[PATCH] Device Tree binding for the 'mv_xor' XOR engine DMA driver

-----Original Message-----
From: Andrew Lunn [mailto:andrew at lunn.ch]
Sent: Monday, November 19, 2012 8:41 AM
To: Lior Amsalem
Cc: Andrew Lunn; Thomas Petazzoni; Vinod Koul; Dan Williams; Saeed
Bishara; Jason Cooper; Gregory Clement; Maen Suleiman; linux-arm-
kernel at lists.infradead.org; Sebastian Hesselbarth
Subject: Re: [PATCH] Device Tree binding for the 'mv_xor' XOR engine DMA
driver

quoted

quoted

The dmatest shows up a second issue:

root at qnap:~# insmod ./dmatest.ko iterations=100
dmatest: Started 2 threads using dma0chan0
dmatest: Started 2 threads using dma1chan0
dmatest: Started 2 threads using dma2chan0
dmatest: Started 2 threads using dma3chan0
dma3chan0-xor0: #3: prep error with src_off=0x13c2 dst_off=0x2c7c
len=0x5d
dma1chan0-copy0: #16: prep error with src_off=0x1f50 dst_off=0x904
len=0x33
dma3chan0-copy0: #22: prep error with src_off=0x14c2 dst_off=0x389b
len=0x6f
dma0chan0-copy0: #31: prep error with src_off=0x10b6 dst_off=0x323c
len=0x2f
dma3chan0-copy0: #49: prep error with src_off=0x17b5 dst_off=0x3eca
len=0x29

The driver refuses any operation where the buffer is less than 128
bytes. The datasheet for Kirkwood says the buffer length must be 8
bytes or more. So maybe we should reduce this 128 limit down to 8?

This limitation (128B) is only in memset operations (set in registers and not

descriptors).

quoted

If I recall correctly the limit is 16B and not 8B.

In any way, this 128 bytes seems like a good logical boundary for XOR
HW enabling. (performance wise)

Hi Lior

Agreed. Anything smaller than that, and you spend more time setting up the
hardware and dealing with interrupts when its finished than doing it in
software.

I had quick look at some of the call sites. Users of the DMA API under the
crypto directory all fall back to software operations when the DMA API
returns an error. Also, raid uses the crypto API for memcpy and XOR. So raid
seems O.K. I could not find any other users of the DMA API for _xor, _copy,
or _memset. So it seems safe to return an error for inefficient usage.

I'm not aware for any other user of the DMA APIs, we use it mainly for RAID5 XOR offload.
So yes, it seems ok to return an error.

      Andrew

Regards,
Lior Amsalem