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

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Date:	Thu, 20 Nov 2008 15:58:13 -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 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));
>
> Could you explain why you're not using dma_map_*() calls
> or the rx_dma (and tx_dma) addresses the caller may pass?

i'm not familiar with said API.  i worked with what was there already.

> 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 ?

> 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.
-mike
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