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

[Vitaly Wool]

Mark Underwood wrote:

> --- vitalhome@xxxxxxxxxx wrote:
>
>  
>
> > Mark,
> >
> >    
> >
> > > > > I still do not see why you are stating this.  Why do you say this?
> > > > >
> > > > >
> > > > >          
> > > > Due to possible priority inversion problems in David's core.
> > > >        
> > > Which you still haven't proven, in fact you now seem to be changing your mind and saying 
> > > that
> > > there might be a problem if an adapter driver was implemented badly although I still 
> > > don't see how
> > > this could happen (the priority inversion I mean not the badly implemented driver ;).
> > >      
> > Truly admiring your deep understanding of the real-time technology, I should remind you 
> > that within the real-time conditions almost each event may happen and may not happen, for 
> > instance, two calls from different context to the same funtion may happen at the same or 
> > almost the same time, and may not happen that way. Therefore I used the word "possible". 
> > Hope I clarified that a bit for you.
> >
> > Please also see my previous emails for the explanation of how priority inversion can 
> > happen. This is not gonna be a rare case, BTW.
> >    
>
> Vitaly, 
>
> First, please can you not change the CC list in the midle of a thread.
Yeah, sorry for that. You see, I was emailing not from my computer.

> OK, looking through the code after a cup of coffe I can see the problem you are pointing out,
> thank you :), for some reason I thought that that code was protected by a spin_lock :/. 
>
> How to fix this? 
>
> David, how would you feel about adding a NOT_DMAABLE flag in the spi_message structure? This
> helper routine could then use this thus solving the one buffer to many callers problem (well
> moving into the adapter driver, but as that serialise's transfers anyway I think this would remove
> the priority inversion problem, Vitaly?) 
>
> The other solution is to do a kmalloc for each caller (would could try to be smart and only do
> that if the buffer is being used). 
>  
And each one of the techniques suggested will make David's core closer 
to ours :)

Vitaly
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