Re: Request for feedback on common data backstore in the kernel

Hi Scott,

The discussion has been moved to "freebsd-arch@xxxxxxxxxxx". Please only reply
there next time.

On Wednesday 26 September 2007, Scott Long wrote:
Hans Petter Selasky wrote:
Hi,

Please keep me CC'ed, hence I'm not on all these lists.

In the kernel we currently have two different data backstores:

struct mbuf

and

struct buf

These two backstores serve two different device types. "mbufs" are for
network devices and "buf" is for disk devices.

Problem:

The current backstores are loaded into DMA by using the BUS-DMA
framework. This appears not to be too fast according to Kip Macy. See:

http://perforce.freebsd.org/chv.cgi?CH=126455

Busdma isn't fast enough for 1Gb and 10Gb network drivers that are
trying to max out their packet rates. It's still fine for storage
drivers and other slow/medium speed device drivers, like USB and
Firewire. I am working on techniques to make the API easier to use
and the implementation fast enough for the aforementioned devices.

That's great!


Some ideas I have:

When a buffer is out out of range for a hardware device and a data-copy
is needed I want to simply copy that data in smaller parts to/from a
pre-allocated bounce buffer. I want to avoid allocating this buffer when
"bus_dmamap_load()" is called.

I think you've missed the point of having architecture portable drivers.
John-Mark describes this further.

I use the bus_dma framework to allocate and sync all DMA memory, and I assume
that is correct.

It also makes little sense to push
the responsibility for handling bounce buffers out of a central
subsystem and back into every driver.

I'm thinking about putting some wrappers around the "bus_dmamap_load()"
function like:

void usbd_rx_buf_load(struct usbd_xfer *xfer, void *buf,
uint32_t framebuffer_offset, uint32_t framebuffer_index, uint32_t len);

void usbd_tx_buf_load(struct usbd_xfer *xfer, void *buf,
uint32_t framebuffer_offset, uint32_t framebuffer_index, uint32_t len);

But I haven't figured out all the details yet. The "usbd_xxx_load()" functions
should automagically figure out what is best to do and it won't be more than
a few lines of code.

--HPS
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