Re: EXPORT_SYMBOL(fat_get_block)

[David Cross]

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. 

> 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. If
there is a different, correct way I am happy to implement it.

> thanks,

>Also, why do this at the FAT level, and not more correctly at the block
> layer level? 
The short answer is that the block level does not have the file system
awareness needed to determine where a file should be stored on the NVM
device. 
The longer answer is to consider the case of an MTP host, which operates
at the file level. It sends the mobile (Linux) device "SendObject" which
tells it to store the file on its file system. When the device receives
this, it has two options:
1) receive all data, buffer it, then write the file. This is typically a
slow process USB->Processor->SDRAM->Processor/DMA engine->Media
2) pre-allocate the file as soon as it knows that it is coming and how
big it is, and then send the block addresses to an external DMA engine
and let it transfer the data from the MTP host directly

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.

>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. 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.  

Please let me know if there are any additional questions or
implementation feedback on this.
Thanks,
David


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