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