Messages in this thread | | | Date | Wed, 15 Feb 2023 12:52:54 +0100 | Subject | Re: Question: partial transfers of DMABUFs | From | Maarten Lankhorst <> |
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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
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