe
Make use of the third free LSB in scatterlist's page_link on 64bit systems.

The extra bit will be used by dma_[un]map_sg_p2pdma() to determine when a
given SGL segments dma_address points to a PCI bus address.
dma_unmap_sg_p2pdma() will need to perform different cleanup when a
segment is marked as a bus address.

The new bit will only be used when CONFIG_PCI_P2PDMA is set; this means
PCI P2PDMA will require CONFIG_64BIT. This should be acceptable as the
majority of P2PDMA use cases are restricted to newer root complexes and
roughly require the extra address space for memory BARs used in the
transactions.

Signed-off-by: Logan Gunthorpe 
Reviewed-by: Chaitanya Kulkarni 
---
 drivers/pci/Kconfig |  5 +
 include/linux/scatterlist.h | 44 -
 2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 133c73207782..5cc7cba1941f 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -164,6 +164,11 @@ config PCI_PASID
 config PCI_P2PDMA
bool "PCI peer-to-peer transfer support"
depends on ZONE_DEVICE
+   #
+   # The need for the scatterlist DMA bus address flag means PCI P2PDMA
+   # requires 64bit
+   #
+   depends on 64BIT
select GENERIC_ALLOCATOR
help
  Enableѕ drivers to do PCI peer-to-peer transactions to and from
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 7ff9d6386c12..6561ca8aead8 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -64,12 +64,24 @@ struct sg_append_table {
 #define SG_CHAIN   0x01UL
 #define SG_END 0x02UL
 
+/*
+ * bit 2 is the third free bit in the page_link on 64bit systems which
+ * is used by dma_unmap_sg() to determine if the dma_address is a
+ * bus address when doing P2PDMA.
+ */
+#ifdef CONFIG_PCI_P2PDMA
+#define SG_DMA_BUS_ADDRESS 0x04UL
+static_assert(__alignof__(struct page) >= 8);
+#else
+#define SG_DMA_BUS_ADDRESS 0x00UL
+#endif
+
 /*
  * We overload the LSB of the page pointer to indicate whether it's
  * a valid sg entry, or whether it points to the start of a new scatterlist.
  * Those low bits are there for everyone! (thanks mason :-)
  */
-#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END)
+#define SG_PAGE_LINK_MASK (SG_CHAIN | SG_END | SG_DMA_BUS_ADDRESS)
 
 static inline unsigned int __sg_flags(struct scatterlist *sg)
 {
@@ -91,6 +103,11 @@ static inline bool sg_is_last(struct scatterlist *sg)
return __sg_flags(sg) & SG_END;
 }
 
+static inline bool sg_is_dma_bus_address(struct scatterlist *sg)
+{
+   return __sg_flags(sg) & SG_DMA_BUS_ADDRESS;
+}
+
 /**
  * sg_assign_page - Assign a given page to an SG entry
  * @sg:SG entry
@@ -245,6 +262,31 @@ static inline void sg_unmark_end(struct scatterlist *sg)
sg->page_link &= ~SG_END;
 }
 
+/**
+ * sg_dma_mark_bus address - Mark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist
+ *
+ * Description:
+ *   Marks the passed in sg entry to indicate that the dma_address is
+ *   a bus address and doesn't need to be unmapped.
+ **/
+static inline void sg_dma_mark_bus_address(struct scatterlist *sg)
+{
+   sg->page_link |= SG_DMA_BUS_ADDRESS;
+}
+
+/**
+ * sg_unmark_pci_p2pdma - Unmark the scatterlist entry as a bus address
+ * @sg: SG entryScatterlist
+ *
+ * Description:
+ *   Clears the bus address mark.
+ **/
+static inline void sg_dma_unmark_bus_address(struct scatterlist *sg)
+{
+   sg->page_link &= ~SG_DMA_BUS_ADDRESS;
+}
+
 /**
  * sg_phys - Return physical address of an sg entry
  * @sg: SG entry
-- 
2.30.2

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