Question: partial transfers of DMABUFs

[Maarten Lankhorst]

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