Re: PATA IDE is slower in newer versions of kernel

[Alejandro Riveira FernÃndez]

El Thu, 16 Sep 2010 17:30:51 -0600
"Joshua Hintze" <joshh@xxxxxxxxx> escribiÃ:

 [ Just CC linux-ide ]

> Hello, 
> 
> I've been digging into this for a couple weeks now. I have an embedded board
> that runs Linux Kernel 2.6.10 that is connected to a SSD over a PATA
> connection. Data is transferred using UDMA4 speeds and I get a nice
> sustained write speed of about 20 MBps which is adequate for us. 
> 
> I have recently taken the plunge to update to a newer version of the kernel
> starting at 2.6.32 (also tried 2.6.33) and what I am seeing writes speeds
> drop to about 18 MBps peak with large jumps going from 8 MBps->16 MBs for
> sustained throughput.
> 
> I decided to read up on the ATA specification and I connected a logic
> analyzer to the PATA bus and here is what is happening...after a large chunk
> of data is written to the device, instead of the device pausing the transfer
> by asserting DDMARDY it actual initiates a device data-out termination by
> pulling DMARQ low.
> 
> The old kernel 2.6.10 responds to this by strobing the STOP line 8 times
> within 800uS and the device releases DMARQ shortly afterwards. On the newer
> kernels the strobing of the STOP line takes near 8mS of time. Since the hard
> drive only release DMARQ after this 8 pulse strobe I believe this is the
> cause of the slower write speeds on the newer kernels. This whole process
> happens thousands of times when writing megs of data. So those extra 7mS
> begin to add up fast.
> 
> My problem is I've been digging through the ide.c/ide-dma.c and other code
> but I'm not exactly sure where the code would jump to upon a device
> initiated termination on a data-out dma transfer.
> 
> My guess is ide_dma_intr(...). Is this correct?
> 
> Any other areas I could look at?
> 
> Thanks in advance.
> 
> 
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