linux-kernel - Re: [PATCH 01/16] Blackfin SPI Driver: ensure cache coherency before doing DMA

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Date:	Thu, 20 Nov 2008 16:57:12 -0500
From:	"Mike Frysinger" <vapier.adi@...il.com>
To:	"David Brownell" <david-b@...bell.net>
Cc:	"Bryan Wu" <cooloney@...nel.org>,
	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	"Vitja Makarov" <vitja.makarov@...il.com>
Subject: Re: [PATCH 01/16] Blackfin SPI Driver: ensure cache coherency before doing DMA

On Thu, Nov 20, 2008 at 16:47, David Brownell wrote:
> On Thursday 20 November 2008, Mike Frysinger wrote:
>> On Thu, Nov 20, 2008 at 15:24, David Brownell wrote:
>> > On Monday 17 November 2008, Bryan Wu wrote:
>> >>                         /* set transfer mode, and enable SPI */
>> >>                         dev_dbg(&drv_data->pdev->dev, "doing DMA in.\n");
>> >>
>> >> +                       /* invalidate caches, if needed */
>> >> +                       if (bfin_addr_dcachable((unsigned long) drv_data->rx))
>> >> +                               invalidate_dcache_range((unsigned long) drv_data->rx,
>> >> +                                                       (unsigned long) (drv_data->rx +
>> >> +                                                       drv_data->len));
>> >
>> > As a rule, you should use the standard kernel interfaces
>> > for such stuff instead of platform-specific calls like
>> > those two.  There are a LOT more developers who can find
>> > and fix bugs that way.
>>
>> could you elaborate on the common calls that would replace these ?
>
> See Documentation/DMA-(mapping,API}.txt ... the "mapping"
> document's section on what memory may be used for DMA is
> not otherwise replicated in the description of the "generic"
> versions of those API calls.
>
> Basically, dma_map_single(), dma_unmap_single() ... and
> remember that the caller may have done the mappings for
> you already.

these arent required to provide coherent memory right ?  if that's the
case, i can take a look at getting things updated.

>> > Also, this patch affects the "not full duplex" branch of
>> > this routine.  DMA here seems unusually convoluted ... but
>> > if you didn't invalidate the cache (RX path) before
>> > flushing it (TX path) and instead did it the other way
>> > aroound, would you actually *need* separate branches?
>>
>> it's convoluted because the hardware sucks.  it cant do full duplex
>> with DMA.  full duplex only works in the non-DMA case.
>
> Ah, ok.  Sucky hardware -- been there, done that, still doing.  ;)
>
> It'd be nice if one of patches snuck in a comment on that
> point:  "Full duplex only works for non-DMA transfers."
> Same rationale:  you may know this hardware inside out,
> but the next person won't.

well, hopefully if they dont know they wont be touching the driver ;).
 i'll add a comment in this code chunk.  thanks for the feedback.
-mike
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