On 4/23/21 3:19 AM, Adrian Larumbe wrote:
Recently at Imgtec we had to provide GLES API buffers with DMA transfer
capabilities to device memory. We had access to a Xilinx CDMA IP module,
but even though the hardware supports scatter-gather operations,
the driver did not. This patch series' goal is to extend the driver
to support SG transfers on CDMA devices.
Hi,
Thanks for the patch series. From an implementation point of view this
looks good. But I'm a bit concerned about dmaengine API compliance.
The device_config() and device_prep_slave_sg() APIs are meant for
memory-to-device or device-to-memory transfers. This means the device
address is fixed and usually maps to a FIFO in the device.
What you are implementing is memory-to-memory, which means addresses are
incremented on both sides of transfer. And this works if you know what
you are doing, e.g. you know that you are using a specific dmaengine
driver that implements the dmaengine API in a way that does not comply
with the specification. But it breaks for generic dmaengine API clients
that expect compliant behavior. And it is the reason why we have an API
in the first place, to get consistent behavior. If you have to know
about the driver specific semantics you might as well just bypass the
framework and use driver specific functions.
And the CDMA seems to support the dmaengine API intended behavior
through what it calls the keyhole feature where the address is not
incremented. And if somebody wanted to use that feature they wouldn't be
able to add support for it because it will break your usage of the
prep_slave_sg() API.
It seems to me what we are missing from the DMAengine API is the
equivalent of device_prep_dma_memcpy() that is able to take SG lists.
There is already a memset_sg, it should be possible to add something
similar for memcpy.
- Lars
