Question: partial transfers of DMABUFs

[Maarten Lankhorst]

Hey,

On 2023-02-15 12:47, Paul Cercueil wrote:
> Hi Maarten,
>
> Le mercredi 15 février 2023 à 12:30 +0100, Maarten Lankhorst a écrit :
>> Hey,
>>
>> On 2023-02-15 11:48, Paul Cercueil wrote:
>>> Hi,
>>>
>>> I am working on adding support for DMABUFs in the IIO subsystem.
>>>
>>> One thing we want there, is the ability to specify the number of
>>> bytes
>>> to transfer (while still defaulting to the DMABUF size).
>>>
>>> Since dma_buf_map_attachment() returns a sg_table, I basically have
>>> two
>>> options, and I can't decide which one is the best (or the less
>>> ugly):
>>>
>>> - Either I add a new API function similar to
>>> dmaengine_prep_slave_sg(),
>>> which still takes a scatterlist as argument but also takes the
>>> number
>>> of bytes as argument;
>>>
>>> - Or I add a function to duplicate the scatterlist and then shrink
>>> it
>>> manually, which doesn't sound like a good idea either.
>>>
>>> What would be the recommended way?
>> Does this need an api change? If you create a DMA-BUF of size X, it
>> has
>> to be of size X. You can pad with a dummy page probably if you know
>> it
>> in advance. But after it has been imported, it cannot change size.
> Yes, the sizes are fixed.
>
>> You don´t have to write the entire dma-buf either, so if you want to
>> create a 1GB buf and only use the first 4K, that is allowed. The
>> contents of  the remainder of the DMA-BUF are undefined. It's up to
>> userspace to assign a meaning to it.
>>
>> I think I'm missing something here that makes the whole question
>> m,ake
>> more sense.
> I want my userspace to be able to specify how much of the DMABUF is to
> be read from or written to.
>
> So in my new "dmabuf enqueue" IOCTL that I want to add to IIO, I added
> a parameter to specify the number of bytes to transfer (where 0 means
> the whole buffer).
>
> The problem I have now, is that the current dmaengine core does not
> have a API function that takes a scatterlist (returned by
> dma_map_attachment()) and a transfer size in bytes, it will always
> transfer the whole scatterlist.
>
> So my two options would be to add a new API function to support
> specifying a bytes count, or add a mechanism to duplicate a
> scatterlist, so that I can tweak it to the right size.

This doesn't have to happen through DMA-BUF. Presumably you are both the 
importer and the exporter, so after you know how much is read, you can 
tell this to the importer that X number of bytes can be read from DMA-BUF Y.

In your case, when enqueing you will get a full SG list, but if you know 
only X bytes are read/written you only have to map the first X bytes to 
your IIO device. The rest of the SG list could be ignored safely.

~Maarten