Re: [PATCH/RFC] SPI: add DMAUNSAFE analog to David Brownell's core

[Vitaly Wool <vwool@ru.mvista.com>]

David Brownell wrote:

> On Wednesday 14 December 2005 5:50 am, Vitaly Wool wrote:
>  
>
> > > > It's way better to just insist that all I/O buffers (in all
> > > > generic APIs) be DMA-safe.  AFAICT that's a pretty standard
> > > > rule everywhere in Linux.
> > > >        
> > > I agree.  
> > >      
> > Well, why then David doesn't insist on that in his own code?
> > His synchronous transfer functions
> >
> > are allocating transfer buffers on  
> > stack which is not DMA-safe.
> >    
>
> I think the very first version did that, but nothing since
> has taken that shortcut.  (Several months now.)  It uses
> a buffer that's allocated on the heap.
>  
You're wrong.
Isn't it a part of your code:

static inline ssize_t spi_w8r8(struct spi_device *spi, u8 cmd)
{
       ssize_t                 status;
       u8                      result;

       status = spi_write_then_read(spi, &cmd, 1, &result, 1);

       /* return negative errno or unsigned value */
       return (status < 0) ? status : result;
}

You're allocating u8 var on stack, then allocate a 1-byte-long buffer 
and copy the data instead of letting the controller driver decide 
whether this allocation/copy is necessary or not. And in 99% cases it's 
not gonna be necessary as any controller driver w/o brain damage will 
transfer this as PIO.
So the overhead for sending/receiving 1 byte will be _very big_ 
(trylock, kmalloc, memcpy). See now what I'm talking about?

>  
>
> > Then he starts messing with allocate-or-use-preallocated stuff etc. etc.
> > Why isn't he just kmalloc'ing/kfree'ing buffers each time these 
> > functions are called 
> >    
>
> So that the typical case, with little SPI contention, doesn't
> hit the heap?  That's sure what I thought ... though I can't speak
> for what other people may think I thought.  You were the one that
> wanted to optimize the atypical case to remove a blocking path!
>  
I meant kmalloc'ing/kfree'ing buffers is spi_w8r8/spi_w8r16/etc.

Vitaly
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