Re: [PATCH/RFC 0/2] simple SPI framework, refresh + ads7864 driver

[Mark Underwood]

--- David Brownell <david-b@xxxxxxxxxxx> wrote:

> > Of course we want to use scatter-gather lists.
> 
> The only way "of course" applies is if you're accepting requests
> from the block layer, which talks in terms of "struct scatterlist".

If you look at the mmci driver (drivers/mcc/mmci.c) then you will see the way that driver
transfers data is do to one sg element at a time. I have added DMA support to this driver which
also does one element at a time (which uses the ARM PL080 DMAC). sg lists should not be for the
SPI layer to worry about. The fact that the PL080 DMAC controller can only transfer (4096-1)K
objects (the size of the object being the source size) should be of no concern to the driver that
uses DMA (either directly for through the SPI subsystem). It is the job of the PL080 driver to
split the requested transfer into several transfers that it can handle.

> 
> In my investigations of SPI, I don't happen to have come across any
> SPI slave device that would naturally be handled as a block device.
> There's lots of flash (and dataflash); that's MTD, not block.
> 
> 
> > The DMA controller 
> > mentioned above can handle only 0xFFF transfer units at a transfer so we 
> > have to split the large transfers into SG lists.
> 
> Odd, I've seen plenty other drivers that just segment large buffers
> into multiple DMA transfers ... without wanting "struct scatterlist".
> 
>   - Sometimes they turn them into lists of DMA descriptors handled
>     by their DMA controller.  Even the silicon designers who talk
>     to Linux developers wouldn't choose "struct scatterlist" to
>     hold those descriptors
> 
>   - More often they just break big buffers into lots of little
>     transfers.  Just like PIO, but faster.  (And in fact, they may
>     need to prime the pump with some PIO to align the buffer.)
> 
>   - Sometimes they just reject segments that are too large to
>     handle cleanly at a low level, and require higher level code
>     to provide more byte-sized blocks of I/O.
> 
> If "now" _were_ the point we need to handle scatterlists, I've shown
> a nice efficient way to handle them, already well proven in the context
> of another serial bus protocol (USB).
> 
> 
> > Moreover, that looks like it may imply redundant data copying.
> 
> Absolutely not.  Everything was aimed at zero-copy I/O; why do
> you think I carefully described "DMA mapping" everywhere, rather
> than "memcpy"?
> 
> 
> > Can you please elaborate what you meant by 'readiness to accept DMA 
> > addresses' for the controller drivers?
> 
> Go look at the parts of the USB stack I mentioned.  That's what I mean.
> 
>  - In the one case, DMA-aware controller drivers look at each buffer
>    to determine whether they have to manage the mappings themselves.
>    If the caller provided the DMA address, they won't set up mappings.
> 
>  - In the other case, they always expect their caller to have set
>    up the DMA mappings.  (Where "caller" is infrastructure code,
>    not the actual driver issuing the I/O request.)
> 
> The guts of such drivers would only talk in terms of DMA; the way those
> cases differ is how the driver entry/exit points ensure that can be done.
> 
> 
> > As far as I see it now, the whole thing looks wrong. The thing that we 
> > suggest (i. e. abstract handles for memory allocation set to kmalloc by 
> > default) is looking far better IMHO and doesn't require any flags which 
> > usage increases uncertainty in the core.
> 
> You are conflating memory allocation with DMA mapping.  Those notions
> are quite distinct, except for dma_alloc_coherent() where one operation
> does both.
> 
> The normal goal for drivers is to accept buffers allocated from anywhere
> that Documentation/DMA-mapping.txt describes as being DMA-safe ... and
> less often, message passing frameworks will do what USB does and accept
> DMA addresses rather than CPU addresses.
> 
> - Dave
> 
> -
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