Question: partial transfers of DMABUFs

[Christian König]

Hi Paul,

Am 15.02.23 um 11:48 schrieb Paul Cercueil:
> 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,

Please don't assume that this is an sg_table. We just used it as 
container for DMA addresses, but this has proven to be a mistake.

There is work underway to replace the sg_table with (for example) just 
an array of DMA addresses.

> 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?

I strongly recommend to come up with a new function which only takes DMA 
addresses and separate segment length.

Regards,
Christian.

>
> Cheers,
> -Paul