Messages in this thread | | | Date | Wed, 4 Jan 2006 10:58:56 +0530 | From | Deven Balani <> | Subject | Re: Ping-Pong Compatible DMA buffer chaining |
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Hi Sir,
Basically My hardware is not allowing to do a scatter-gather list. Might be my next version of Hardware _could_ allow me. So I decided to come out with a _Work Around_.
please find attached sg_pingpong.txt.
My hardware has two Buffers B0 and B1 for both Tx and Rx path. I had to fill the first buffer initially and then _continue_ a walk through the sg list with help of buffer completion interrupts of B0 & B1.
Can you suggest any inputs for this scenario.
Regards, Deven
On 1/3/06, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote: > On Llu, 2005-12-26 at 11:35 +0530, Deven Balani wrote: > > 1) The SATA Host Controller supports a DMA Logic which has Ping Pong Buffers. > > How Can I allocate a linear contiguous buffer and give it to the > > Buffer Descriptors of the Host Controller. I believe consistent alloc > > would help me in this regard. But How could I do a chaining of Buffers > > in a Ping Pong Scenario which has Buffer Descriptors. > > Depends entirely on your hardware. > > > 2) The libata is using Scatter-Gather List to send Device Identify > > Command. Is it necessary to have a Scatter-Gather List for non-PCI ARM > > platforms. Can I bypass this mechanism. > > It is neccessary as the user space virtual memory will be fragmented in > physical space (unless you are using MMUless Linux when it will be far > less fragmented). > > If the platform is an MMUless one then you can fill in the host > structure and indicate that you support a maximum scatter gather list > size of one. This will break up I/O's when neccessary to keep within > that limit but this will hurt your performance by causing a lot more > requests on a non MMUless system. > > Alan > >
-- "A smile confuses an approaching frown..." For conversion from SG list to ping pong:
Generate new sglist based on original sglist but taking into account MAX_DMA_BUF_SIZE bytes as the maximum per buffer. This will simplify your continuance logic significantly.
For each entry in Sgentries set entry in progress to true while entry in progress if entry.buffersize larger than MAX_DMA_BUF_SIZE Add a new entry to SGentries1 a buffer of MAX_DMA_BUF_SIZE Decrement entry.buffersize by MAX_DMA_BUF_SIZE Increment entry.pointer by MAX_DMA_BUF_SIZE else Add a new entry to SGentries1 with a copy of current entry set entry in progress to false endwhile endwhile
while more SGentries1 and at least one buffer free populate first active buffer with entry Remove entry from SGentries1 toggle active buffer endwhile
if SGentries1 is not empty enable DMA buffer complete interrupt endif
Start DMA.
In your DMA buffer complete interrupt:
if SGentries1 is not empty populate the completed buffer activate newly populated buffer endif check the active buffer (the other one) to see if it just completed if this other one just completed if Sgentries is not empty populate this buffer endif endif if SGentries1 is STILL not empty at this point enable DMA buffer interrupt else disable DMA buffer interrupt endif
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