Is adding offset to dma address is valid operation?

Is adding offset to dma address is valid operation?

[Harsh Jain]

Hi,

Can we add  offset to dma address received from IOMMU in scatter/Gather list before sending it to H/W.

Address to HW = sg_dma_address(sg) + offset, where 0 < offset < sg_dma_len(sg).

I need this operation to make sure our H/W does not receives entry of size greater than 2K.

Regards

Harsh Jain

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Re: Is adding offset to dma address is valid operation?

[Robin Murphy]

Hi Harsh,

On 26/03/18 10:55, Harsh Jain wrote:
> Hi,
> 
> Can we add  offset to dma address received from IOMMU in scatter/Gather list before sending it to H/W.
> 
> Address to HW = sg_dma_address(sg) + offset, where 0 < offset < sg_dma_len(sg).

sg_dma_address() should already account for sg->offset (i.e. the start 
of the buffer within the page. If there's a DMA API implementation 
failing to respect that then that's a bug.

> I need this operation to make sure our H/W does not receives entry of size greater than 2K.

If however it's just that you want to fragment a single SG entry into 
multiple commaneds/descriptors/etc. internally to your driver, then I 
can't see that being an issue as long as you don't visibly modify the 
scatterlist itself. If there's a genuine length and/or alignment 
limitation in the hardware, though, it would be better to set up 
dev->dma_parms appropriately to describe them, such that the subsystem 
responsible for generating the scatterlist respects the appropriate 
constraints in the first place (and if it doesn't, then that's another 
bug to fix).

Robin.
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Re: Is adding offset to dma address is valid operation?

[Harsh Jain]

On 26-03-2018 17:15, Robin Murphy wrote:
> Hi Harsh,
>
> On 26/03/18 10:55, Harsh Jain wrote:
>> Hi,
>>
>> Can we add  offset to dma address received from IOMMU in scatter/Gather list before sending it to H/W.
>>
>> Address to HW = sg_dma_address(sg) + offset, where 0 < offset < sg_dma_len(sg).
>
> sg_dma_address() should already account for sg->offset (i.e. the start of the buffer within the page. If there's a DMA API implementation failing to respect that then that's a bug.
Thanks.
Here "offset" means size of last dma entry I had mapped to H/W structure. eg if dma length is 9K offset will be 0, 2K, 4K,6K,8K.
>
>> I need this operation to make sure our H/W does not receives entry of size greater than 2K.
>
> If however it's just that you want to fragment a single SG entry into multiple commaneds/descriptors/etc. internally to your driver, then I can't see that being an issue as long as you don't visibly modify the scatterlist itself. If there's a genuine length and/or alignment limitation in the hardware, though, it would be better to set up dev->dma_parms appropriately to describe them, such that the subsystem responsible for generating the scatterlist respects the appropriate constraints in the first place (and if it doesn't, then that's another bug to fix).
>
Yes, I wants to fragment single entry into multiple such that it is less than 2K. eg. if sg->length  == 5k , I will split into 3 entries
addr1 = sg_dma_address(sg),     len =2K
addr2 = sg_dma_address(sg) + 2K,     len = 2K
addr3 = sg_dma_address(sg) +4K,     len = 1K


> Robin.

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Re: Is adding offset to dma address is valid operation?

[Harsh Jain]

On 26-03-2018 17:15, Robin Murphy wrote:
> Hi Harsh,
>
> On 26/03/18 10:55, Harsh Jain wrote:
>> Hi,
>>
>> Can we add  offset to dma address received from IOMMU in scatter/Gather list before sending it to H/W.
>>
>> Address to HW = sg_dma_address(sg) + offset, where 0 < offset < sg_dma_len(sg).
>
> sg_dma_address() should already account for sg->offset (i.e. the start of the buffer within the page. If there's a DMA API implementation failing to respect that then that's a bug.
>
>> I need this operation to make sure our H/W does not receives entry of size greater than 2K.
>
> If however it's just that you want to fragment a single SG entry into multiple commaneds/descriptors/etc. internally to your driver, then I can't see that being an issue as long as you don't visibly modify the scatterlist itself. If there's a genuine length and/or alignment limitation in the hardware, though, it would be better to set up dev->dma_parms appropriately to describe them, such that the subsystem responsible for generating the scatterlist respects the appropriate constraints in the first place (and if it doesn't, then that's another bug to fix).
Can dma_parms  handle 2K size limit ? Because dma_map_sg() returns <= passed nents. But in my case nents will increase.

>
> Robin.

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Re: Is adding offset to dma address is valid operation?

[Robin Murphy]

On 26/03/18 13:31, Harsh Jain wrote:
> 
> 
> On 26-03-2018 17:15, Robin Murphy wrote:
>> Hi Harsh,
>> 
>> On 26/03/18 10:55, Harsh Jain wrote:
>>> Hi,
>>> 
>>> Can we add  offset to dma address received from IOMMU in
>>> scatter/Gather list before sending it to H/W.
>>> 
>>> Address to HW = sg_dma_address(sg) + offset, where 0 < offset <
>>> sg_dma_len(sg).
>> 
>> sg_dma_address() should already account for sg->offset (i.e. the
>> start of the buffer within the page. If there's a DMA API
>> implementation failing to respect that then that's a bug.
>> 
>>> I need this operation to make sure our H/W does not receives
>>> entry of size greater than 2K.
>> 
>> If however it's just that you want to fragment a single SG entry
>> into multiple commaneds/descriptors/etc. internally to your driver,
>> then I can't see that being an issue as long as you don't visibly
>> modify the scatterlist itself. If there's a genuine length and/or
>> alignment limitation in the hardware, though, it would be better to
>> set up dev->dma_parms appropriately to describe them, such that the
>> subsystem responsible for generating the scatterlist respects the
>> appropriate constraints in the first place (and if it doesn't, then
>> that's another bug to fix).
> Can dma_parms  handle 2K size limit ? Because dma_map_sg() returns <=
> passed nents. But in my case nents will increase.

The point is that dma_map_sg() should still never increase nents, 
because the guy *creating* the scatterlist is also expected to respect 
dma_parms where it exists, such that nents would be appropriate to start 
with. That said, it does look like there are some places (e.g. the block 
layer) which may not cope properly with limits less than PAGE_SIZE, so I 
guess the reliability of dma_parms might depend on how much you want to 
go spelunking in core code :/

Robin.