linux-kernel - Re: About dma_sync_single_for

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Message-ID: <CACYmx9O_TJoSmhiai_Mv09JE3HZ+-V=w8KobTn3zPXtkWbojkw@mail.gmail.com>
Date:	Tue, 31 Jul 2012 09:27:49 +0200
From:	Karl Beldan <karl.beldan@...il.com>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: About dma_sync_single_for_{cpu,device}

On 7/31/12, Clemens Ladisch <clemens@...isch.de> wrote:
> Karl Beldan wrote:
>> To tx a chunk of data from the SoC => network device, we :
>> - prepare a buffer with a leading header embedding a pattern,
>> - trigger the xfer and wait for an irq
>> // The device updates the pattern and then triggers an irq
>> - upon irq we check the pattern for the xfer completion
>>
>> I was expecting the following to work:
>> 	addr = dma_map_single(dev, buffer, size, DMA_TO_DEVICE);
>
> Of both the CPU and the device write to the buffer, you must use
> DMA_BIDIRECTIONAL.
>
Hi Clemens,

This does not work (tested) :  seems to me BIDIRECTIONAL would just
add invalidate, and invalidate before the ram has been updated, as
stated, does not work.
In fact the immediately following sync_for_device is intended to cater
for what DMA_BIDIRECTIONAL would provide (though it is not
implementation agnostic), only  invalidating a smaller address range.

Regards,

Karl
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