Re: EXPORT_SYMBOL(fat_get_block)

[Greg KH]

On Fri, Aug 13, 2010 at 12:17:39PM -0700, David Cross wrote:
> On Fri, 2010-08-13 at 12:01 -0700, Greg KH wrote:
> > On Fri, Aug 13, 2010 at 11:43:12AM -0700, David Cross wrote:
> > > On Fri, 2010-08-13 at 10:54 -0700, Greg KH wrote:
> > > > On Fri, Aug 13, 2010 at 10:45:39AM -0700, David Cross wrote:
> > > > > Hello Hirofumi,
> > > > > I would like to export this symbol from the vfat code and add this patch
> > > > > to the Linux kernel. You are listed as the MAINTAINER for FAT and VFAT.
> > > > > As such, I need your approval to do it. 
> > > > > The reason that I need to export this symbol is to allow for file based
> > > > > DMA in which the file needs to be pre-allocated. The pre-allocated block
> > > > > addresses are passed to a DMA engine which then directly transfers the
> > > > > data to non-volatile storage.
> > > > > Please let me know if you have any objections to exporting this symbol
> > > > > or if you need additional information from me concerning this change
> > > > > request. I am happy to answer any questions that you may have about this
> > > > > in the meantime.
> > > > 
> > > > Wait, _I_ have an objection to exporting this symbol.
> > > > 
> > > > I really don't think it is needed at this point in time, as we really
> > > > need to figure out exactly what your driver is doing to want to need to
> > > > call this function.
> > > 
> > > OK, I am trying to answer all questions on this topic that I am getting,
> > > but honestly I have not gotten a lot so far. 
> > 
> > That's because not many people have noticed the code yet :)
> > 
> > > > So please, let's not export it for now, when we get to the point that we
> > > > all agree that this driver is doing the "correct" thing, then we can
> > > > export the symbol, ok?
> > > 
> > > Sure, I think that the driver is doing the correct thing, but that is
> > > mostly because I have not found another way to solve this issue.
> > 
> > What specifically is the issue here that you are trying to solve?
>   We are trying to solve the issue of all data needing to go through the
> processor, even when its ultimate destination is non-volatile media.
> 	
> > And why has no other driver ever needed this type of functionality
> > before?
>   I am not sure, but my guess is because this represents a novel solution to 
> a performance problem.

What exactly are the performance issues with doing this from userspace,
vs. the FAT hack?

> > We have a userspace MTP driver for Linux, using gadgetfs, right?  So
> > none of this is applicable from what I can tell.
> Yes, the g_mtp development has started, but it is not integrated yet
> last I checked. Most of the applications for this driver have used
> gadgetfs as well in order to handle the protocol itself. So, I think it
> is applicable.

No, there's another MTP stack already released that works just fine on
Linux.  You can find it at:
	http://wiki.meego.com/Buteo

> > > The West Bridge driver goes for option two for performance reasons. In
> > > doing this, it needs to get information from the file system on where to
> > > store the file.
> > 
> > Look at how Linux already handles MTP for how to handle this properly, I
> > don't think there is any need to do any of this from within the kernel.
> I somewhat familiar with how Linux handles MTP. The current model is
> CPU-centric and all data goes through the main processor from what I
> have seen. This is a working solution, but not always a very fast one. I
> agree though that this would not need to be done within the kernel if we
> had a userspace method for file allocation and commitment.

Again, what's wrong with using the processor here?  What else does it
have to do at this point in time?

> > > >What happens if this isn't a FAT partition on the >device?
> > > Good question. So far, it has been stored on a FAT partition in most use
> > > cases because the user typically wants the option to enumerate the
> > > device as mass storage as well or be able to see content on a PC if the
> > > SD card is removed. However, there is no reason that this could not be
> > > done with ext2 or other filesystems on non-removable media.
> > 
> > Like NTFS?  How are you going to handle that when you run out of size
> > due to the limitations of FAT?  Hint, that's not going to work with this
> > type of solution...
> Isn't this also a userspace problem? When I run out of space on my Linux machine,
> the message "no space left on device" pops up. Why is this solution any
> more prone to size limitations compared with any other?

No, my point is that for larger disk sizes, you can't use FAT, you have
to use NTFS to be interoperable with other operating systems.  Your
solution will not handle that jump to larger storage sizes as you are
relying on FAT.

> > > This would require some work to figure out how to implement a
> > > pre-allocation interface with it. Alternatively a more general
> > > pre-allocation interface could be added (falloc()) to implemented file
> > > systems to make this process easier.  
> > 
> > Userspace handles this quite easily already, see above.
> I don't see how userspace handles this from the comments above. I do
> understand that there is a userspace MTP driver, but I don't see a
> method for pre-allocation of files from the information above. Am I
> missing something?

Yes, the pre-allocation is done in userspace, and then the data is
copied to the filesystem then.  The kernel doesn't have to have any
filesystem specific hacks in it to try to handle this at all.

Take a look at the above link for what you might want to do instead.
Because of this, I'm guessing that a lot of this code can be removed
from the driver, right?

thanks,

greg k-h
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