Re: O_DIRECT address restrictions

["Rick Smith" <rgsmith72@xxxxxxxxxxx>]

Yes, I am referring to memory physically on the video card itself (PCI), not 
system RAM. I am aware of the page alignment requirements of O_DIRECT under 
XFS, but usually I get an "invalid argument" (EINVAL) error when this 
requirement is not satisfied instead of the "bad address" (EFAULT) error 
that I am getting. Looking at the XFS code, there are some places where it 
bails with EFAULT after checking alignment, so I will look further into 
those. I have an API that returns the address of the actual video memory 
buffer on the card and I would like to be able to use O_DIRECT to blast data 
directly there in one step without a kernel buffer and a DMA step. Like I 
said, without O_DIRECT I can do the I/O to the video memory, but I need the 
speed of O_DIRECT. Is this possible?

Rick

> From: "Jeremy Jackson" <jerj@xxxxxxxxxxxx>
> To: "Rick Smith" <rgsmith72@xxxxxxxxxxx>, <linux-xfs@xxxxxxxxxxx>
> Subject: Re: O_DIRECT address restrictions
> Date: Fri, 9 May 2003 10:55:52 -0400
>
> Hmm interesting... when you say mmaped video memory, do you mean PCI/AGP
> memory *on* the video card, or system RAM?  I believe some of the replies
> indicating that it should work were talking about system RAM?
>
> I think the normal case is DMA from scsi card or ide or whatever to/from
> system RAM.  I don't think (but I don't know that much) that DMA from a 
> scsi
> card to another PCI card is the normal case.  Can anyone comment furthur?
>
> Jeremy
> ----- Original Message -----
> From: "Rick Smith" <rgsmith72@xxxxxxxxxxx>
> To: <linux-xfs@xxxxxxxxxxx>
> Sent: Thursday, May 08, 2003 6:16 PM
> Subject: O_DIRECT address restrictions
>
>
> > Hello,
> >      What are the memory address restrictions when using the O_DIRECT 
> flag
> > with XFS? I am trying to perform I/O directly from the filesystem to a
> piece
> > of mmap'ed video memory and I get the "bad address" error when I attempt
> to
> > do so with O_DIRECT. Without the O_DIRECT flag I am able to perform the
> I/O
> > but is it must too slow. Using O_DIRECT with other system memory seems 
> to
> > work fine, but I would like the option of transferring data directly to 
> a
> > video card without a bounce buffer and a DMA step. Is direct I/O 
> available
> > to any memory address that satisfies the alignment requirement or must
> this
> > address be below the MAX_DMA_ADDR?
> >
> > Any help appreciated. Thanks.
> >
> > Rick Smith
> >
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