Hi,
I'm currently working on a Linux USB emulation layer for FreeBSD. In that
regard I have some issues that makes the stack peform non-optimal due to its
API design.
The main issue is that DMA'able memory is allocated per USB host controller on
Linux and not per USB host controller and pipe combination. Due to that I
have to pre-allocate memory for all pipes to avoid blocking behaviour when
one submits an URB. Further I have to split all large transfers into a fixed
buffer size. If the fixed buffer is too large I will waste memory. If it is
too small I will get lousy performance.
What I would suggest is that when you allocate an URB and DMA'able memory, you
have to specify which pipe {CONTROL, BULK, INTERRUPT or ISOC} it belongs to.
What do you think?
The reason is that in the new USB stack on FreeBSD, the USB transfer
descriptors are allocated along with the data-buffer, so that when you
unsetup an USB transfer, absolutely all memory related to a transfer is
freed. This also has a security implication in that when you have
pre-allocated all buffers and all USB host controller descriptors, you will
never get in the situation of not being able to allocate transfer descriptors
on the fly, like done on Linux.
--HPS
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