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

[Lior Amsalem]

> -----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
> 
> > > 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).
> > 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