Preferred kernel API/ABI for accelerator hardware device driver

[Alon Ziv]

Hi

I am looking for advice on the proper design of an API for a device driver
for hardware acceleration (e.g. a JPEG decoder or a cryptographic function
accelerator).  These hardware modules don’t appear to map properly to the
standard device-driver paradigm (i.e., a “device file” that communicates
with userspace using read()/write() system calls), as we need to pass input
data into the kernel and receive transformed output directly. The only way I
see to use standard IO operations on such drivers is by copying the data
into temporary kernel buffers, which is (a) a pain and (b) horribly
inefficient.

Currently the only way I see around this limitation is to implement all
communications with such a driver using ioctl() operations, but this isn’t a
very satisfactory solution—for example, it does not provide a way to do
something similar to “writev()” (gathering several buffers for a single
processing operation).

Should I just take the plunge and implement a new “iov(fd, iov_in,
iovcnt_in, iov_out, iovcnt_out)” system call?
Does anyone have a better idea for an interface that will allow zero-copy
interfacing to a transformation driver from userspace?

                -az



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