Question: partial transfers of DMABUFs

[Paul Cercueil]

Hi Maarten,

Le mercredi 15 février 2023 à 12:52 +0100, Maarten Lankhorst a écrit :
> 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.

Yes, I do that already as it is an argument in my ioctl.

> 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.

Yes. But I don't know how to "ignore the rest of the SG list".

- dma_buf_map_attachment() does not have a parameter to specify that I
only need the first X bytes mapped;

- if I map the whole thing, dmaengine_prep_slave_sg() does not have an
option to specify that I only want the first X bytes transferred.

-Paul