Re: Direct transfer between SCSI and PCI

[Steve Rottinger]

The SCSI device would use it's DMA controller to copy the data, just as it
would if the buffer was in the processor's RAM.  If I understand
correctly, if I open up the SCSI device using the O_DIRECT option, the
driver will DMA directly to/from the user buffer specified by the
write()/read() functions. Instead, of that buffer being an address
pointing to the processors RAM, I would like that buffer to point to the
PCI address of the buffer on my PCI device. Currently, when I try this on
an mmapped buffer to my PCI device, the write() routine returns a "Bad
address" error.

-Steve


> On 9.3.2009 14:29, Steve Rottinger wrote:
>>As part
>> of my project, I need to be able to directly transfer data from a large
>> buffer that resides on
>> a custom PCI communications device to a SCSI disk without going through
>> the CPU's main
>> memory.  Initially, it seemed like opening up the raw SCSI device in
>> O_DIRECT mode would
>> be the perfect solution.  However, through some experimentation, and
>> reading some posts from
>> a few years back, it appears that O_DIRECT only works on buffer's that
>> reside in processor's
>> main memory.   Does anyone have any suggestions as to how to get this to
>> work?
>
> How would you want your scsi device to get the data? I suppose they are
> stored somewhere on the PCI device memory connected to its local bus or
> alike. At best it can be visible as a PCI bar but I doubt it is. As you
> described it here, there is no way, how would the SCSI device see the
> data.
>
> Or do you want to move the data from the PCI device via DMA implemented
> in it directly to the SCSI device? How would you do this? I guess SCSI
> devices do standard S/G DMA so that they read data from host memory by
> themselves.
>
> Maybe if you elaborate on what exactly do you mean by the idea?
>


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