On 2023-02-15 13:00, Paul Cercueil wrote:
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.
sg_split apppears to allow you to split it? I'm not 100% sure whether it
leaves the original SG untouched, but you can try to put it in between
those 2 calls to get a smaller SG to pass to prep_slave_sg.
~Maarten
