[maemo-developers] WLAN Horrible Roaming Performance (N800, OS2008), Software or Hardware Problem ?

[From: Frantisek Dufka]

Kalle Valo wrote:
>> Also CPU usage is very high because of busyloop when waiting till
>> DMA transfer is done. Tasklet, which executes the code can't be
>> easily preempted, as far as I understand kernel documentation. Maybe
>> it is possible to split tasklet into several parts, one of them
>> could be responsible for initiating DMA transfer, the other could be
>> activated on DMA transfer completion? This all is important for
>> video streaming as any excessive CPU resources consumption by WLAN
>> driver negatively impacts video playback performance.
> 
> Sorry, I'm not familiar with OMAP 1710 McBSP, so I can't comment.
> 

I think you don't have to. From the code (sm_drv_spi_io.c) it looks like 
McBSP is setup to use dma transfer with callback when it finishes

omap_request_dma(OMAP_DMA_MCBSP2_TX, "McBSP TX", dma_tx_callback, ...
omap_request_dma(OMAP_DMA_MCBSP2_RX, "McBSP RX", dma_rx_callback, ....

and the dma_tx/rx_callback() code just sets variable 
spi_dma.dma__tx/rx_done to 1.

But the code that sends/receives the frame busyloops for it like this

         omap_start_dma(spi_dma.dma_rx_ch); 
 

         omap_start_dma(spi_dma.dma_tx_ch); 
 

         while(!spi_dma.dma_rx_done) { 
 

                 udelay(5); 
 

         } 
 

 
 
                     while(!spi_dma.dma_tx_done) { 
 

                 udelay(5); 
 

         } 
 

 
 
                    spi_dma.dma_rx_done = 0; 
 

         spi_dma.dma_tx_done = 0; 
 



So there is this nice dma architecture with callback used but the code 
still spins up the cpu waiting for the done flag instead of sleeping.

So you need to be familiar with the driver and tell us if it is possible 
to sleep inside cx3110x_spi_dma_read and cx3110x_spi_dma_write. And one 
also needs to be familiar with kernel programming and waiting primitives 
to suggest how to sleep and wait for the callback (if possible in this 
context) and how to wake up the sleeping code from the dma callback.

Frantisek