Re: [PATCH 2.6-git] SPI core refresh

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

David Brownell wrote:

> > Yeah thus we don't have an ability to allocate SPI messages on stack as 
> > you do, that's what votes for your approach. Yours is thus a bit faster, 
> > though I suspect that this method is a possible *danger* for really 
> > high-speed devices with data bursts on the SPI bus like WiFi adapters: 
> > stack's gonna suffer from large amounts of data allocated.
> >    
>
> No, you're still thinking about a purely synchronous programming model;
> which we had agreed ages ago was not required.
>  
Ah yes. But wait... I've got an important question here.
For instance, let's take your MTD driver. You're allocating a message 
structure on stack and passing it then down to spi->master->transfer 
function.
The benefit you're talking about is that you don't have to use 
heavyweight memory allocation. But... the transfer is basically async so 
spi->master->transfer will need to copy your message structure to its 
own-allocated structure so some memory copying will occur as this might 
be an async transfer (and therefore the stack-allocated message may be 
freed at some point when it's yet used!)
So your model implies concealed double message allocation/copying, 
doesn't it?
And if I'm wrong, can you please explain me this?

Vitaly
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