[spi-devel-general] Re: [PATCH 2.6-git] SPI core refresh

[Vitaly W. <vw...@ru...>]

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