Re: [PATCH] enhanced reimplemented of the kfifo API

[Stefani Seibold <stefani@seibold.net>]

Am Freitag, den 15.01.2010, 15:38 -0800 schrieb Ira W. Snyder:
> On Fri, Jan 15, 2010 at 11:05:07PM +0100, Stefani Seibold wrote:
> > Am Freitag, den 15.01.2010, 14:01 -0800 schrieb Ira W. Snyder:
> > > On Fri, Jan 15, 2010 at 10:30:01PM +0100, Stefani Seibold wrote:
> > > > 
> > > > > I'm glad to see this example, it really illustrates how to use the new
> > > > > DMA functionality of the kfifo API.
> > > > > 
> > > > > Is there any reason why a very large scatterlist could not be used? I
> > > > > have a driver that uses a large scatterlist (~4000 entries, allocated as
> > > > > a struct sg_table). I implemented my own copy_from_user() functionality
> > > > > into this scatterlist, but I'd love to use the kfifo DMA API instead.
> > > > > After filling the scatterlist, I use the usual DMA API's to transfer it
> > > > > to my device.
> > > > > 
> > > > 
> > > > No, but the API will only return max. two entries.
> > > > 
> > > 
> > > Ah, I see. __kfifo_alloc() uses kmalloc() internally. The kfifo API will
> > > not work for my purposes then, since I want to allocate a 16MB chunk of
> > > memory, and I'm very uncomfortable doing that with kmalloc(). I'll stick
> > > with my custom scatterlist code, which uses alloc_page() to fill in the
> > > scatterlist with order-0 allocations.
> > > 
> > 
> > I think you can use vmalloc() instead and assign this buffer with
> > kfifo_init().
> > 
> 
> Yes, that might be an option. I presume I could still use dma_map_sg()
> on the resulting scatterlist.
> 
> > > Another nitpick in the code: I've noticed that you use sgl++ in the
> > > setup_sgl() function. That should become "sgl = sg_next(sgl);" so that
> > > this code can work with an struct sg_table as well.
> > 
> > No, its by definition an array. You cannot pass as struct sg_table to
> > the kfifo_dma_* functions.
> > 
> 
> A struct sg_table is a container for an underlying chain of struct
> scatterlists, allowing for easier allocation. See the definition of
> struct sg_table and the sgl member (include/linux/scatterlist.h line
> 11).
> On Fri, Jan 15, 2010 at 11:05:07PM +0100, Stefani Seibold wrote:
> > > Am Freitag, den 15.01.2010, 14:01 -0800 schrieb Ira W. Snyder:
> > > 
> > > > Ah, I see. __kfifo_alloc() uses kmalloc() internally. The kfifo API will
> > > > not work for my purposes then, since I want to allocate a 16MB chunk of
> > > > memory, and I'm very uncomfortable doing that with kmalloc(). I'll stick
> > > > with my custom scatterlist code, which uses alloc_page() to fill in the
> > > > scatterlist with order-0 allocations.
> > > > 
> > > 
> > > I think you can use vmalloc() instead and assign this buffer with
> > > kfifo_init().
> > > 
> > 
> > Yes, that might be an option. I presume I could still use dma_map_sg()
> > 
> IIRC, struct sg_table allocates arrays of struct scatterlist in
> PAGE_SIZE chunks, and chains them together.
> 
> If you change from "sg++" to "sg_next()", you should be able to work
> with chained scatterlists. You can only have a single struct page * in
> one struct scatterlist. This would mean that if I tried to use
> kfifo_dma_out_prepare() to give me 16MB worth of data, I would need 4096
> seperate struct scatterlist entries. One for each struct page * in that
> 16MB chunk.
> 

No, this would require an initialized scatterlist array and the
kfifo_dma_* function does not expect this. And i am not sure why you
need 4096 seperate struct scatterlist entries for a continues memory, i
need only one. The scatterlist structure contains the page, the offset
and the length. The length can be more the a page size.

Stefani