linux-kernel - Re: [patch 1/5] Staging: VME Framework for the Linux Kernel

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Date:	Mon, 10 Aug 2009 21:38:49 +0200
From:	"Emilio G. Cota" <cota@...ap.org>
To:	Martyn Welch <martyn.welch@...anuc.com>
Cc:	Greg K-H <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	devel@...uxdriverproject.org,
	Sebastien Dugue <sebastien.dugue@...l.net>
Subject: Re: [patch 1/5] Staging: VME Framework for the Linux Kernel

Martyn Welch wrote:
> That's one reason for the helper function. If VME bridges are added  
> which sit of on other buses then vme_alloc_consistent can be extended to  
> support this without requiring VME device drivers to be altered to  
> reflect the fact that pci_alloc_consistent might not work for all VME  
> bridges.

But it corresponds to the bridge's code (not to the generic layer) to
know whether the bridge sits on top of PCI or not; hence my comment.

> The DMA controller in the tsi-148 can do PCI to PCI; PCI to VME; VME to  
> PCI; VME to VME; Patterns to  VME and Patterns to PCI transfers. How do  
> you specify all those options without providing a structure where over  
> 50% of the fields aren't used for any given transfer?

I think a struct where not _every_ field is *always* used is fine.
However requiring your drivers to know about what your bridge
looks like is a mistake; just give them something thin that doesn't
assume anything about what's behind it, and if it's not supported,
return an appropriate error message.

> Every bridge I have seen is capable of link-list execution. The API  
> provides the ability to do scatter-gather style DMA transfers, this  
> could be used as part of a "zero-copy" type driver for high speed VME  
> capture devices. How would you support the link-list execution with a  
> single call?

Let me say it again: "Drivers should know *nothing* about the
underlying bridge".
They should work with *any* bridge; or do we care about the PCI
bridge we're plug our PCI device on when writing a driver for it?

> I was also looking at the ability to queue DMA transfers if the  
> controller was currently busy - if the consensus is that this is  
> overkill I will happily remove this.

AFAICT the tsi148 has several DMA channels; in our driver we just
try to find a free channel to proceed with the transfer; if there
aren't any then we return EBUSY.
Drawing an analogy with current 'streaming DMA' transfers, we're
told we should check whether they failed or not, so I think
trying to be too clever here it's not worth it.

> I implemented it using param array, I agree that the arguments might get
> quite long with 10 devices, especially if multiple parameters are
> required, but I don't see why it shouldn't work.

The fact that 'it works' doesn't make it less ugly or messy.
When referring to code, these two words usually mean "there's
a better way"; so let's just find it.

E.
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