linux-kernel - Re: [PATCH] fsldma: use PCI Read Multiple command

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Date:	Mon, 27 Apr 2009 13:22:46 -0700
From:	David Hawkins <dwh@...o.caltech.edu>
To:	Timur Tabi <timur@...escale.com>
CC:	Liu Dave-R63238 <DaveLiu@...escale.com>,
	Ira Snyder <iws@...o.caltech.edu>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH] fsldma: use PCI Read Multiple command

Hi Timur,

>> Would you like some sort of summary of this info for a commit
>> message?
> 
> That's probably overkill.  I just want a sentence or two that tells
> someone looking at the code casually that the behavior of reading PCI
> memory might be different than what they expect.

Could you help us with the wording you'd like to see in the code.
Did you want to see something in the header comments, or something
near the register settings?

How about something like this in place of the existing PCI_RM
comment:

PRC_RM - PCI read multiple
   The default PCI read command used by the DMA controller is
   PCI Read (PCI command 6h). When the burst length is 32-bytes
   or longer, PCI Read Line (PCI command Eh) is used (undocumented
   feature of the controller). Using PCI read multiple
   (PCI command Ch) results in high-performance across PCI
   bridges. DMA transfers to non-prefetchable PCI registers
   should not result in prefetched reads, even when using
   the PCI read multiple command.

Cheers,
Dave

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